Patterns and Perspectives in Environmental Science NATIONAL SCIENCE BOARD 1972 /CO. Patterns and Perspectives in Environmental Science Report Prepared for the National Science Board National Science Foundation 1972 **T V- < at UJ Q. % UJ -.2 - 4 1 1 , 1 , 1 , 1 , 1 1 1 1 7 80 90 1900 10 20 30 40 50 60 7 3 YEAR The mean observed temperature variation for the northern hemisphere has here been adjusted for the time lag of the system, the warming effect of CO,, and the effect of both stratospheric (volcanic) and tropospheric dust. The dust effect ex- plains 80% of the variance of the adjusted temperature, with 63% due to strato- spheric and 17% due to tropospheric dust. The resulting curve shows what tempera- tures would be observed under conditions of direct solar radiation with cloudless skies, although some residual errors remain. (Compare Figures III-4, 8, and 9) 71 PART III — CLIMATIC CHANGE numbers and control of energy. Thus man can, and probably has, modified the climate of the earth. The Climates of the Past Century From late in the nineteenth century until the middle of the twentieth, the mean temperature of the earth rose. During this time the carbon dioxide content of the atmosphere rose enough to explain the global temperature rise — apparently the first global climatic modification due to man. At the same time, local production of particulate pollution was starting to increase rapidly due to mechanization and industrialization. By the middle of the twentieth century, these trends — amplified by a general population ex- plosion and a renewal of volcanic activity — increased the worldwide particulate load of the atmosphere to the point where the effect of these particulates on the global albedo more than compensated for the carbon di- oxide increase and world temperatures began to fall. The total magnitude of these changes in world or hemispheric mean temperature is not impressive — a fraction of a degree. However, the difference between glacial and non- glacial climates is only a few degrees on the worldwide average. Actually, it is not the mean tem- perature of the earth that is impor- tant, but rather the circulation pattern of the atmosphere. This is stronglv dependent on the temperature differ- ence from the tropics to the poles. The same man-modifiable factors that affect the mean temperature of the globe-albedo and carbon dioxide — even if applied uniformly over the globe — will have the effect of chang- ing the meridional temperature gradi- ent and thus the circulation pattern and resultant weather pattern. It is this change of pattern that is of prime concern. Dzeerdzeerski in the Soviet Union, Kutzbach in the United States, and Lamb in England have all pro- duced different kinds of evidence that the circulation patterns have changed in the past two decades. In turn, the local climates show change — some regions wetter, some drier, some colder, some warmer — though some remain unchanged. The most striking changes have been where the effects of the change are cumulative, such as the slightly changed balance between evaporation and precipitation in East Africa which has caused the level of great lakes such as Victoria to rise markedly. Another case is the balance between ice wastage and production that has changed enough in the last decade to bring drift-ice to the Icelandic shores to an extent unknown for a century. It would be most useful to know what the cumulative ecological effect of these local or regional changes might be. Since biological selection in re- sponse to environmental changes usu- ally requires a number of generations to show the total effect of the change, it is probably too soon to know the total ecological impact of the present change. Here we can only look to the past to see what is possible. The Lesson of History The advent of radiocarbon dating has given a new dimension to the study of the variety of paleobotany known as palynology. It is now pos- sible to put an absolute time-scale on the record of environmental change contained in the pollen assemblages recovered from bogs and lake sedi- ments. In the context of the present discussion, the most startling result is the rapidity with which major envi- ronmental changes have taken place. If we examine the most carefully studied and best-dated pollen profiles, we find that the pollen frequencies often show a quasi-exponential change from, for example, an assemblage that might indicate boreal forest to an assemblage typical of mixed hard- woods. Calling the time required for half the change to occur the half-life of the transition, it appears that such major changes in vegetation may have half-lives of a couple of centuries or less. (Greater specificity must await analyses with much finer time-resolu- tion than has been generally used.) Since the plants integrate the climate, the half-life of the climatic change must be shorter still! With the agricultural land use of the world still reflecting the climatic pattern almost as closely as the native vegetation did, a major shift in cli- matic pattern within a century could be disastrous. Unlike the past, migra- tion into open lands is not possible: there are none, and forcible acquisi- tion of agricultural land with a favor- able climate is not acceptable. Only in a few nations would a combination of regional variety and advanced tech- nology allow an accommodation to a major climatic change. What We Need To Know Faced with the possibility that we are well into a climatic change of ap- preciable magnitude, of man's mak- ing, there appear a number of ques- tions to which answers are urgently needed. Since in the past there have been rapid changes in climate due to natural causes, such as major changes in vol- canic activity, what is the probability of increased volcanism in the next few decades adding to the pollution of the atmosphere made by man and thus speeding up the present climatic change? How far will the present climatic change go? It appears that the change from a glacial climate to a nonglacial climate occurred with great rapidity. Would the opposite change occur as fast? What chance is there, on a rela- tively short time-scale, to control the sources of turbidity? 72 CAUSES Ol CHANGE If we have reverted to the climate characteristic of the early 1800's, what displacements in the world agricul- tural pattern will occur in the next decade? The answers to these and a host of related questions will require a much more sophisticated knowledge of cli- mate and the man-environment sys- tem than we now possess. Time is short and the challenge to science is clear. Environmental Change in Arid America One of the great controversies in ice-age paleoecology is how to explain the virtually simultaneous coast-to- coast extinction of large mammals in North America around 11,000 years ago. We know, for example, that ele- phants once existed even in the pres- ently arid lands of the West. Paleon- tologists have commonly recovered the bones of Mammuthus columbi in arid America, along with bones of other extinct large mammals, includ- ing horses, camels of two extinct genera, extinct bison, and ground sloth. Did the climate change suddenly? Fossil elephants and the like inevitably provoke visions of a wetter climate and a more productive ecosystem than today's arid land will support. But the fossil-pollen record has indi- cated otherwise. Fossil Pollen and Other Forms of Evidence The technique of fossil-pollen anal- ysis has proved of unique value in determining what the vegetation and, by implication, the primary produc- tivity of arid America must have been during the period when this region, along with the rest of the continent, supported large numbers of native large mammals. Pollen is a very popular fossil be- cause it is produced in quantity by certain plants and, thanks to its acid- resistant outer wall or shell, is pre- served in many types of sediments. Unlike fossils of larger size, pollen is usually dispersed evenly throughout a deposit rather than aggregated in one or a few distinct beds. Under relatively uniform sedimentation, as determined by closely spaced radio- carbon dates, one can estimate the intensity of the local pollen rain through time, as Davis has done in a study of vegetation history at Rogers Lake, Connecticut. Different vegeta- tion zones shed different amounts of pollen — a tundra much less than a forest, for example. This is revealed by the fossil pollen extracted through hydrofluoric-acid treatment of lake muds. In many deposits, especially in arid lands, absolute values cannot be esti- mated. The relative amounts of the dominant pollen types in a deposit can be compared with the pollen content of sediments presently being deposited in areas of natural vegetation. Literal interpretation of the relative pollen percentage cannot be made — i.e., 10 percent pine pollen does not mean that 10 percent of the trees in the stand were pines. But the pollen spectrum of all types identified in a fossil count can be matched, through computer programs or simple direct comparison, with the pollen rain of modern natural communities. This method works especially well in west- ern United States, where there are extensive areas of relatively undis- turbed vegetation. In this way, any major or increasing number of minor changes in vegetation through time can be detected. As opportunity allows, the fossil- pollen record can be compared with other forms of evidence. Macrofossil remains of plants, including seeds and leaves, are found in certain lake muds. They have been reported in remark- able abundance in ancient wood-rat middens of certain desert regions by Wells. The oldest rat's nests studied by Wells are over 30,000 years in age, essentially older than can be deter- mined by the radiocarbon method. The Climatic Record of Western America The fossil record of radiocarbon- dated deposits covering the last 30,000 years in western America indicates an initial cool, dry period becoming colder and wetter by 20,000 to 16,000 years ago. At this time, there were ponderosa-pine parkland and pinyon- juniper woodland at elevations about 3,300 feet below their present lower limits on western mountains. The fate of prairie, both short and tall grass- land, is unknown. The present prairie region was occupied by spruce in the north and pine in the south. This suggests that arid America, like other regions, was affected by the late Pleistocene cooling associated with ice advance over Canada. Around 12,000 years ago the cli- mate changed rather rapidly, becom- ing warmer and drier, until conditions were only slightly cooler and wetter than now. Modern vegetation zones have occupied their present positions, with minor fluctuations, continuously for the last 8,000 years. Thus, the record shows that the environment of western America in- habited by mammoth, camels, native 73 PART III — CLIMATIC CHANCE horses, and bison at the time of their extinction 11,000 years ago was not vastly different from what we know at present. Why, then, did the ani- mals die? Fossil pollen and other evidence from the radiocarbon dating of extinct Pleistocene faunas seem to indicate that no environmental defects will explain this phenomenon. One must look elsewhere. And the only new variable in the American ecosys- tem of the late-glacial period is the arrival of skilled Stone Age hunters. These events of thousands of years ago have major implications for mod- ern-day range management. Implications for Modern Range Management In part, the concept of the West as a "desert" is based on the fact that grass production is indeed quite low. But the dominant woody plants found across the one million square miles of western America — the creosote bush, sagebrush, cactus, and mesquite — do yield large amounts of plant dry- matter annually. Primary productiv- ity data on these western shrub com- munities are less abundant than one might wish. Nevertheless, such data as do exist indicate that shrub com- munities in southern Arizona may yield 1,400 kilograms per hectare a year, considerably more than adjacent grassland under the same climate (12 inches of precipitation annually). Observers have overlooked or writ- ten off this annual production, per- haps because it is often avoided by domestic livestock. Indeed, fifty years of range management in the West has been aimed at destroying the woody plants to make way for forage more palatable to cattle. The effort has been singularly futile and should be abandoned. The Future of Western Meat-Pro- duction — The dilemma faced by the range industry in arid America is that beef can be produced faster, more efficiently, and at less expense in the southeast or in feedlots. If this fact is accepted, one can make a case for keeping large areas of arid America as they are, at least until much more is known about primary production of the natural communities and until some value for Western scenery can be agreed upon. Some large, wealthy ranchers have already recognized this and have disposed of their cattle. More should be encouraged to do so. If a meat-producing industry is to be established in the marginal cattle lands in the West, it should be based on new domestic species, animals that are better adapted to arid environ- ments than cattle and that are adapted for efficient browsing rather than grazing. Potential New Domesticates — One obvious source for potential new domesticates is Africa, where arid ranges that barely sustain cattle are supporting thrifty herds of wilde- beest, kongoni, zebra, giraffe, and kudu. In size and general ecology, the African species bear at least general resemblance to the extinct Pleistocene fauna of the Americas. They did not invade the New World during the ice ages because they failed to range far enough north to be able to cross the Bering Bridge, the only natural method of intercontinental exchange open to large herbivores. Many natu- ral faunal exchanges of arctic-adapted herbivores did occur over the Bering Bridge in the Pleistocene. Some, but not all, of the invaders re-adapted to warmer climates of the lower latitudes. In summary: (a) Studies of fossil pollen and other evidence of the last 30,000 years reveal no environmental defects that might explain the extinc- tion of many species of native New World large mammals 11,000 years ago. (b) The only known environ- mental upset at the time of large ani- mal extinction was the arrival of Early Man. (c) The cattle industry of west- ern America is marginal, being main- tained for reasons of its mystique, not for its economics, (d) If a more pro- ductive use of the western range is desirable, experiments with other species of large mammals should be begun now, as indeed they have been on certain ranches in Texas, New Mexico, Mexico, and Brazil. 74 PART IV DYNAMICS DFTHE ATMOSPHERE-OCEAN SYSTEM 1. OCEANIC CIRCULATION AND OCEAN- ATMOSPHERE INTERACTIONS Oceanic Circulation and the Role of the Atmosphere The ocean circulation is one of the primary factors in the heat budget of the world. The circulation is impor- tant not only internally to the ocean but also to the overlying atmosphere and, indeed, to the climate of the entire earth. Together the sea and the air make a huge thermal engine, and it is not possible to understand either without having some compre- hension of the other. Any studies of ocean circulation must inevitably in- volve this coupling with the atmos- phere. The Present State of Understanding Studies of ocean circulation have progressed a long way in the past fifty years. Measurements of the characteristics of the ocean at great depths have produced at least a gen- eral sense of the major deep circula- tions. And extensive theoretical de- velopments over the same period have given us some glimmering as to why the circulations are what they appear to be. Ocean Variability — Both the ob- servational and theoretical studies have dealt mostly with a steady-state ocean or the long-term mean of an ocean. (See Figure IV-1) During the past few years, however, some data have been accumulated that allow us to speculate a bit about the variability of the ocean. Like mean circulation, variability is closely coupled to the atmosphere, and variations in ocean circulation may lead to, or stem from, variations in atmospheric phenomena. For example, one of the critical parts Figure IV-1 — SEA-SURFACE TEMPERATURE AVERAGE EMPERATURE IN C NORMAL COLD ER- I I £ WARMER The figure shows sea-surface temperatures represented as deviations from global average values of the sea-surface temperature. The global average value for each 5° latitude band is marked at the right-hand edge of the world map. Note the extent of the cold equatorial water in the Pacific (from the coast of South America westward halfway across the Pacific) and the warm water west and north of the United Kingdom. 77 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM of the heat engine is the Norwegian Sea, an area where warm saline sur- face water from the Gulf Stream is cooled by contact with the atmos- phere, made dense, and returned to the open Atlantic as dense deep water in such quantity as to create a recog- nizable subsurface layer extending throughout the Atlantic, Antarctic, Indian, and Pacific oceans. In this case, the power to drive this thermo- haline engine comes from heat ex- change with the atmosphere. Warming of the surface waters in low latitudes and cooling in high lati- tudes creates easily recognizable ef- fects on the circulation of the ocean. The effect of this exchange on the atmosphere is equally important, not just locally — in that the coast of Norway remains ice-free — but also in the larger sense of general effects on the world atmospheric climate. The budget of this heat exchange and the details of its various expenditures must be learned if the earth's climate is to be understood. Seasonal and nonseasonal variations of the heat exchange, and their causes and ef- fects, must be studied. The Gulf Stream is both a cause and an effect of this exchange. It would exist in any case as a conse- quence of the wind-driven circulation in the trade-wind and westerlies areas, as do, in a weaker form, its South Atlantic, North Pacific, and South Pacific counterparts. (The heat and water sink of the far North Atlantic requires a vaster flow in the Gulf Stream than in the other western boundary currents.) But variations in the strength of the Gulf Stream may be either causes or consequences of variations in heat exchange in the Norwegian Sea. Although the effects of these variations may be severely damped by the time the waters enter the immense reservoir of the abyssal ocean, there is no certainty that their effects on the far reaches of the ocean are negligible. Some of the most interesting varia- tions yet observed in the ocean are in the North Pacific, where bodies of surface water thousands of miles in diameter remain warmer or colder than their seasonal means for periods ranging from three months to over a year. Such features seem to be char- acteristic of the North Pacific. Thus, a typical map of surface temperature is not one that is very near the norm everywhere, with many small highs and lows; instead, the whole North Pacific may consist of three to five large areas of deviant temperature. Such features have been noted only in the past fifteen years. They are beginning to receive the attention of meteorologists, as well as oceanogra- phers, since their consequences for the atmospheric climate cannot be discounted in attempting to under- stand and predict the world's weather. Prediction — Our present under- standing of the ocean is barely suffi- cient to account for the major cir- culations in a general way. Some preliminary attempts are now being made to predict specific features of ocean behavior, most of them being based on the persistence of deviations from the mean. That is, if an area shows an abnormally high surface temperature in one month, this anom- aly is apt to endure or persist for several months more and to diminish to the norm slowly. Strictly speaking, this is not prediction but merely the extrapolation of a present feature. More ambitious predictions are being contemplated, but they are still in very early stages. Advances in Instrumentation Devices to measure ocean currents have improved greatly over the past ten years. They have been used to monitor changes in position of the Gulf Stream, to measure its deep flow, and to investigate some of the principal inferences about deep cir- culation in the Pacific and Atlantic oceans. Considerable improvement has also been achieved in instruments for measuring water characteristics. Moored buoys of various kinds have been developed for deep-water use within the past decade. They are used for monitoring certain character- istics of the ocean and atmosphere, including wind, air, and sea tempera- ture, subsurface temperature, waves, and, possibly, water velocity. These measurements can either be recorded and recovered by vessels or trans- mitted immediately by radio to ap- propriate shore bases. The future may see interrogation and retransmission of signals by satel- lite. The advantages of such monitor- ing stations would include relatively inexpensive operation (compared to weather ships) and the ability to gather data from regions that are out- side normal shipping lanes but may be extremely pertinent to ocean and weather studies. Deficiencies in the Data Base The data base for study of the ocean consists of measurements of water characteristics in various loca- tions and depths at different times and measurements of currents, waves, tides, and ocean depths. In some areas and some seasons, this data base is adequate for a long-term mean to be established; it is not continuous enough in time, however, to allow for adequate study of variations from the long-term mean. In other areas and seasons, the data base barely exists. High-latitude areas in winter have hardly been explored. Our knowl- edge of the deep arctic is extremely limited. Some few winter data are available from the antarctic region. The deeper parts of the ocean may be better represented in the present data base than the surface parts, since the deeper parts show less time-variation than the upper layers. Other parts of the data base in- volved in investigating ocean circu- lation include atmospheric-pressure observations and wind measurements, air temperature and the like. These, 78 OCEANIC CIRCULATION AND OCEAN-ATMOSPHERE ITONS too, are limited both in time and space. Major shipping lanes are fairly well measured in many seasons. Among the more systematically meas- ured areas are the North Sea, the California Current system, and the Kuroshio Current. But data from the areas that ships avoid, either because of bad weather conditions or because they do not represent profitable ship routes, are generally sparse. Not only is the arctic poorly represented even with atmospheric information, but also the South Pacific and large parts of the South Atlantic. Very few areas in the world are represented by a data base sufficient to allow for sea- sonal and nonseasonal variations. Numerical models of the ocean are also still in an early stage of develop- ment. What is Needed A proper understanding of air-sea interchange and of deep flow are among the most urgent tasks of oce- anic circulation research. We need to determine which data are critical, ob- tain them, and use them in mathe- matical modeling of the ocean. Topics of practical importance to man, re- quiring urgent study, include fisheries production in the world ocean; this is related to ocean circulation, since the latter controls the availability of plant nutrients. Better understanding of the Arctic Ocean is crucial to proper evaluation of its possibilities as a commercial route for surface vessels or subma- rines. Better knowledge of the deep circulation and the rates of exchange of ocean water — both from the sur- face to the bottom and from the deeper parts of one ocean to the deeper parts of another — is particu- larly important in the light of new concerns over contamination and pol- lution. While the ocean can act as a reservoir to absorb, contain, and re- duce much of the effluent now being produced, it is not of infinite capacity nor can it contain materials indefi- nitely without bringing them back onto the surface. Time-Scale — It is not possible to lay out a time-scale for many of the things that must be investigated. For the problem of describing the mean ocean, another ten or fifteen years might be sufficient. In that period of time, it would be feasible to collect the additional data needed without substantially expanding the facilities. In order to accomplish this, however, the various institutions capable of carrying out the requisite measure- ments would have to devote a greater part of their time to this subject — and this may not be desirable. Developing a data base to study the time-variable ocean is a different sort of problem. Since our under- standing of the nature of time-varia- tions is still in a primitive stage, we must first learn how to observe the phenomena and then begin a system- atic series of observations in the ap- propriate places. Progress has been made in learning how to do this from buoy deployments in the Pacific and Atlantic oceans. These are prelimi- nary, however, and must be greatly augmented before we can really un- derstand even the scale, much less the nature, of the anomalies being observed. Understanding of this kind usually advances step by step from one plateau to another, but the steps are highly irregular both as to height and duration, and a feasible time- scale cannot be estimated. Necessary Activity — On the one hand, the scale of the problems dis- cussed here suggests large-scale, large-area, heavily instrumented re- search carried out by teams of in- vestigators. On the other, the history of ocean circulation research has shown that some of the greatest con- tributions were made by individuals — e.g., Ekman transport, Stommel's westward intensification, Sverdrup transport. A balance is required be- tween large-scale programs compa- rable to the space program and indi- vidual small-scale projects. One of the first needs is to train people able to work on problems of both the ocean and the atmosphere. The two fields have been far too sepa- rated in most cases. People trained in mathematics and physics are avail- able, but the average student finds it difficult to acquire a working back- ground in both the oceanic and at- mospheric environment; indeed, many people trained in physics and mathe- matics have limited backgrounds in either environment, relying on theory without adequate knowledge of the structure of the two systems. On Predicting Ocean Circulation Nonspecialists tend to think of ocean circulation systems as being primarily a matter of geographical exploration. We are not going to dis- cover many new undercurrents, how- ever. Nor will simple-minded "moni- toring" of ocean currents teach us much. Twenty years of looking for — and not finding — relations be- tween changes in patterns of applied wind stress and the total transports of currents like the Gulf Stream where it passes through the Florida Straits warn us that the chain of cause and effect in the ocean is rather 79 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM complicated and that the primary problem is to make more profound our understanding of the ocean as a hydrodynamical phenomenon. What We Know — and Don't Know It has been pointed out that there has been a really effective growth of understanding of ocean surface waves only in the last decade. And ocean surface waves are probably the most easily observable and dynamically linear of ocean phenomena. Internal waves and oceanic turbulence are not so easily observable, and treatments of these phenomena are a thin tissue of preliminary theory largely unsup- ported by observation. Studies limited to rather high-frequency phenomena actually represent the kind most nearly duplicable in the laboratory. There is a small body of theory concerning oceanic circulation, but it deals only with the climatological mean circulation. The role of medium- scale eddy processes in ocean circula- tion is completely unknown, although current measurements indicate that they can be very important — as, for example, they are in the general circu- lation of the atmosphere. A two- pronged development of mathematical modeling and fairly elaborate field in- vestigation is going to be necessary to develop much further our under- standing of the hydrodynamical in- teraction of these eddies and the mean circulation. (A working group of the Scientific Committee on Ocean Research of the International Coun- cil of Scientific Unions recommended a "Mid-Ocean Dynamics Experiment" (MODE).) Considering the three- dimensional detail of velocity struc- ture and its development in time that such a measurement program will entail, it seems clear that a major input from the engineering commu- nity will be needed.) Technological Limitations Oceanography is not presently competent technologically to tackle the tasks of measurement that are necessary in trying to tinravel the dynamical features of large-scale mo- tions. The difficulty is simply that one needs to map variables like velocity rather densely in large volumes (per- haps 2 miles deep and 300 miles on a horizontal side) for rather long pe- riods (perhaps a year) with sufficient accuracy that reliable statistics can be calculated for complicated functions like triple correlation products. Many different modes of motion are occur- ring simultaneously, and we need to be able to separate one mode from another in order to compute interac- tions. Therefore, a great variety of arrays of sensors need to be arranged in different configurations and on different scales for gathering the kind of data required from the ocean. Some test portions of the ocean will need to be heavily instrumented in a manner more sophisticated than pres- ent small-scale observational opera- tions can achieve. It is safe to say that solutions of problems of internal waves, the general circulation and eddy processes, and such important local processes as coastal upwelling are simply going to have to wait until major new instrumental arrays be- come available. There is a limit beyond which in- ferior technique cannot go. It needs to be made very clear what a helpless feeling it is to be on a slow-moving ship, with a few traditional measuring techniques like water bottles and pingers on hand, trying to keep track of a variable phenomenon like an eddy that won't hold its shape. A faint idea of the elusiveness of the phenomenon can be conveyed to any- one who has tried to pick up mercury with his fingers or who has watched a teacher trying to keep track of her pupils on an outing to a public park. But the ocean environment is so much larger, so much harder to see, that we don't bring many of "our children" home. Measurement in large-scale ocean physics illustrates this limit very well. Further theoretical devel- opment is simply going to have to wait upon adequate measurement technique. The theoretical difficulties are not serious; mathematical model- ing can be worked by machine once sufficient insight has been gained as to what is actually going on in the ocean. The Need for Mathematical Models Some advances in climate control, pollution evaluation, and numerical weather forecasting might be achieved simply by extending present land- based meteorological networks into the ocean by means of buoys. Per- haps a superficial knowledge of tem- perature on a coarse grid in the upper 100 meters of the ocean will be useful to meteorologists. But this will not provide the basis for a quantitative, rational, ocean-prediction system. In order to be able to predict the mechanism of the ocean it is neces- sary to have numerical-mathematical models that have been verified by comparison with actually observed case histories of oceanic motion. Be- cause there are several modes of such motion, these experiments or com- parisons have to be made on several different scales. But to date they have not been made. They are beyond our technical means. Actually, it is too early to try to design an oceanic monitoring system; some experimental measuring systems are needed first — aimed squarely at providing input for mathematical numerical modeling of the basic hy- drodynamical processes at work. Suc- cessfully tested models could evolve into successful prediction schemes. If sufficient resources were mus- tered to start a good crew of instru- ment engineers on a sample program of measurement, sufficient progress might be made in carrying out one sample comparison of theory and observation to catalyze progress on the other necessary experiments. One has the feeling that the science is locked in a dead-center position, and that a mighty shove is going to be needed to get it rolling. 80 OCEANIC CIRCULATION AND OCEAN-ATMO 1 1 IONS Hydrodynamic Modeling of Ocean Systems Waves and currents in the ocean can be organized into many different categories depending on horizontal dimension and the time-scale of vari- ability. Some of these categories are strongly interconnected, others al- most independent. In Figure IV-2 an attempt is made at classification, along with an indication of the principal ways in which each phenomenon has an impact on human activities. (The emphasis in this outline is on ocean- circulation phenomena; surface waves, tides, and storm tides are treated only briefly, although thev are admittedly important subjects from the stand- point of practical disaster-warning systems.) Present Status Wind Waves and Tidal Waves — The numerical models presently used to predict surface waves are essentially refinements of earlier operational models developed by the U.S. Navy; they have proved valuable to ship- ping. New computer models, how- ever, allow a much more detailed in- corporation of the latest experimental and theoretical advances in the study of wave generation. Furthermore, or- biting satellites may soon be able to provide a good synoptic picture of the surface sea state all over the globe. Given an accurate weather forecast, computer models would then be able to predict future sea states. Indeed, it may turn out that the ultimate limi- tation to wave forecasting will involve the accuracy of the weather forecast rather than the wave-prediction model itself. Operational models for predicting tidal waves (tsunamis) have been de- veloped for the Pacific, where the danger of earthquakes is greatest. As soon as the epicenter of an earth- quake is located by seismographs, the model can predict the time a tidal wave will arrive. Such warning sys- tems are being developed by the National Oceanic and Atmospheric Administration (NOAA) and the Japanese Meteorological Agency. Storm Surges and Tides — Most of the research in developing numerical models to predict storm tides has been carried out in Europe, in con- nection with flooding in the North Sea area. In the United States, storm surges caused by hurricanes ap- proaching the Gulf Coast have gener- ated the most interest. The results of these model studies appear promising. Graphs and charts based on the model calculations may be used by Weather Service forecasters in mak- ing flood warnings. The models will also be useful in the engineering de- sign of harbor flood-walls and levees. In time, computer models will prob- ably replace the expensive and cum- bersome laboratory models of harbors now used by coastal engineers. Figure IV-2 — CLASSIFICATION OF WAVES AND CURRENTS Time-Scale Local Intermediate Global Short (minutes) Surface Waves (shipping, shore erosion, offshore drilling) Tidal Waves (tsunamis) (safety of shore areas) Intermediate (hours/days) Ocean Turbulence and Mixing (pollution, air-sea interaction Storm Surges (safety of shore areas, hurricane damage) Tides (navigation) Long (months/years) Near-Shore Circulation (pollution) Circulation of Inland Seas (Great Lakes pollution, polar pack-ice models) Circulation in Ocean Basins (long-range weather forecasting, fisheries, climatic change) The chart classifies waves and circulations as functions of time and distance. 81 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM Ocean Circulation — Over the past decade, three-dimensional numerical models for calculating ocean circula- tion have been developed by the So- viet Hydrometeorological Service and NOAA. The methods used are sim- ilar to those of numerical weather forecasting. Given the flux of heat, water, and momentum at the upper surface, the model predicts the re- sponse of the currents at deeper levels. The currents at deeper levels in turn change the configuration of temperature and salinity in the model ocean. Although active work in develop- ing these models is being conducted at several universities, the only pub- lished U.S. calculations are based on the "box" model developed at NOAA's Geophysical Fluid Dynamics Laboratory. This model allows the inclusion of up to 20 levels in the vertical direction and a detailed treat- ment of the bottom and shore con- figuration of actual ocean basins. Cox's calculation of the circulation of the Indian Ocean is perhaps the most detailed application yet at- tempted with the NOAA "box" model. Using climatic data, it was possible to specify the observed dis- tribution of wind, temperature, and salinity at the surface as a function of season. The model was then able to make an accurate prediction of the spectacular changes in currents and upwelling in response to the changing monsoons that were measured along the African coast during the Indian Ocean Expedition of the early 1960's. Application of the Model to Prac- tical Problems — The numerical mod- els designed for studying large-scale ocean circulation problems can be modified to study more local circula- tion in near-shore areas or inland seas such as the Great Lakes. Thus, numerical models may be useful for the many problems in oceanography in which steady currents play a role. A partial list includes: (a) long-range weather forecasting; (b) fisheries fore- casting; (c) pollution on a global or local scale; and (d) transportation in the polar ice-pack. Needed Advances The Data Base — Standard oceano- graphic and geochemical data provide a fairly adequate base for modeling the time-averaged, mean state of the ocean. The data base for modeling the time-variability of the ocean is extremely limited, however. Infor- mation on large-scale changes in ocean circulation as well as the small- scale variability associated with mix- ing in the ocean have not been gath- ered in any comprehensive way. Future progress in ocean modeling will depend on more detailed field studies of ocean variability. Such studies will establish the data base for the formulation of mixing by small-scale motions which must be included in the circulation model. Information on large-scale variability will provide a means for verifying the predictions of the models. Technical Requirements — The most promising approach appears to be the different arrays of automated buoys that have been proposed as part of the International Decade of Ocean Exploration (IDOE) program. Coarse arrays covering entire ocean basins, as well as detailed arrays for limited areas, will be required. Another technical requirement for ocean modeling is common to a great many other scientific activities: the steady development of speed in elec- tronic computers and the steady de- crease in unit cost of calculations. Manpower Training — Numerical models of currents have now reached a point where they can be of great value in the planning of observational studies and the analysis of data col- lected at sea. The models can be used in diagnostic as well as predictive modes. This is particularly true of the buoy networks proposed as part of the IDOE. In order to do this, however, more oceanographers will need to be trained to use the numeri- cal models and to carry out the com- putations. This action will have to be taken quickly if numerical models are to have much signficance in IDOE programs. Application of Ocean Modeling in Human Affairs As pointed out by Revelle and others, a large fraction of the added carbon dioxide (CO-) generated by the burning of fossil fuels is taken up by the oceans. However, few details are known concerning the ocean's buffering effect and how long it will continue to be effective. The ability of the ocean to take up CO- depends very much on how rapidly surface waters are mixed with deeper water. More detailed studies of geo- chemical evidence and numerical modeling are essential to get an un- derstanding of this process. A start in numerical modeling of tracer dis- tributions in the ocean has been made by Veronis and Kuo at Yale University and Holland at the NOAA Geophysical Fluid Dynamics Labora- tory. Another urgent task is to make an assessment of the effect of CO- and particulate matter in the atmosphere on climate. Present climatic knowl- edge does not allow reliable quan- titative predictions of the "green- house effect" due to CO- or the screening out of direct radiation by particulate matter. Published esti- mates have been based on highly simplified models that treat only the radiational aspects of climate. But no climate calculation is complete without taking into account the cir- culation of both the atmosphere and the ocean. Some preliminary climatic calculations have been carried out with combined numerical models of the ocean and atmosphere. But greater effort is required to develop 82 OCEANIC CIRCULATION AND OCEAN-ATMOSPHERE INTERACTIONS more refined ocean models if these climatic calculations are to be reliable enough to be the basis for public policy decisions on pollution control. Time-Scale of Significant Ad- vances — Since published papers on three-dimensional ocean circulation models have only recently begun to appear, rapid development should continue for at least another five years along present lines. In that time, ocean models should have reached about the same level of development as the most advanced atmospheric numerical models today. Within five years, at least the feasi- bility of application of numerical modeling to small- and large-scale pollution studies, long-range weather forecasting, and hydrographic data analysis should be well established. Another five years will probably be required to work out standard pro- cedures for using numerical ocean circulation models in these applica- tions. Effects of Antarctic Water on Oceanic Circulation Except for a relatively thin (slightly less than one kilometer) warm surface layer in the tropics and subtropics, the ocean is basically cold and fairly high in dissolved oxygen content. Ninety percent of the ocean is colder than 8 centigrade, with an oxygen content generally from 50 to 90 per- cent of the saturation level. This warm surface layer, because of its high stability, acts as an impervious cap over the cold abyssal water, blocking renewal (by the usual tur- bulent transfer methods) of the oxy- gen that has been consumed by various biological processes. warm and low-oxygen-content cir- cumpolar deep water (CDW) slowly flows southward and upward. Even- tually, it reaches the near-surface layers at the wind-produced Antarc- tic Divergence. Here, the intense thermohaline alteration resulting from the sea-air interaction converts the CDW into "antarctic surface water" (AASW), which is cold (near freez- ing, —1.6° to —1.9° centigrade) and relatively fresh. Some of the CDW is converted by more intense thermohaline alterations due to ice formation into a fairly dense con- tinental shelf water. At certain times, this shelf water drops to the sea floor where, on mixing with additional CDW, it forms the "antarctic bottom water" (AABW); neither the times nor the exact locations of the vertical motion are adequately known. The AABW has worldwide influence. It reaches far into the northern hemi- sphere in the western Atlantic and Pacific oceans. Though we do not know how the shelf water is produced, three meth- ods appear to be likely: (a) sea-ice formation; (b) freezing, melting, or a combination of these at the floating Why, therefore, is the bulk of the ocean so cold and highly oxygenated? In studying the relationship of tem- perature to salinity in the cold abyssal waters of the world ocean, one is struck by its similarity to that found in antarctic waters. This suggests that the oceanographic processes oc- curring in antarctic waters influence, in a direct way, the physical and chemical properties of much of the ocean's abyssal water. One may think of the antarctic region as a zone in which the abyssal waters can "breathe," renew their oxygen sup- ply, and release to the atmosphere the heat received at more northern lati- tudes. The Antarctic Water Masses Figure IV-3 — ANTARCTIC WATERS AND THEIR CIRCULATION POLAR FRONT ZONE ANTARCTIC DIVERGENCE ZTlUE^NTARCTirr^ \ A /?/ "^AiTARCTIC IURFACI WATER h(>' ICE SHELF S S™ E -^^7i~.., ^ f. * A ( \ S7"7^\5\- se*s^^ — r „rU" \L T-, rf ■ TTl^ rn -Ttn ^rwfn__ rmJL 383 3mm 771 1 526 5 9136 197 8 4399 1959 "h-i-JL hi M 1596 6 I960 1961 m-i n 759 2 J hTI rh-rTHT-rrl rfh. 30" 28 9° 278° 200° 100° o C 30° 289° mm 400 1962 1962 1964 1965 1966 1967 * A ■ V / . \ . ">*IR ^ We* f J y^' 1 V x."*^ — ' s^ 1 V — \ n 1 n urn ^rfKT 4Th-rr " , n-T tl 401 6 mm 712 5194 1432 8 i □ ! Q 100 28 9° 278° mm 500 400 300 200 100 The figure shows a time-series of monthly air and sea temperatures and monthly precipitation amount as measured at Canton Island from 1950 through 1967. 85 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM two years' periodicity, especially dur- ing the 1960's; at other times the rhythms were less regular. The mechanism of the equatorial air-sea rhythms is illustrated in Fig- ure IV-5, which shows that a six- month, smoothed time-series of atmospheric pressure in Djakarta, Indonesia (6°S. 107 E.), exhibits the same long-period trends as the sea- surface temperatures measured at Canton Island and by ships crossing the equator at 165W. When the barometric pressure in Djakarta is lower than normal, the equatorial easterlies heading for the Indonesian low become stronger than normal; this automatically intensifies the Pacific equatorial upwelling and cools the sea surface. The parallelism of the time-series of Djakarta pressure and Canton Island sea temperature is thereby assured. If wind profiles are observed along the equator at two opposite phases of the air-sea rhythm, as exemplified by November 1964, with its cool ocean and aridity, and November 1965, with its warm ocean and abun- dant rainfall at Canton Island, it is found that in November 1964 the equatorial easterlies swept uninter- ruptedly from South America past Canton Island toward a deeper-than- normal Indonesian low, whereas in November 1965 they stopped short of reaching Canton Island. The equa- torial upwelling — a by-product of the equatorial easterlies — extended almost to Indonesia in November 1964, while being confined to a much smaller area east of Canton Island a year later. Concomitantly, the equatorial rainfall was confined to the neighborhood of Indonesia in No- vember 1964; the following year it expanded from the west to beyond Canton Island, while Indonesia suf- fered serious drought. The propulsion of the air-sea rhythms resides in the atmospheric thermally driven equatorial circula- tion over the Pacific, which has its heat source (by condensation) in the rising branch, and heat sink (by radiative deficit insufficiently com- pensated by scarce precipitation) in its descending branch near South America. The oceanic counterpart to this atmospheric circulation is, in part, the westward surface drift and Figure IV-5 — WALKER'S "SOUTHERN OSCILLATION" The diagram shows the similarities in trend ot the time-series of sea temperature and pressure measured at and near the equator in the southern hemisphere. The dotted curve that follows that for Djakarta is based on data from Singapore. The rapid oscillations of the sea-temperature curve measured at the equator in 1958 and 1959 result from more frequent ship crossings — and hence a greater density of short- period detail — rather than from any unusual natural activity. the subsurface return flow and, addi- tionally, the circulation consisting of an upwelling thrust at the equator and sinking motion to the north and south of the equator. These ocean circulations are wind-driven and in- trinsically energy-consuming, but they exert a powerful feedback upon the atmosphere by slowly varying the areal extent of warm water at the equator and thereby varying the ther- mal input for the global atmospheric circulation. In November 1964, when cool up- welling water occupied almost the whole Pacific equatorial belt, the at- mosphere received less heat than in November 1965, when the upwelling had shrunk back into a smaller east- ern area. Consequently, the tropical atmosphere swelled vertically from 1964 to 1965. This swelling was most conspicuous over the Pacific at 160 W. longitude. Moreover, the swelling of the tropical atmosphere had spread all around the global tropical belt between 1964 and 1965, a global adjustment that is inevitable, since pressure gradients along the equator must remain moderate. North and south of the swelling atmosphere in the tropical belt, the gradient of 200-millibar heights in- creased from November 1964 to November 1°65, which indicated increasing westerly winds in the globe-circling subtropical jet streams. This can best be documented in the longitude sector from the area of Pacific equatorial warming eastward across North America and the At- lantic to the Mediterranean. The corresponding change at sea level could be seen most dramatically over Europe, where the moving low- pressure centers abandoned their normal track by way of Iceland to Scandinavia and, instead, in Novem- ber 1965 moved parallel to the strengthened subtropical jet stream and invaded central and southern Europe. Other associated rearrangements involved the arctic high-pressure sys- 86 OCEANIC CIRCULATION AND OCEAN-ATMOSPHL tern, which in November 1965 was displaced toward northern Europe and, consequently, on the Alaskan side of the pole left room for the moving low-pressure systems from the Pacific to penetrate farther north than normal. So much for a description of the air-sea rhythms. Supporting evidence is available from a few other case histories. The motivation for con- tinued research on the equatorial air- sea rhythms is the desire to develop skill in forecasting climatic anomalies. Current Scientific Knowledge The data base is, unfortunately, scanty. As mentioned earlier, Canton Island is the only place where a continuous record of the near-equa- torial air-sea interaction was main- tained; even there, scientific knowl- edge of the air-sea rhythms, extending vertically to great heights in the atmosphere, must be based mainly on a study of the years from 1950 through 1967. Oceanographic cruises in the equa- torial belt have been few and far between in space and time. The EASTROPAC Program, a series of internationally coordinated cruises in the eastern tropical Pacific and trans- equatorial cruises in the mid-Pacific, sponsored by the U.S. National Ma- rine Fisheries Service (NMFS), Hono- lulu, has been the best oceanographic effort to date to explore air-sea in- teraction in the critical area where the air-sea rhythms originate. Eess sophisticated, widely scattered ob- servations are available from com- mercial ships. Those collected by the NMFS in Honolulu from commercial ships that ply the route from Hawaii to Samoa have provided a time-series of equatorial sea temperature at 165°W., together with the corre- sponding sea-temperature series at Canton Island. The two records agree rather well as far as the long rhythms are concerned. Organized reporting of sea and air temperatures from commercial ships crossing the east and central part of the Pacific tropical zone is in good hands with the NMFS in La Jolla, California; the monthly maps issued by that institution are at present the best source of informa- tion on tropical air-sea rhythms. The Status of Instrumentation ■ — An important technical improvement in the ocean data reported from commercial ships will come soon. Selected ships will be equipped with Expendable Bathy-Thermographs (XBT) to enable them to monitor the varying heat storage in the ocean down to the thermocline. Anchored buoys can provide the same information as XBT-equipped commercial ships and will have the advantage of delivery data for long time-series at fixed locations. The buoys that can be permanently fi- nanced should preferably be placed to fill the big gaps between fre- quented shipping lanes. Also, their locations should be selected where ocean temperatures are likely to vary significantly, for instance along the equator. Infrared radiometers on satellites can be adjusted to record sea tem- perature in cloud-free areas, but the accuracy of such measurements can- not quite compare with careful ship- or buoy-based observations. The great contributions of the satellites to tropical studies are — presently and in the future — the TV-mapping of cloud distribution, the temperature measurements of the top surface of cloud, and, under favorable condi- tions, the movement of individual clouds and cloud clusters. Fixed installations on tropical is- lands will continue to be important for research on ocean-atmosphere in- teraction. Aerological soundings, in- cluding upper wind measurements, are best done from islands; moreover, fundamental measurements like the time variations of the topography of ocean level can only be done with a network of island-based tide gauges. The latter job does not call for very expensive equipment, and the tide gauges can be serviced as part-time work by trained islanders; the aerological work, on the other hand, calls for a technologically skilled staff on permanent duty. Replacements for Canton Island as an aerological observatory would be relatively expensive, but yet cheaper than was Canton, if islands with stable native population were selected for observatory sites. The two British islands of Tarawa (l°2l'N. 172°56'E.) and Christmas (1°59'N. 157°29'W.) would be ideal choices. Mathematical Modeling — A crude modeling of an asymptotically ap- proached "steady state" of an equa- torial ocean exposed to the stress of constant easterly winds has been pro- duced by Bryan, of the Geophysical Fluid Dynamics Laboratory, NOAA. A corresponding, quickly adjusting atmospheric model of the equatorial circulation, such as observed over the Pacific, was described in 1969 by Manabe, also of the Princeton NOAA team. Presumably, the ocean and atmos- pheric models can soon be joined for a simulation of the equatorial air- sea rhythms. Even without mathe- matical formulation, the rhythm can be crudely visualized to operate as follows: The cooling phase of the rhythm begins when the equatorial easterlies of the eastern Pacific start increasing and thereby start intensifying the upwelling. This increases the tem- perature deficit of the eastern end of the oceanic equatorial belt com- pared to its western end. The asso- ciated feedback upon the atmosphere shows up in an increased east-west temperature contrast, which produces an increment of kinetic energy in the equatorial atmospheric circula- tion. This, in turn, feeds back into 87 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM increasing upwelling and ocean-cool- ing over an increasing area. A corresponding chain reaction can he visualized for the phase of the rhythm characterized by decreasing easterly winds, decreasing upwelling, and increasing equatorial ocean warming. Hence, a slow vacillation between the two extreme phases of equatorial atmospheric circulation, rather than a stable steady-state equatorial circulation, becomes the most likely pattern. Simulation experiments are pres- ently being planned on a global basis, encompassing both ocean and at- mosphere; they will bring more pre- cise reasoning into the explanation of the equatorial air-sea rhythms and, hopefully, into the interpretation of their teleconnections outside the tropics. Both the Princeton team, un- der Smagorinsky, and the team at the University of California, at Los Angeles, under Mintz and Arakawa, are progressing toward that goal. Requirements for Scientific Activity Continued empirical study of the tropical air-sea rhythms, in past and in real-time records, should accom- pany and support modeling efforts of theoretical teams. The knowledge gained on tropical air-sea rhythms and their extratropical teleconnec- tions so far rests on the study of only a limited number of case his- tories. Much more can be learned by studying the whole sequence of years 1950-67, during which Canton Island was available as an indicator of the air-sea rhythms. These years include the International Geophysical Year period, which happened to exhibit some extreme climatic anomalies and also had better-than-normal global data coverage. Such investigations are relatively cheap. The main expense goes into the plotting and analysis of world maps of monthly climatic anomalies in several levels up to the tropopause. Such a system of climatic anomaly maps would be the empirical tool for tracking the mechanism of the tele- connections. Liaison with EASTRO- PAC and other post-1950 Pacific tropical oceanographic research would become a natural outgrowth of the "historical" study. The 1970's is to be the era of the International Decade of Ocean Ex- ploration (IDOE) as well as that of the Global Atmospheric Research Program (GARP). The study of trop- ical air-sea rhythms belongs within the scope of both of these worldwide research enterprises and, indeed, will serve to tie the two together. The ultimate goal of IDOE-plus-GARP should be to model the atmosphere and the world oceans into one com- prehensive system suitable for elec- tronic integration. That endeavor should produce meaningful progress toward climatic forecasting by the end of the 1970's. 88 2. ATMOSPHERIC CIRCULATION Modeling the Global Atmospheric Circulation An understanding of the structure and variability of the global atmos- pheric circulation requires a knowl- edge of: 1. The quality and quantity of radiation coming from the sun. 2. The atmospheric constituents — not only the massive ones, but also such thermodynamically active components as water va- por, carbon dioxide, ozone, and clouds as well as other partic- ulates. Furthermore, one must understand the process by which these constituents react with the circulations and their radiative properties — i.e., ab- sorption, transmission, scatter- ing, and reflection. 3. The processes by which the atmosphere interacts with its lower boundary in the trans- mission of momentum, heat, and water substance over land as well as sea surfaces. The behavior of the atmosphere cannot be considered independ- ent of its lower boundary be- yond a few days. In turn, the lower boundary can react sig- nificantly. Even the surface layers of the oceans have im- portant reaction times of less than a week, while the deeper ocean comes into play over longer periods. Hence, the evolution of the atmospheric circulation over long periods requires consideration of a dy- namical system whose lower boundary is below the earth's surface. 4. The interactions of the large- scale motions of the atmos- phere with the variety of smaller-scale motions normally present. If these smaller scales have energy sources of their own, as is the case in the at- mosphere, the nature of the interactions will be consider- ably complicated. In principle, mathematical models embodying precise statements of the component physical elements and their interactions provide the means for numerically simulating the nat- ural evolution of the large-scale at- mosphere and its constituents. Suc- cessful modeling would have potential applications in a number of areas: long-range forecasting; determination of the large-scale, long-term disper- sion of man-made pollutants; the interaction of these pollutants in in- advertently altering climate; the in- fluence of intentionally tampering with boundary conditions to arti- ficially modify the climate equilib- rium. No doubt there are a variety of other applications of a simulation capability to problems that may not yet be evident. Current Status Efforts to model the large-scale atmosphere and to simulate its be- havior numerically began more than twenty years ago. As additional re- search groups and institutions in the United States and elsewhere became involved, steady advances in model sophistication followed. These came from refinements in numerical meth- ods as well as from improved formu- lations of the component processes. Today's multi-level models account for a variety of interacting influences and processes: large-scale topographic variations; thermal differences be- tween continents and oceans; varia- tions in roughness characteristics; radiative transfer as a function of an arbitrary distribution of radiatively active constituents; large-scale phase changes of water substance in the precipitation process; interactions with small-scale, convectively un- stable motions; the thermal conse- quences of variable water storage in the soil; and the consequences of snow-covered surfaces on the heat balance. More recently, combined models have taken into account the mutual interaction of the atmosphere and ocean, including the formation and transport of sea-ice. Although many of these elements are rather crudely formulated as cogs in the total model, it has been pos- sible to simulate with increasing detail the characteristics of the observed climate — not only the global wind system and temperature distribution from the earth's surface to the mid- stratosphere, but also the precipita- tion regimes and their role in forming the deserts and major river basins of the world. Attention is beginning to be given to the simulation of climatic response to the annual radia- tion cycle. Detailed analyses of such simula- tions in terms of the flow and trans- formation of energy from the primary solar source to the ultimate viscous sink show encouragingly good agree- ment with corresponding analyses of observed atmospheric data. Such models have also been applied to observationally specified atmospheric states in tests of transient predict- ability. Even within the severe limita- tions of the models, the data, and the computational inadequacies, it has been possible to simulate and verify 89 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM large-scale atmospheric evolutions of the order of a week. These advances give promise that, as known deficien- cies are systematically removed, the practical level of the large-scale pre- dictability of the atmosphere can converge on a theoretical determin- istic limitation of several weeks. Models have also been used in some, more limited applications. For example, an attempt was made to simulate the long-term, large-scale dispersion of inert tracing material, such as radioactive tungsten, which had been released at an instantaneous source in the lower equatorial tropos- phere. The results were surprisingly good. Only limited attempts have been made to apply extant models to test the sensitivity of climate to small external influences. The reason is that one normally seeks to detect departures from fairly delicately bal- anced states. It is often beyond the current level of capability to simulate an abnormal response that is com- parable in magnitude to the natural variability noise level. Observational Problems The present large-scale data base is essentially dictated by the extent of the operational networks created by the weather forecast services of the world. The existing network is hardly adequate to define the north- ern-hemisphere extratropical atmos- phere; it is completely inadequate in the southern hemisphere and in the equatorial tropics. For example, there are only 50 radiosonde stations in the southern hemisphere in contrast to approximately 500 in the northern. The main difficulties arise from the large expanses of open ocean which, by conventional methods, impede de- termination of the large-scale com- ponents of atmospheric structure responsible for the major energy transformations. This critical defi- ciency in the global observational data store makes it difficult to define the variability of the atmosphere in enough detail to discern systematic theoretical deficiencies. Furthermore, the data are inadequate for the spec- ification of initial conditions in the calculation of long-range forecasts. Recent dramatic advances in in- frared spectroscopy from satellites promise significant strides in defining the state of the extratropical atmo- sphere virtually independent of loca- tion. (See Figure IV-6) However, the motions of the equatorial tropical at- mosphere lack strong rotational cou- pling, making the observational prob- lem there more acute. Independent wind determinations may be needed as well as the information supplied by a Nimbus 3 (SIRS sensor) type satel- lite. It is not yet known to what ex- tent balloon-borne instrumentation or measurements from ocean buoys will be needed to augment satellite obser- vations, especially in the lower tropos- phere. This will depend on just how strongly the variable character- istics of the atmosphere are coupled. A more precise knowledge would per- mit relaxing observational require- ments for an adequate definition of its structure. Figure IV-6 — SIRS SOUNDING T 5 Id (T to UJ 1/4 is sufficient for stability, and Ri ^ 1/4 is neces- sary, but not sufficient, for instability. The entire process has been dem- onstrated by Thorpe in laboratory fluid experiments and by Woods in thin, hydrostatically stable sheets in the summer thermocline of the Medi- terranean Sea. Both of these experi- ments show the development of beau- tifully formed billows, or K-H waves which roll up into vortices and finally break. And both demonstrate the gen- eral validity of the critical Ri sC 1/4. Evidence from tlie AtmospJiere — Ludlam has observed the existence of the K-H instability mechanism in the atmosphere by the presence of billow clouds, but only rarely are the com- bination of cloud and stability con- ditions just right to produce the lovely roll vortices in the clouds that are seen in the laboratory and the sea. The observation of their com- mon presence in the atmosphere has awaited the use of ultrasensitive ra- dars capable of detecting the weak perturbations in refractive index (due to temperature or humidity perturba- tions) which mark sharp inversions. Using three powerful radars at Wal- lops Island, Virginia, Atlas and his colleagues first reported the radar de- tection of clear air turbulence at the tropopause; Hicks, Angell, Hardy, and others have reported K-H waves and turbulence in clear air layers marked by static stability, large wind shear, and small Richardson number. Undoubtedly the most striking evi- dence of the K-H process as a cause of WIT, and of its common occur- rence at interval fronts, are the ob- servations made possible by the use of a unique new ultrasensitive FM-CW (Frequency Modulated Continuous Wave) microwave radar at the Naval Electronics Laboratory Center, San Diego. This radar is capable of one- meter vertical resolution, roiij; hundredfold increase over that pre- viously available with radars of com- parable sensitivity. With this new tool, it has been reported that K-H waves are a virtually ubiquitous fea- ture of the marine inversion over San Diego at altitudes up to about one kilometer. Indeed, the atmospheric K-H waves observed in this manner are commonly as beautiful in form as those produced in the laboratory and observed in the sea. (See Figure IV- 10) It is worth noting that the unex- pectedly classical form of the waves, and their great frequency of occur- rence within the marine inversion, recommends the southwest coast of the United States as an atmospheric laboratory for studies of WIT. What the Data Show — The fact that the observed K-H waves are fre- quently restricted to exceedingly thin layers, sometimes only a few meters in depth, and rarely with amplitudes as large as 100 meters, explains why the previously available high-sensi- tivity radars of poor resolution could not identify them. In other words, the K-H wave structure was simply too small to be seen and the echoes appeared merely as thin, smooth lay- ers marking the base of the inversion. The new data also indicate that, though K-H wave activity may be in progress, the associated turbulence will not be intense unless the waves grow to large amplitude prior to breaking. This has been demon- strated by the erratic perturbations of the height of the radar-detected layer, indicative of moderate turbulence, which resulted from the breaking of K-H waves of 75-meter amplitude. In general, waves of significantly smaller amplitude appear not to pro- duce appreciable turbulence. Work now in progress shows that the turbulent kinetic energy following the breaking of the roll vortex of a K-H wave is directly proportional to the kinetic energy of the vortex im- 109 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM Figure IV-10 — WAVES AND TURBULENCE IN THE CLEAR ATMOSPHERE Height (m) 400 - 300 - 200 1920 1930 TIME (PDT) AUGUST 6, 1969 (Illustration Courtesy of the American Geophysical Union ) Radar echoes from the clear atmosphere reveal a group of amplifying and breaking waves in the low-level temperature inversion at San Diego, California, as observed with a special FM-CW radar. Waves are triggered by the sharp change of wind speed across the interface between the cool, moist marine layer and the warmer, drier air aloft. They move through the radar beam at the speed of the wind at their mean height, about 4 knots, so that crests appear at succes- sive stages of development. In the second wave at 1919 PDT cooler air from the wave peak drops rapidly as the breaking begins. By 1929 PDT the layer has become fully turbulent, and the radar echo subsequently weakens. Note, too, the secondary waves near the crests at 1919.5, 1922, and 1926 PDT; these secondary waves give rise to microscale turbu- lence, which causes the echo layers to be detected. The resulting turbulence would be weak, as detected by an air- craft. Waves of this type occur regularly in the low-level inversion, and are believed to be similar to those which cause the severe turbulence occasionally encountered by jet aircraft at high altitude. mediately prior to breaking. The r.m.s. velocity of a vortex, Vrms = 0.707 Aoj = 0.707 A(tV/(z) (2) where A is the amplitude of the roll or wave, to its angular rotation rate or vorticity, and cV/cz the wind shear, thus provides a simple estimate of the expected turbulence; prelimi- nary tests support this hypothesis. Moreover, it is of particular interest that the high-resolution radar data provide direct measures of A and its rate of growth as well as of 5V/?z, the shear. Similarly, the turbulence intensity may be deducted from the r.m.s. perturbations in the echo-layer height subsequent to breaking. (As yet, the inherent doppler capability of the FM-CW radar, which would pro- vide direct measurements of both vertical motion and roll vorticity, has not been implemented.) Unresolved Problems — If Equa- tion (2) is validated by experiments now in progress, we may contemplate the prediction of WIT from measure- ments and predictions of maximum wave amplitude and shear. But this assumes that we shall be able to pre- dict the latter. At this writing, the relationship of the maximum wave amplitude to the thermal and wind structure of the environment is not understood. Present K-H wave the- ory is limited to small-amplitude waves and their initial growth rates; clearly, the theory needs to be ex- tended to finite-amplitude waves. But rapid progress is more likely to come from experiments in the real atmos- phere, such as those already men- tioned, which involve somewhat more complex wind and temperature pro- files and interactions than are likely to be tractable in finite-amplitude theoretical models. In this regard, it should also be noted that the critical Richardson number, Ri,- < V-i, which might be regarded as a predictor of WIT, refers only to the initial growth stage of K-H instability. Since the high-reso- lution radar shows breaking K-H waves with amplitudes as small as 5 meters (with negligible resulting tur- bulence) and as large as 100 meters (with appreciable turbulence), a seri- ous question is raised as to the verti- cal scales over which thermal stabil- ity and shear — and so Ri — need to be measured. Surely, the present data imply that Ri must be observed on scales of a meter or less to account for the small-amplitude waves. But it is not so clear that measurements with resolution of 10 to 100 meters or more, such as those available from present-day radiosondes, would be adequate to predict the occurrence of larger-amplitude waves. What, for 110 CLEAR \ . , lENCE example, happens to a growing un- stable wave in a thin stratum when it reaches a dynamically stable layer in which Ri is significantly greater than Vi ? We do not know. This is one of many important questions that needs to be answered by further re- search. Other aspects of the new radar ob- servations that are relevant to flight safety as well as to aircraft investiga- tions of WIT and to its predictability, are: (a) the sharp vertical gradations in turbulence intensity (i.e., some- times the turbulence is restricted to a stratum no more than a few tens of meters thick) and (b) the inter- mittancy of K-H waves and turbu- lence. It is not surprising that one air- craft experiences significant turbu- lence while the next one encounters none in the same region. While the radar observations demonstrate that the base of the inversion and subsidi- ary sheets within it are the seat of K-H wave activity, their breaking is self-destructive in that the shear and stability to which they owed their origin are decreased, and Ri thus in- creased above its critical level. Ac- cordingly, the breaking action acts as an escape valve to release the pressure for K-H activity, and turns the waves and turbulence off. On the other hand, the larger-scale atmospheric processes work to restore the initial conditions, and new K-H waves are triggered. All this speaks to the difficult ques- tions of aircraft experiments directed to observing the initial conditions for WIT, the energy budget involved, and, indeed, its entire life cycle. Pre- cisely where and when should the measurements be made and how are they to be interpreted in the light of WIT's great spatial and temporal vari- ability? Clearly, such experiments should preferably be conducted si- multaneously with a radar capable of "seeing" the waves and turbulence di- rectly. Prospects for Prediction — The prior discussion raises serious doubts as to the ultimate achievement of pinpoint forecasts of WIT in either space or time. While one may expect, eventu- ally, to be able to predict the medium- to large-scale processes that work to develop and sharpen internal fronts and shear, many presently unobserv- able small-scale phenomena (gravity waves, orographic lifting and tilting, convective motions, and such) will operate to reduce Ri to its critical value locally and trigger wave activity here and there. Accordingly, while we may expect significant improve- ments in the predictability of the heights of internal surfaces, and thus in the heights at which WIT is likely, and probably in the predicted in- tensity as well, the actual forecast will probably remain a probabilistic one for many years to come. We should therefore direct a good share of our attention to the remote-probing tools that are capable of detecting both the internal surfaces and the occurrence of waves and turbulence. As in the case of radar detection of thunder- storms, such observations are likely to provide the best short-term predic- tions of WIT for the foreseeable fu- ture. Instrumentation for Detecting WIT Although we have spoken exten- sively of the capability of ultrasensi- tive high-resolution radar techniques in detecting WIT, a few additional re- marks need to be made concerning actual warning devices. Ground-Based Devices — High- resolution FM-CW microwave radar is an obvious candidate for this task. At present, however, it is limited to a detection range (based on over-all sensitivity in detecting clear air in- versions) of about 2 kilometers. An increase of range to 15 kilometers is attainable with available state-of-the- art components. This would accom- plish the detection of clear-air WIT throughout the depth of the tropo- sphere. A network of such st, across the nation, with fixed, ver- tically pointing antennas, is econom- ically feasible. Fortunately, the sig- nificant internal fronts at which WIT occurs are horizontally extensive, so that detection of waves and turbu- lence at one or more stations would indicate the layers affected and the likelihood of WIT at the same height (or interpolated height for sloping layers) in between stations. (Note that we emphasize the need for observa- tions with a high degree of vertical resolution, capable of detecting the suspect layers and measuring the amplitude and intensity of breaking waves.) Airborne Radar — With regard to the use of high-resolution FM-CW microwave radar on board aircraft for purposes of detecting and avoiding WIT along the flight path, the 15- kilometer range capability would be inadequate to provide sufficient warn- ing even if a high-gain antenna of the required dimensions (10' to 15' effec- tive diameter) could be accommodated in the aircraft. Moreover, since the vertical resolution in such a use-mode would correspond to that of the beam dimension rather than the available high-range resolution, the radar could not discern wave amplitude and heights with precision. However, the use of such a radar in both down- ward- and upward-looking directions (from large antennas fitted within the fuselage structure) does appear feas- ible. Clear-air WIT could then be avoided by detecting the heights of internal surfaces and K-H wave ac- tivity above and below flight level and assuming continuity of layer slope. Whether or not such a system should be adopted depends on cost/ benefit/risk ratios. The installation of a $100,000 radar seems warranted when aircraft carry more than 200 passengers. Certainly, it should be adopted for experimental purposes in connection with WIT research. The potential benefits of airborne high- resolution radar to both military and commercial aviation could then be better evaluated. Ill PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM High-Resolution Acoustic Radar is another candidate for clear-air WIT detection from ground-based stations. Such radars have detected thin inter- nal surfaces and stable and breaking wave activity to heights of 2 kilo- meters. The potential to reach 15 kilometers in the vertical direction can probably be realized, although the effect of strong winds aloft on the refraction of the acoustic beam re- mains an open question. Unfortu- nately, acoustic radar cannot be used on board fast-flying aircraft because of the slow speed of sound and the high acoustic noise levels. Future Teclmologx/ — Finally, a real hope still remains for the develop- ment of a coherent laser radar (or LIDAR) sufficiently sensitive to de- tect the small background concentra- tions of aerosols in the high tropos- phere and capable of measuring turbulence intensity through the dop- pler velocities. Although a theoretical feasibility study of such a device in 1966 indicated that the then available LIDARs could not accomplish the task, more recent developments in laser technology may now make such a system feasible. The National Aero- nautics and Space Administration is presently conducting research and de- velopment along these lines. A Note on Acoustic Monitoring As is well known, the propagation of sound waves through the atmos- phere is strongly affected by wind, temperature, and humidity. The pos- sibility therefore exists that measure- ments of the propagation of sound waves could be used to derive infor- mation on important meteorological parameters. The potential of these methods has been analyzed and some experimental results published. It has shown that acoustic echoes can readily be ob- tained from the atmospheric turbu- lence and temperature inhomogenei- ties always existing in the boundary layer of the atmosphere. The equip- ment required is relatively simple; it involves a radar-like system in which pulses of acoustic signal, usually about 1kHz in frequency, are radi- ated from an acoustic antenna, with echoes from the atmospheric structure obtained on the same or on a second acoustic antenna. This field of acoustic echo-sounding of the atmosphere is very new and appears to hold considerable promise for studies of the boundary layer of the atmosphere — i.e., the lowest sev- eral thousand feet. Specifically, re- search is now being undertaken to identify its usefulness for the quanti- tative remote measurement of wind, turbulence, humidity, and tempera- ture inhomogeneity. If, as expected, the technique is shown capable of measuring the structure of the bound- ary layer and the vertical profiles of these meteorological parameters, it will represent a major breakthrough in remote measurement of the atmos- phere, which should be of great value to meteorological observations and research. Its primary application is likely to be in the monitoring of meteorological parameters in urban and suburban areas, for use by air- pollution and aviation agencies. In addition, it is already providing the research worker with totally new in- sight into the detailed structure and processes controlling the atmospheric boundary layer in which we live. 112 5. URBAN EFFECTS ON WEATHER AND CLIMATE Urbanization and Weather For centuries, man has speculated that major battles, incantations, large fires, and, lately, atomic explosions could affect weather, although he made no serious scientific attempts to modify weather until 25 years ago. Except for a few localized projects involving precipitation increases and fog dissipation, however, man's in- tentional efforts have yet to pro- duce significant, recognized changes. Rather, the major means whereby man has affected weather have been inadvertent — through his urban en- vironment. Growing Awareness of the Problem As long ago as 700 years or more, London had achieved a size great enough to produce a recognizable ef- fect on its local weather, at least in terms of reduced visibility and in- creased temperature. Since major ur- ban areas became prevalent in Europe following the Industrial Revolution, Europeans have directed considerable scientific attention to this problem of urban-induced weather change. Now that major urban-industrial com- plexes exist in many countries, world- wide attention has grown rapidly, particularly in the United States, where the growth of megalopolitan areas during the past ten to thirty years has brought with it increasing public and scientific awareness of the degree and, in some cases, the seri- ousness of urban effects on weather. Recent studies documenting signif- icant urban-related precipitation in- creases in and downwind of Chicago, St. Louis, and industrial complexes in the state of Washington have further focused scientific and public attention on the urban-weather topic and its considerable potential. Certainly, even the casual observer is aware that visibility is more fre- quently restricted in a major urban complex than in rural areas, and that this has come from smoke, other con- taminants, increased fog, and their additive, smog. Most Americans are now aware that the temperature with- in a medium-to-large city is generally higher at any given time of the day or season than it is in rural areas. This temperature effect has been rec- ognized and measured for many years, since its measurement, at least at the surface, is relatively easy. "Heat islands" for many cities of various sizes have been well documented. Urban areas also act as an obstacle to decrease winds near the surface, to increase turbulence and vertical motions above cities, and to create, occasionally, a localized rural-urban circulation pattern. There have been enough descriptive studies, further- more, to reveal that many other weather conditions are also being changed, often dramatically, by urban complexes. Although available re- sults indicate that urban-induced weather changes are restricted to the cities and their immediate downwind areas and have little effect on macro- scale weather conditions, the "urban flood" and advent of the megalopolis could conceivably lead to significant weather changes over large down- wind regions. Value Judgments — The question of desirability of the weather changes wrought by urbanization has only re- cently been considered. The fact that many of the urban-induced changes have occurred gradually has not only made them difficult to measure quan- titatively within the natural variabil- ity of weather, but has also made them less obvious and, therefore, un- wittingly accepted by the urban dweller. Now that urbanization is nearly universal, American citizens have suddenly become aware of many of the urban-induced weather changes. In general, such changes as increased contaminants, higher warm- season temperatures, lower winds, added fog, increased thunder and hail, added snowfall, and decreased visibil- ity are considered undesirable. Cer- tain urban-related weather changes are desirable, however, including warmer winters and additional rain- fall to cleanse the air and to add water in downwind agricultural areas. In summary, then, with respect to their effects on weather, urban areas sometimes act as volcanoes, deserts, or irregular forests; as such, they pro- duce a wide variety of weather changes, at least on a local scale, and these changes can be classed as bene- ficial or detrimental depending on the locale and the interests involved. Type and Amount of Weather Change The changes in weather wrought by urbanization include all major surface weather conditions. The list of elements or conditions affected in- cludes the contaminants in the air, solar radiation, temperature, visibil- ity, humidity, wind speed and direc- tion, cloudiness, precipitation, atmos- pheric electricity, severe weather, and certain mesoscale synoptic weather features (e.g., it has been noted that the forward motion of fronts is re- tarded by urban areas). (See Figure IV-11) The degree of urban effect on any element will depend on the climate, 113 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM Figure IV— 1 1 —WEATHER CHANGES RESULTING FROM URBANIZATION Cold Warm Annual season season (percent) (percent) (percent) Contaminants + 1000 +2000 +500 Solar Radiation -22 -34 -20 Temperature +3 + 10 +2 Humidity -6 -2 -8 Visibility -26 -34 -17 Fog + 60 + 100 +30 Wind Speed -25 -20 -30 Cloudiness +8 +5 +10 Rainfall + 11 + 13 + 11 Snowfall ±10 ±10 — Thunderstorms +8 +5 + 17 The table summarizes changes in surface weather conditions attributable to urban- ization. Changes are expressed as percent of rural conditions. nearness to major water bodies, on topographic features, and city size and components of the industrial complex. Furthermore, the amount of effect on the weather at any given time depends greatly on the season, day of the week, and time of day. Thus, urban solar radiation is de- creased much more in winter than summer; is decreased on weekdays; and is decreased more in the morning than in the afternoon. Temperature increases resulting from the heating of urban structures are much greater in winter than in summer; hence, the average urban air temperature in win- ter is 10 percent higher than that in rural areas, whereas in summer it is only 2 percent higher. However, ur- ban temperatures during certain sea- sons and weather conditions can be as much as 35 percent higher or 5 percent lower than nearby rural tem- peratures. It should be emphasized that op- posite types of changes in certain weather conditions are produced at different times. For example, fog is generally increased by urbanization, although certain types of fogs are ac- tually dissipated in large cities. Wind speeds are generally decreased, but they increase in some light wind con- ditions. Snowfall is generally in- creased by urban areas, but under certain conditions the city heat actu- ally melts the descending snow, trans- forming it into rain. Current Scientific Status Most studies of urban effects on weather have been descriptive and based on surface climatic data. Fur- thermore, only a few studies have at- tempted to investigate the causative factors and the physical processes in- volved in urban-produced weather changes. Without careful investiga- tions of the processes whereby urban conditions affect the weather, there is little hope for developing an adequate understanding and, hence, predictive capabilities. Data Base — The present data base is woefully inadequate for studies of most urban-affected weather ele- ments. Two-dimensional spatial de- scriptions of urban effects on weather elements are now adequate only for temperature patterns. Data for weather changes in the vertical are totally inadequate for temperature as well as for all other weather elements. Descriptive types of urban-weather studies based on existing historical records tend to be seriously limited in their spatial information. For instance, studies of urban-rural fog differences have typically been based on surface values from a point in the central city and one at the airport; although these may indicate a 30 percent difference, they fail to describe the horizontal distribution of fog over the urban or rural environs. Unfortunately, adequate descrip- tions of the surface weather changes are not available for most metropoli- tan areas of the United States. Study of the urban-weather relationships in the United States has been much more limited than that in Europe be- cause the surface weather-station net- works in and around American cities have been too sparse. Information useful for such practical problems as city planning can be developed for major U.S. metropolitan centers only on the basis of thorough comparative studies of data from denser urban- rural surface networks than currently exist around most American cities. Instrumentation — Satisfactory tools to perform needed monitoring and study of urban-induced weather changes are available. Major advances in the development of airborne equip- ment to measure meteorological vari- ables and aerosols provide the poten- tial for obtaining the vertical data measurements needed to develop time-dependent, three-dimen- sional descriptions of the weather ele- ments around cities. Field studies of the airflow and vertical temperature distributions at Cincinnati and Fort Wayne, Indiana, have used these new instruments and techniques in pio- neering research. Theory and Modeling — The basic theoretical knowledge and formulas exist for understanding the atmos- pheric chemistry and physics in- volved in urban-weather relation- ships. Ultimately, studies of the urban factors that affect weather elements will provide the inputs 114 URBAN EFFECTS ON ' IATE needed to model the urban-weather system. However, this will require three-dimensional, mesoscale numeri- cal models (not currently available) and computers (soon to be available) with the capacity to handle them. Practical Implications of Urban-Induced Weather Change Regional Planning — The factors that produce undesirable weather changes clearly need to be assessed, and hopefully minimized, in planning and building new urban areas and redeveloping old ones. For instance, the ability of large urban-industrial complexes to produce thunderstorms, heavy rains, and hailstorms in and downwind of the complexes has par- ticular importance in hydrologic de- sign for urban storm drainage and in agricultural planning. Pollution — Knowledge of the urban-induced wind and rainfall changes apt to occur with various weather conditions is also required for determining whether these changes will materially affect pollu- tion levels. The generally expected decrease in winds and poorer ventila- tion are certainly undesirable, but ur- ban-increased rainfall is beneficial in this connection. Such knowledge would also help in improving local forecasting, thus enabling man to do better planning of his outdoor ac- tivities. Weather Modification — Study of the exact causes of various urban- produced weather changes can also be expected to help man in his efforts to modify weather intentionally. In particular, the study of the conditions whereby urban complexes affect pre- cipitation processes could generate needed information about the weather conditions appropriate for seeding, the types and concentrations of ef- fective seeding materials, and poten- tial rainfall changes expected beyond the areas of known urban-related increases. Continuing disagreements over evaluation of man-made changes and the types of physical techniques and chemical agents of modification reveal the need for proper study of these aspects during urban field in- vestigations and analyses. The economic aspects of this prob- lem are hard to assess but are surely significant. Reduced visibility, more fog, and added snowfall directly and indirectly restrict human activity. The damages to health, property, and crops resulting from added contami- nants, less sunshine, higher tempera- tures, and less ventilation can be serious. National economic losses at- tributable to urban-induced weather changes are inestimable. Requirements for Scientific Activity The interactions of urban-produced weather changes with such matters as agriculture and hydrology, and with ecology, are only partly understood, since the inadvertent aberrations are frequently within the limits of natural variability of weather. For instance, the increase in crop yields resulting from urban-increased rainfall could be easily and accurately assessed, whereas the effect on crop yields of increased deposition of urban con- taminants into soils cannot currently be assessed without special studies. Our knowledge and understanding of the interactions of weather changes with man and society are almost totally lacking. The legal and social ramifications are barely understood, although the threats of damage to property, crops, health, and safety from such changes as increased con- taminants, more fog, less sunshine, and higher temperatures are now clear. Certainly, the responses to inadvertent weather changes provide an opportunity to study and assess potential human reaction to planned weather modification. The only means of fully assessing the urban- modification effect of each weather element in a given locale, however, is to measure all elements in three dimensions. Adequate measurement and under- standing of the interactions between urban factors and atmospheric con- ditions that produce, for example, a 10 percent rainfall increase in one urban complex should lead to rea- sonably accurate predictions of the precipitation changes in most com- parable cities where routine measure- ments of the urban factors exist or could easily be performed. Indeed, major projects to study the urban conditions that change weather ele- ments are sorely needed at several cities, each of which should be repre- sentative of basically different North American climates and urban com- plexes so that the results could be extrapolated to other cities. A min- imum national effort would consist of a thorough field project in one city that is representative in size and climate of several others. Such a project would be more meaningful if relevant interdiscipli- nary projects involving the physical and social sciences were conducted simultaneously. To achieve meaningful, three- dimensional measurements of weather and urban conditions will require marshalling of instrumentation and scientific effort to create dense net- works of surface instruments heavily supplemented by vertical measure- ments obtained by aircraft, balloons, and remote probing devices. The scientific skills, personnel, and fa- cilities necessary to explain and pre- dict most facets of this topic exist, but they have yet to be focused on it. Answers exist in relation to sev- eral basic questions concerning the urban-weather topic, but more con- centrated study is needed in the next five years. No serious effort has been made to describe the interac- tion between urban-induced weather changes and man, and this, too, is urgently needed. If performed, these studies should provide information adequate to modify some of the un- desirable weather changes within ten years. 115 PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM The Influence of Urban Growth on Local and Mesoscale Weather The fact that large human settle- ments change the atmospheric con- ditions in their immediate vicinity has been recognized for over a cen