Scientific studies1''2''3 based on the Earth Radiation Budget Experiment (ERBE) have yielded unanticipated fundamental insights into the problem of greenhouse effect and global warming. Continuedobservations of the earth radiation budget with sufficient precision and accuracy are critical for identifymg the role of human activities on climate change. 2.

Understanding the cause of differences among general circulation model pro jections of carbon dioxideinduced climatic change is a necessary step toward improving the models. An intercomparison of 17 atmospheric general circulation models for which sea surface temperature perturbations were used as a surrogate climate change showed that there was a roughly threefold variation in global climate sensitivity. Most of this variation Is attributable to to differences in the models'' depictions of cloudclimate feedback a result that emphasizes the need for improvements In the treatment of clouds in these models if they are ulti mately to be used as climatic predictors.

Cloud radiative forcing (CRF) is the difference between the radiative flux (at the top of the atmosphere, say) which actually occurs in the presence of clouds, and that which would occur if the clouds were removed but the atmospheric state were otherwise unchanged1'2. We also use the term CRF to denote warming or cooling tendencies due to cloud-radiation interactions. Cloud feedback is the change in CRF that accompanies a climate change. It is useful to distinguish among three aspects of the CRF: the "planetary CRF" acting at the top of the atmosphere, the "surface CRF" at the Earth's surface, and the "atmospheric CRF," which acts on the atmosphere itself and is the difference between the planetary and surface CRFs. The planetary CRF, which can be observed from satellites, can be thought of as the sum of the surface CRF and the atmospheric CRF. Because clouds do not absorb much solar radiation, the atmospheric CRF (hereafter, ACRF) is almost entirely due to interactions of the clouds with infrared radiation. The surface CRF involves important contributions from both solar and infrared radiation. Its solar component is strongly correlated with the solar portion of the planetary CRY, and so may be relatively easy to infer from space observations. The infrared surface CRF is much more difficult to observe. This paper presents climate model results concerning the magnitudes, distribution, causes, and consequences of the atmospheric and surface CRF. A review of some earlier studies is combined with a brief presentation of new, previously unpublished results.

The Nimbus-7 Earth Radiation Budget (ERB) instrument began recording data on November 16 1978 and it is now in its 12th data year. The principle products are nine years of global albedo outgoing longwave and net radiation plus continuing solar irradiance measurements. The presence of an associated six-year Nimbus-7 global cloud data set enhances the usefulness of the ERB products. The chief purpose of this paper is to describe some uses that have been made of the data. These include studies in: regional energy heat budgets the improvement of climate and weather prediction models interannual climate variations shortwave bidirectional reflectance from Earth atmosphere scenes and solar physics. The evolution of the data products with time and some algorithm and calibration problems are also briefly discussed. Despite numerous problems the experiment continues to prove its usefulness. 1.

Processed ERBE data are used in several studies in which NMC global analyses and forecasts are examined and evaluated. In the first study ERBE net radiation values for four months are avergaged and integrated to provide global net energy transport. Energy transport by atmospheric circulations is obtained using NMC analyses and results provide a measure ofreasonableness of the analyses. In the second and third experiments ERBE values of outgoing longwave radiation are used as ''topside truth'' to examine the both spinup problem and the radiation algorithms of the global forecasts. 1.

Experiments of increasing sophistication have intensively observed the Earth''s radiation budget over the last 2 decades. During the first years of these measurements there were many types of sensors and satellite orbits. The earliest measurements used very simple thermal devices and assumed that the Earth reflected and emitted isotropically. Including crude bidirectional reflectances and limb-darkening models improved the results. Likewise calibration improved. The Nimbus-7 Earth Radiation Budget (ERB) measurements used pyroelectric and thermopile detectors with on-board calibration. All of the measurements showed that the Earth was darker than expected. In the most recent observations by the Earth Radiation Budget Experiment (ERBE) there have been three major improvements. First ERBE measurements were better calibrated than previous ones. Second the experiment used systematic observations of angular distributions and scene identification from the Nimbus-7 data to improve estimates of instantaneous flux. Third the ERBE algorithms for time interpolation and averaging were more sophisticated than the measurements made before. In the closing years of this century we expect to produce a new set of measurements of the Earth''s radiation budget. This set will come from an investigation of " Clouds and the Earth''s Radiant Energy System" (CERES) a part of the Earth Observing System (EOS) . The CERES investigation will use the ERBE heritage of high-accuracy scanner measurements. CERES will also improve our understanding of the role of clouds in the radiation budget by retrieving

Orbital measurements of the earth''s longwave emitted radiation, and the sun''s radiation reflected by the earth are being made by scanning radiometers on three spacecraft platforms in both high altitude sun-synchronous polar (833 km) and low altitude (600 km) equatorial orbits. These ERBE instruments were carried aboard two TIROS ATN satellites (NOAA-9 and -10) in December 1984, and September 1986, and on the Earth Radiation Budget Satellite launched from Space Shuttle mission 41-G in October 1984. Electronic problems with the scanners on all platforms have halted the reception of data after surpassing the scanner design life. The ERBE instruments have been designed to measure the radiance from earth with an absolute radiometric error of less than one percent. The scanner instrument consists of three broadband radiometer channels, shortwave, 0.25 micron to 3.5 microns, longwave 5.0 to 50 microns; and total, 0.25 microns to more than 50 microns, and use thermistor bolometers as sensing elements. This paper describes the design and operation of the ERBE scanner, an overview of the ground calibration approach, the in-flight calibration stability, and an analysis of on-orbit anomalous behavior.

The Earth Radiation Budget Experiment (ERBE) scanning radiometers are producing measurements of the incoming solar, earth/atmosphere-reflected solar, and earth/atmosphere-emitted radiation fields with measurement precisions and absolute accuracies, approaching 1 percent. ERBE uses thermistor bolometers as the detection elements in the narrow-field-of-view scanning radiometers. The scanning radiometers can sense radiation in the shortwave, longwave, and total broadband spectral regions of 0.2 to 5.0, 5.0 to 50.0, and 0.2 to 50.0 micrometers, respectively. Detailed models of the radiometers'' response functions were developed in order to design the most suitable calibration techniques. These models guided the design of in-flight calibration procedures as well as the development and characterization of a vacuum-calibration chamber and the blackbody source which provided the absolute basis upon which the total and longwave radiometers were characterized. The flight calibration instrumentation for the narror-field-of-view scanning radiometers is presented and evaluated.

The large decrease in specific heat of pure metals achieved at liquid helium temperatures can be used to greatly reduce the time constant and increase the sensitivity of spectrally nonselective electrical substitution radiometers used for satellite remote sensing o fhte earth''s radiations. Single-element and array receivers with a natural time constant below 25 msec and a sensitivity below 10 nW have been constructed and servocontrolled to measure radiations from below 300 nm to beyond 40 microns, to better than 1 percent absolute accuracy. Flight of such an experiment on the Atlas pallet would provide an important proof of concept for future cryogenic radiometry at high accuracy, sensitivity and spatial resolution from the Space Station, from polar platforms and possibly from geosynchronous platforms, as flight qualified 2 K cryocoolers are developed in the next decade. The use of cryogenic radiometers in radiometric calibrations at NIST and elsewhere, and their possible applications for ground-based calibrations of the Eos remote sensing instrument are described.

Scanning radiometers aboard satellites have been employed for over a decade to measure the Earth radiation budget. The measured radiance is converted to flux at the top of the atmosphere by applying angular dependence models (ADM''s) that are a function of the viewing geometry and the scene type. This paper examines the significance of the number of ADM''s used a measure of the effectiveness of the ADM''s and the implications of requiring the ADM''s to satisfy reciprocity. The overall significance of the ADM''s is determined by analyzing the same satellite data with a single Lambertian model a single mean model and the 12 Earth Radiation Budget Experiment (ERBE)1 ADM''s. The ERBE ADM''s will undergo reexamination and possible improvement before they are applied to the future Clouds and the Earth''s Radiant Energy System (CERES) radiation measurements. To measure the improvement of the models we develop a test of " goodness" for the models which includes normality reciprocity and uniformity of flux estimates determined from different viewing geometries. Alongtrack scan data are used for this test of the ADM''s. We discuss the task of constraining the standard ERBE ADM''s to satisfy the reciprocity condition. Earth Radiation Budget Satellite (ERBS) data are used to demonstrate the effect of these modified models. 1. FORMULATION OF ADM''S The radiance L at a point in Wm2sr1 is given by L (1) where M is the flux in

In February 1989 NASA announced the projects it will fund for an eighteen month definition phase study as part of its planned Earth Observing System program. One of these projects was the " Clouds and the Earth''s Radiant Energy System Instrument" (CERES-I). The instrument is composed of two scanning components each very similar in design to the successful ERBE scanner. One operates continuously in the ERBE " cross-track" scan mode while the other rotates the scan plane through 180 degrees every 30 to 45 seconds. A computer simulation code for ERB measurement developed at NOAA/NESDIS has been used to evaluate the potential performance of the cross-track component of this instrument. Specifically we have estimated the error in ERB instantaneous flux measurements for five different CERES-I designs and for two different earth surface resolutions 1 and 2. 5degree latitude and longitude regions. This study is limited to the effects of spatial sampling errors by assuming that the Earth/atmosphere system reflects radiation isotropically. A subsequent study will treat the effects of angular sampling errors and evaluate the value of the rotating azimuth scanner in minimizing this error. To allow for a margin in meeting user requirements (RMS error 10 W*M2) it has been concluded that a triangular (bi-directional) scanner with a field-of-view of 1. 8 degrees (half the ERBE design) averaged into 2. 5 degree target areas is the preferred design (RMS error

Cloud properties for individual ERBE scanner fields of view are obtained from simultaneous collocated imagery data. The cloud properties are used to assess the performance of the ERBE maximum likelihood estimate of cloud conditions which ultimately influences estimates of the radiative fluxes at the top of the atmosphere. During EOS the analysis of MODIS observations within the fields of view of the CERES instruments promises to yield in addition estimates of surface radiative fluxes as well as the vertical distribution of radiative heating within the atmosphere. 1.

It is widely recognized that cloud classification schemes based upon multispectral signatures and clustering measures are severely limited over snow- and icecovered surfaces. This is due to the similarity of cloud and snow/ice spectral signatures in both visible and infrared wavelengths. Infrared threshold techniques are limited in particular by persistent surface inversions and warm lowlevel clouds. However pattern recognition schemes based upon the combination of spectral and textural signatures can be used effectively for cloud discrimination over high albedo surfaces. This study is based primarily upon AVHRR LAC imagery but with some results from LANDSAT high spatial resolution scenes. A large number of textural features are investigated including the Gray Level Difference Vector (GLDV) and Sum and Difference Histogram (SADH) approaches various features proposed by Garand the Gray Level Run Length (GLRL) spatial coherence " footprints and spectral histogram measures. Twenty arctic surface and cloud classes are identified using two different classificatiofl approaches: 1) the traditional stepwise discrirninant analysis and 2) neural network analysis. Principal component analysis of textural measures is used to eliminate those measures which contribute little to class separability. The neural network feed-forward back-propagation approach produces the highest classification accuracy and does so with a relatively small training set. However the main limitation is the long training times required. A new hybrid architecture using a modularized competitive learning layer inserted before the feed-forward backpropagation layer developed by Lee

A multispectral scanning radiometer has been used to obtain measurements of the reflection function of marine stratocumulus clouds at 0. 75 1 . 65 and 2. 1 6 rim. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE) conducted off the coast of southern California during July 1987. Multispectral images of the reflection function were used to derive the optical thickness and effective particle radius of stratiform cloud layers on four days. In addition to the radiation measurements in situ microphysical measurements were obtained from the University of Washington Convair C-131A aircraft. In this paper we compare remote sensing results with in situ observations which show a good spatial correlation for both optical thickness and effective radius. These comparisons further show systematic differences between remote sensing and in situ values with a tendency for remote sensing to overestimate the effective radius by 23 pm independent of particle radius. The optical thickness in contrast is somewhat overestimated for small optical thicknesses and underestimated for large optical thicknesses. Marginal probability density functions of optical thickness and effective radius have been derived from our remote sensing results. The joint probability density function of optical thickness and effective radius shows that the effective radius increases as the optical thickness increases for optically thin clouds in contrast to optically thick clouds for

From about 10 months of spectral data obtained by the Infrared Interferometer Spectrometer (IRIS) flown on Nimbus 4 satellite in 1970, we have examined the extinction characteristics of thin ice crystal clouds over the oceanic convective rain belts in the tropics and midlatitudes. The optical depth of these clouds in the IR window region is < 3 and their spectral features are consistent with cloud particle size of 10 Sum. These optically thin ice crystal clouds are extensions of the anvil clouds generated by deep convective systems. From the IRIS data the frequency of occurrence of these thin clouds is estimated to be about 30 % over the warm waters surrounding Indonesia. As a result these clouds can contribute significantly to the radiative heating of this region. The IRIS data also indicated the existence of optically thin ice clouds that extend from the upper troposphere into the lower stratosphere, in the polar regions, during winter and early spring. The spectral features of these polar cirrus clouds differ somewhat from that of the optically thin cirrus clouds in the tropics. From theoretical simulation of the infrared spectra in the 8-25 jim region we infer that these polar clouds have a vertical stratification in particle size, with larger particles (12 .,um) in the bottom of the cloud and smaller ones (< 1 um) aloft. Radiative transfer calculations also suggest that the equivalent ice water content of these polar clouds is of the order of 1 to 2 milligram per cmi. It is suggested that the 'ozone hole' in the region of Weddel Sea is linked, at least in part, to these ice crystal clouds.

The surface radiation budget of the Earth is important in climate research because it gives an understanding of the distribution of solar and IR radiation exchanges at the surface. Data on the distribution and exchange of these surface radiations are needed over the entire Earth on a long-term basis. Global satellite data coupled with highly developed models now provide estimates of the surface radiation budget which are near the accuracy required for climate research. A description of surface radiation research its current state and planned programs are presented. 1.

The surface radiation budget (SRB) and the atmospheric radiative flux divergence (ARD) are vital components of the weather and climate system. We explain the importance of radiation in a complex international scientific endeavor the Global Energy and Water Cycle Experiment (GEWEX) of the World Climate Research Programme (WCRP) . The radiative transfer techniques and satellite instrumentation that will be used to retrieve the SRB and ARD later in this decade with the Clouds and the Earth''s Radiant Energy System (CERES) are discussed CERES is a component of the Earth Observing System (EOS) satellite program. Examples of consistent SRB and ARD retrievals made with Nimbus-7 and International Satellite Cloud Climatology Project (ISCCP) data from July 1983 are presented. 1.

A major objective of the Clouds and Earth Radiant Energy System (CERES) is the computation of vertical profiles through the atmosphere of the divergence of radiation flux with global coverage. This paper discusses the need for radiation divergence and presents some options for its inference from CERES measurements and other data from the Earth Observating System (EOS). 1 .

For the set of equations governing the motions of the atmosphere the knowledge by measurement of some of the terms of the energy equation is very challengin, especially those relative to the radiative and phase transition terms corresponding to heat sources or sinks. At the end of the 1950 decade a lot of interest 2, 3 was focussed on the determination of radiation flux divergence, its profiles and diabatic effect in relation with atmospheric dynamics. The so called "blackball" instrument was used on a large amount of soundings but turned out to be a misconcept. A detailed review of the different aspects about the radiative flux divergence, its measurement evaluation and theoretical calculation, the incertitudes and interest as a research field has been published by K. Y. Kondratyev whose conclusions and suggestions are still pertinent. In this presentation, the intention is to show that although delicate the radiation flux divergence can be measured directly and if kept in the context of the energy equation at microscale in the lower troposphere is directly related to atmospheric turbulence. The direct measurement method described will later be applied systematically to explore and quantify the behaviour of the radiation flux divergence in the lower troposphere where the ground albedo, the water content and the clouds play an important role on the atmospheric motions through their interaction with the atmospheric radiation field. A good understanding of this behaviour is of much interest for the research about the volume radiation flux divergence global distributions and variability that is part of the CERES project objectives.

Satellite measurements from the Earth Radiation Budget Experiment (ERBE) are providing important quantitative data on the diurnal variability of broadband shortwave and longwave radiation. The results derived from the combination of the Earth Radiation Budget Satellite (ERBS) and NOAA-9 indicate that the largest diurnal variations in longwave radiation occur typically over deserts and over land areas which experience intense convective activity. Maximum values of the albedo diurnal amplitude factor are over oceans. Seasonal and cloud cover variations have important effects on the diurnal cycles of Earth's radiation budget. ERBE results derived for individual regions are in substantial agreement with the diurnal results derived from the Geostationary Operational Environmental Satellite (GOES) measurements.

The relationship between narrowband and broadband thermal radiances is explored to detennine the accuracy of outgoing longwave radiation derived from narrowband data Infrared window (1 1. 5 jim) thta from the Geostationary Operational Environmental Satellite (GOES) are correlated with longwave (5. 0 -50. 0j. tm) data from the Earth Radiation Budget Experiment (ERBE). A simple quadratic fit between the narrowband and longwave fluxes results in a standard error of the estimate of 4. 5-5. 3 for data which are matched closely in time and space. Use of matched regional flux data with temporal differences up to a half hour yield standard errors of4. 6-5. 9. About one fourth of the magnitude of the error may be atiributed to limb-darkening and temporal differences in the matched fluxes. The relationship shows a significant dependence on the relative humidity of the atmosphere above the radiating surface. Although this dependency accounts for only about 2 of the standard error it reduces the monthly mean regional errors by more than 10. Data taken over land produced a slightly different relationship than data taken over water. The differences appear to be due to the higher altitudes of the land radiating surfaces. Cloud amount and height also influence the narrowband-broadband relationship. Inclusion of these parameters does not affect the standard errors but it reduces the monthly mean regional errors by another 10. Better humidity and temperature data and

The outgoing longwave radiation is an indicator of the overall state of the Earthatmosphere system and its diurnal variation reveals how the system responds to the solar diurnal forcing. The Earth Radiation Budget Experiment (ERBE) is the first attempt to use a multi-satellite system to obtain improved time sampling necessary for determining diurnal variation. We have used 3-hourly Meteosat data to improve the time sampling still further as compared to the four observations per 24 hours obtained during most of the ERBE lifetime. Regression techniques and radiative transfer calculations allow us to convert the relatively narrow-band Meteosat infrared window (IRW: 11 pm) and " water vapor" (WV: 6. 3 m) channel data into estimates of broad-band longwave (LW) radiant exitances. We compare the diurnal variations obtained from these data with the ERBE diurnal cycle estimates. Results show that the ERBE diurnal modelling algorithms generally perform very well and that most of the discrepancies that are observed would not have occurred if the system had been complete with scanners functioning together on 3 satellites as originally planned. 1.

The first year of broadband Earth Radiation Budget Experiment (ERBE) data is analyzed for top-of-theatmosphere regional variations of outgoing longwave (LW) flux and planetary albedo for total scene as well as clear-sky conditions. The annual variation of radiative parameters is examined for February 1985 through January 1986 for selected regions, latitude zones, and the entire globe. Results show significant seasonal variations for both LW fluxes and albedo. A broad longwave flux maximum (with a relative minimum corresponding to the Intertropical Convergence Zone in the middle) covers the Tropics and the sub-tropics with its center moving about 200 latitude between seasonal extremes. Minimum albedo (about 20%) occurs within 15° of the Equator. In the Tropics and midlatitudes, there is a tendency toward higher albedos during the summer. Larger albedos at the higher latitudes are caused by solar zenith angle effects and by increased snow and ice cover. Net warming occurs between 35°N and 35°S latitude near the equinoxes and in a 90°-wide latitude band at the solstices centered around 35° latitude in the summer hemisphere. This energy surplus at lower latitudes coupled with an energy deficit in the poleward regions is the primary driver of atmospheric circulations. For the year, the global net radiation is nearly in balance. Clouds were found to have a net cooling effect on Eartlfs climate for all seasons. The annual mean net cloud radiative cooling for the globe from ERBE is 18 Wm-2.