Robert S. Kandel

The ScaRaB project: earth radiation budget observations from the meteor satellites

France, Russia, and Germany are cooperating in the construction and calibration of a Scanning Radiometer for Radiation Balance (ScaRaB), to be flown on the Soviet/Russian Meteor-3 series polar orbiting weather satellites, with launches scheduled annually beginning in summer 1993. The instrument comprises 2 broad channels (total, solar) for radiation budget, and 2 narrower bands for scene identification, with spatial resolution 60 km at nadir. We describe the instrument design and calibration procedures. The data processing algorithms follow the general lines adopted in the NASA Earth Radiation Budget Experiment (ERBE) so as to ensure continuity in the long-term series of data. We describe the data products which will be archived and made available to the international scientific community.

The ScaRaB Earth Radiation Budget Scanning Radiometer

In order to ensure that Earth Radiation Budget measurement from space continues through the 1990s, France, Germany and the USSR are developing a Scanning Radiometer for Radiation Balance (ScaRaB) to be flown on a series of METEOR-3 Soviet polar orbiting weather satellites. The instrument described in this paper comprises two broad channels (0,2 μm to 50 μm, 0,2 μm to 4 μm) for radiation budget, and two narrower bands (0,5 μm to 0,7 μm, 10,5 μm to 12,5 μm) for scene identification, with spatial resolution at nadir of order 50 km. The set of on-board sources includes black-body simulators for the calibration of long-wave channels, and both tungsten filament lamps and mosaic array diffusers for short-wave calibration. A specific filtering scheme cancels the influence of thermal environment during calibration of the 0,2 μm to 50 μm channel on solar diffusers and lamps. Hemispherical choppers associated with pyroelectric detectors ensure a high immunity to low frequency noise and thermal transients.

Satellite observation of the Earth Radiation Budget and clouds

We review briefly the needs of climate research for satellite observations of atmospheric and surface properties. We consider in particular the need for observations of the Earth radiation budget and clouds, the fundamental physical constraints on such observations, and the advances both in instrumentation and in data processing methodology. We note how our concepts of the role of clouds and radiation in the climate system have been refined and in some cases revised as a result of the availability of satellite observations. The ongoing (NASA) Earth Radiation Budget Experiment and the (WCRP) International Satellite Cloud Climatology Project have begun to provide new results of considerable importance. Following a description of projects for the near and not-so-near term future, we conclude with recommendations regarding the directions to be taken in planning for the global climate satellite monitoring system of the year 2000.

Regional Diurnal Albedo Climatology and Diurnal Time Extrapolation of Reflected Solar Flux Observations: Application to the ScaRaB Record

Abstract A regional (2.5° × 2.5° resolved) diurnal (hourly) albedo climatology for low and midlatitudes is derived for each month from the 5⅓-yr narrow-field-of-view data record obtained from the Earth Radiation Budget Satellite (ERBS). It is used in a quasi-operational diurnal interpolation/extrapolation procedure (DIEP) to calculate regional monthly means of the reflected shortwave radiation flux (RSR) from instantaneous albedo observations. This climatological approach (CDIEP) replaces the questionable assumption of diurnally constant cloud conditions made in the conventional DIEP by assuming a diurnal variation of cloudiness corresponding to the mean long-term diurnal variation of the planetary albedo. Validation of CDIEP, using the three-satellite Earth Radiation Budget Experiment (ERBE) data for December of 1986, indicates that on regional scales monthly time sampling errors for single satellite products are generally reduced but not completely removed in comparison with the currently applied diurna...

Improved Diurnal Interpolation of Reflected Broadband Shortwave Observations Using ISCCP Data

Abstract The multisatellite Earth Radiation Budget Experiment (ERBE) was designed to provide complete temporal coverage of the solar-reflected and earth-emitted radiation. Following operation of ERBE scanners on as few as one and as many as three satellites between November 1984 and February 1990, narrow-field-of-view earth radiation budget measurements were resumed in March 1994 by the Scanner for Radiation Budget (ScaRaB) mission and in December 1997 by the first Clouds and the Earth’s Radiant Energy System (CERES) instrument, each time on a single satellite. Due to sparse temporal sampling, diurnal variations must be accounted for in order to establish accurate unbiased daily and monthly mean radiant exitance. When the ERBE diurnal interpolation algorithm is used alone, large discrepancies appear between monthly mean radiative fluxes obtained from single- and multisatellite data. The authors extend the algorithm by accounting for diurnally varying cloud cover using International Satellite Cloud Climato...

Understanding and Measuring Earth’s Energy Budget: From Fourier, Humboldt, and Tyndall to CERES and Beyond

This historical perspective on the determination of Earth’s energy fluxes, beginning with the classical description of climate, outlines the establishment of the basic physics of the Earth climate system in the nineteenth century. After recalling the early twentieth century ground-based attempts to determine the Earth’s energy budget, I review the growing contributions of observations from space to quantifying these exchanges. In particular, space observations have shown that variations of solar luminosity have been extremely small (of order 0.1%) over past decades and probably past centuries and that they play practically no role in present-day climate variations or variations that may be expected in coming decades. Overall geographical structure, diurnal and seasonal cycles, and some of the interannual and interdecadal variations of Earth’s energy exchanges with the Sun and space are now quite well determined, but much remains to be done regarding, on the one hand, fluxes at the surface and, on the other hand, variations of clouds. Improvements are essential if scientific assessment of anthropogenic climate change risk is to keep up with the changes themselves.

Determination of longwave anisotropic emission factors from combined broad- and narrow-band radiance measurements

Abstract The conversion of measured broad-band radiances into radiative fluxes requires application of angular corrections: In the Earth Radiation Budget Experiment (ERBE), the longwave anisotropic emission factors (AEF) were tabulated for different viewing zenith angles, seasons, latitude bands and scence types including four different cloud cover classes. Investigating an alternative approach for the ScaRaB (Scanner for Radiation Balance) instrument, we use a radiative transfer model to parametrize the AEF as a function of viewing zenith angle and a single other variable - ‘atmospheric pseudo-absorptance’ - extracted from simultaneously measured broad-band longwave (LW) and infrared (IR) atmospheric window (10.5–12.5μm) radiances. For validation, we use simultaneous co-located NOAA-9 AVHRR-ERBE data. Compared to the ERBE AEFs the ScaRaB parametrization provides more realistic AEFs as a function of scene brightness temperature which is related to cloud top height. Analysis of classified cloud data indicates that there are only a few extreme cases in which additional anisotropy due to broken clouds will affect the usefulness of our parametrization. We also consider enhanced anisotropy of semitransparent cirrus. Model and data show that although not explicitly treated in our procedure, the parametrization gives good results.

Diurnal variation of earth radiation budget components above Africa and the neighboring Atlantic Ocean; Changes between 1983 and 1985, estimated from METEOSAT observations adjusted to ERBS data

Abstract METEOSAT observations in the thermal infrared “window” and “water vapor” channels, as well as in the visible channel, reveal diurnal variations over large areas which remain significant in the monthly means. The variations in the infrared correspond to diurnal cycles in the surface skin temperature (over land) and in cloud cover (over both land and sea) at various levels, and they must appear as a more or less significant diurnal variation in the integrated longwave emission to space of the Earth-atmosphere system. The diurnal cycle in the reflected shortwave radiation is influenced by these meteorological variations as well as by the astronomical cycle and the anisotropic reflectance. These must be taken into account in studies of Earth Radiation Budget variations. Using nearly simultaneous and spatially coincident pixel data from the ERBE scanner on ERBS and from METEOSAT in November 1984, we construct provisional transfer functions relating the narrow-band METEOSAT infrared observations to the longwave radiant exitance at the top of the atmosphere. We apply these transfer functions to the METEOSAT ISCCP B2 data sets for the summers of 1983–1985, and compare the resulting longwave radiant exitance estimates, with particular attention to the diurnal variation, which should be relatively insensitive to the inaccuracy inherent in applying the provisional (November 1984) transfer functions to the 1983–1985 data.

Characterization of flight model 1 of the Scanner for Radiation Budget (ScaRaB) instrument in the solar spectral domain

Abstract The Scanner for Radiation Budget (ScaRaB) is an instrument for the determination of Earth Radiation Budget parameters from space. On-board calibration is necessary during the experiment. For this purpose, various calibration sources (lamps and blackbody-simulators) are integrated in the ScaRaB instrument. Before launch, all on-board sources have to be calibrated on ground. This paper deals with the ground calibration in the solar spectral domain. The ground calibration standard for the lamps is a reference diffusor which is illuminated by the sun. The absolute radiance of the diffusor is calculated from its absolute reflectivity and the incoming solar irradiance. The latter is measured with a pyrheliometer. The total error of the ground measurements of the filtered radiance of the on-board lamp sources is estimated as 1.4 to 4% (depending of the calibration source) of the nominal value. However, the lamp sources show aging effects during ground testing from 0.5% to 4% of the nominal value.

Earth radiation balance and climate: Why the moon is the wrong place to observe the Earth

Abstract Increasing “greenhouse” gases in the Earths atmosphere will perturb the Earths radiation balance, forcing climate change over coming decades. Climate sensitivity depends critically on cloud-radiation feedback: its evaluation requires continual observation of changing patterns of Earth radiation balance and cloud cover. The Moon is the wrong place for such observations, with many disadvantages compared to an observation system combining platforms in low polar, intermediate-inclination and geostationary orbits. From the Moon, active observations are infeasible; thermal infrared observations require very large instruments to reach spatial resolutions obtained at much lower cost from geostationary or lower orbits. The Earths polar zones are never well observed from the Moon; other zones are invisible more than half the time. The monthly illumination cycleleads to further bias in radiation budget determinations. The Earth will be a pretty sight from the Earth-side of the Moon, but serious Earth observations will be made elsewhere.