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HIGH-PRIORITY NEEDS WITHIN PRESENT SYSTEMS An extensive R$D program in support of operational weather satellites is already in existence. This program has been thoughtfully constructed over a period of many years by a capable, dedicated group of meteorologists and engineers. A substantial group of R£D initiatives is well under way; others are under consideration. In these circumstances, it would be surprising indeed if the Panel on Weather and Climate should find major deficiencies in the program as a result of this comparatively limited review. Many of the following recommendations are, therefore, designed to emphasize high-priority needs in areas where work is already under way. It is clearly recognized that many of the recommendations will require an extensive R£D commitment over an extended period of time. IMPROVED SOUNDING OF THE ATMOSPHERE The primary contribution of operational satellites to weather forecasting is the determination of the vertical structure of the atmosphere, especially temperature and humidity. Present IR soundings indicate the temperature struc- ture with an accuracy of about 2°C or 3°C; an accuracy approaching that of radio- sondes (about l°C) would be more useful. The resolution of present satellite observations in clear air is approximately 40 km, a figure satisfactory for synoptic forecasts. However, difficulties are encountered when substantial cloud cover exists. The use of microwave sounders will to a considerable extent overcome these problems. The successful operation of a non-scanning microwave sounder on NIMBUS-5 opens the way to development of a combined IR and microwave sounding system and, indeed, such a system is being developed for operational use. Neither the IR nor the microwave sounder, however, locates temperature inversions well, especially the important inversion at the tropopause (the boundary between the troposphere and the stratosphere). A limb-scanning IR sounder is being prepared for launch on NIMBUS-F in late l974. A test of this instrument on a balloon flight has proved its ability to locate the height of the tropopause and to obtain an accurate temperature profile. With respect to humidity profiles, the total water vapor in a column is measurable to about 20% accuracy by present vertical sounders, and a future sounder will include two channels for water vapor determination. By l978 it should be possible to construct a crude two-parameter vertical profile with 11
an accuracy of 20% to 30%. The sounding, however, will be limited to the middle and lower troposphere, since the upper troposphere is too dry to emit adequate signals in this band. A limb-scanning sounder may make it possible to extend the humidity profile upward from the middle troposphere to the stratosphere. RECOMMENDATION - The Panel recommends that the vertical temperature sounding capability be improved to approach more closely a 1°C accuracy requirement. The Panel strongly supports the concept of a combined IR and microwave sounder for future operational systems and recommends that the possibility be kept open of adding limb-scanning sounders as well. IMPROVED GEOSTATIONARY SATELLITES Geostationary meteorological satellites will become increasingly important in the operational meteorological system. Observational coverage is virtually continuous. These satellites are ideal for frequent monitoring of such phenomena as local severe storms; generally, these platforms are capable of repeated and detailed examination of portions of the earth's atmosphere which are of particu- lar interest or concern. Data from geostationary satellites are of key importance to synoptic forecasts; such information will also be valuable in the display of present-weather information to a wide variety of users. Four important improvements should be made in the geostationary satellite system: IR sounders should be incorporated into the existing satellite system as soon as possible. Three-axis-stabilized geostationary satellites should be used to replace the existing spin-stablized operational satellites. This improvement would make it possible to view the earth continuously, with a gain in viewing efficiency by a factor of l0 to 20. Microwave sounders should be added to three-axis-stabilized geostationary satellites as soon as possible. A low-cost local readout system should be developed to bring the information directly to many users, public agencies and private organizations. RECOMMENDATION - The Panel recommends that high priority be given to a long-term R&D effort to provide a new generation of geostationary satellites. These improved satellites should be three-axis-stabilized, have the capability to view selectively sectors of the earth's disk, be provided with a low-cost local readout system, and incorporate advanced capability in temperature and humidity sounding. l2
OPERATIONAL RADIATION-BUDGET OBSERVATIONS Preparation of synoptic forecasts and climate predictions requires substan- tially improved understanding of the radiation budget of the earth. Budget determination includes the measurement of solar radiation, the determination of the earth's albedo, and the measurement of outgoing IR emissions. These measure- ments will ultimately be used to establish parameters for the energy budget in terms of other measurements of the state of the atmosphere, such as the nature and distribution of cloud cover and the vertical structure of temperature and humidity. Considerable work has already been undertaken in the development of the necessary sensors, and experimental determinations of the earth's energy budget will be taken from NIMBUS-F. However, accurate measurements from space of the earth's radiation balance will not be achieved easily. A host of special prob- lems exists, such as the effect of viewing angle on albedo measurements. In addition, the use of polar-orbiting sun-synchronous satellites would introduce consistent (diurnal) errors into the energy-budget determination. The elimina- tion of such problems will require particular care. RECOMMENDATION - The Panel recommends that an operational capability to measure the radiation budget of the earth be developed as quickly as possible. Studies should be undertaken to determine the kind of multiple-satellite, multiple-orbit system required to obtain good samplings of local times and of reflectance angles, to permit accurate determination of the average planetary albedo. The operational system must provide long-term (that is3 over decades) continuity and consistency of radiation measurements. MONITORING OCEANS The ability of the oceans to store, redistribute, and release heat makes them a prime factor in climate prediction. It is therefore vital that the sea- surface temperature and the heat content of the upper layer of the ocean be observed routinely. Also required are estimates of surface winds over the ocean and of near-surface currents. These data are vital to the development of models of climate. Present measurements of sea-surface temperature, while encouraging, are not accurate enough to be of major use. However, future system improvements promise to provide the accuracy required. Considerable effort must be expended to develop a wind-measuring capability. The estimation of near-surface currents poses an even more difficult problem that has yet to be addressed. RECOMMENDATION - The Panel recommends that an operational capability be undertaken as quickly as possible to measure climatically important oceanic variables. l3
MONITORING ATMOSPHERIC CONSTITUENTS, PARTICULATE LOADING, AND AEROSOLS Theoretical studies have indicated that changes in the composition of the atmosphere with regard to minor constitutents, particulate loading, and aerosols can have a major effect on climate. The measured concentration of carbon dioxide has increased about 25% in the last century. Substantial variations in stratospheric ozone concentration are also suspected. Particulate loading and aerosol concentration are also believed to be increasing, although reliable data are lacking. Clearly, the intelligent management of man's activities requires careful monitoring of the atmosphere in order to establish a baseline, to detect trends, and to detect climatic changes. Satellite remote- sensing techniques provide a possible major contribution to such a monitoring program. A number of promising techniques are to be tested on NIMBUS-G to demonstrate their effectiveness. RECOMMENDATION - An operational capability to monitor climatically important atmospheric constituents, particulate loading, and aerosols should be undertaken as quickly as possible. ACCURATE DETERMINATION OF OCCURRENCE AND INTENSITY OF PRECIPITATION, SOIL MOISTURE, AND WATER CONTENT OF SNOW Microwave emission has been used experimentally to delineate areas where heavy precipitation is occurring. Such observations are most easily made over the ocean where there is substantially lower surface emission at these frequen- cies. Results have been sufficiently encouraging to warrant the development of an R§D program to measure precipitation on a world-wide basis. Such observations would be particularly valuable not only in meteorology but also in a variety of other disciplines, such as water resources and agriculture. Soil moisture and the water content of snow may also be measurable through the use of microwaves. The difficulties of instrumentation are many. Nevertheless, these types of observations, which can best be made from meteorological satellites, would provide invaluable information. An appropriate R§D program should be initiated to determine the feasibility of such measurements. RECOMMENDATION - The Panel recommends an R&D effort to produce sensing and processing systems to provide information on the occurrence and intensity of precipitation and on the moisture content of soil and snow cover. IMPROVED DETERMINATION AND EVALUATION OF THE ACCURACY OF EARTH OBSERVATIONS FROM SPACE It is essential that users have access to detailed information concerning the accuracy of meteorological measurements derived from satellite sensor systems. A series of carefully planned ground-air-verification experiments will be needed to compare the satellite-based observations with those obtained by the various land-based techniques. Information should also be provided on the horizontal and vertical resolutions of the sensor systems and the derived observations. l4
RECOMMENDATION - The Panel recommends that observations obtained or derived from satellite sensors be checked regularly against those from ground-based observing methods with known degrees of accuracy. Results of such verification tests should be published and furnished to users along with the data. Informa- tion on the effective horizontal and vertical resolutions of the observations should be provided as well. l5
