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J. P. Duvel
and
R. S. Kandel

Abstract

Simple geometric considerations suggest that in the same way as broken cloud cover appears practically total near a ground observer's horizon, the anisotropy of the longwave radiation emitted to space from an area of broken cloud should be substantially enhanced.

Using a simple model consisting of parallel bands of “black” clouds of rectangular cross section, imbedded in an otherwise horizontally homogeneous vertically stratified atmosphere, we evaluate the longwave radiance emergent in different directions, averaged over the cloud pattern, and find this expectation to be confirmed in a wide range of cases. At large zenith angles, the emergent radiance can exhibit considerable dependence on azimuth, relative to the direction of the band structure. Although such azimuthal dependence practically disappears for many 3-dimensional broken cloud fields (e.g., hexagonal cells), the enhanced zenith angle dependence remains. This can be a source of bias in cloud extractions or Earth radiation budget determinations based on geostationary satellite data, since zenith angle is directly related to geographical location of the point under observation.

With our model cloud arrays, and considering radiative transfer in the atmosphere, we have evaluated the bias in the broad-band longwave (3–100 μm) radiant exitance values derived, assuming horizontal homogeneity, from either broad-band or narrow-band (10.5–12.5 μm) radiance data. We find that the bias for near-nadir observations can be as large as 20% (40 W m−2) in extreme cases involving bands of opaque high-level cloud over warm ground. It may remain substantial in situations which are encountered in nature. We examine the dependence of the broken cloud effects on the geometrical parameters of the field.

We discuss how such effects might be detected and how they can influence determinations of Earth radiation budget components and of cloud cover. We note that “brokenness” will be a factor in cloud-radiation feedback.

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J. F. Duvel
and
R. S. Kandel

Abstract

For any given area, diurnal variation of the emitted longwave radiation reveals the response of the surface and atmosphere to the astronomical diurnal forcing. However, the diurnal rhythm can be masked locally by changes in cloud cover linked to passage of weather systems. To eliminate this weather “noise” we construct histograms of the pixel radiance counts from the METEOSAT IR window channel, for regions of dimensions 250 to 1000 km, and we study their evolution with time. We find that considerable information on the diurnal variation, both of surface temperature and of cloud cover at all levels, can be obtained from short sequences (3 days) of hourly METEOSAT data.

In West Africa, diurnal variation of clear-sky emergent radiances, corresponding to amplitudes of 30–50 K in surface skin temperature, is found not only in the Saharan desert regions but also during the dry season in the Sahel–Sudan–Guinea zone. In the August wet season, in this last area, the amplitude is lower, while diurnally varying cloud cover is evident at middle and upper levels. We find that the bias in sun-synchronous satellite estimates of cloud cover can he as high as 25–30%, depending on the time of observation.

Over Brazil, surface temperature cycling is present but smaller than observed in West Africa. Apparent cloud top height is found to undergo a regular systematic decrease through the late night and morning, although as in other tropical land regions, high-level cloud exhibits an early-evening maximum.

Using the histogram diurnal variation to discriminate between low cloud and sea, we find that low cloud over the central and eastern South Atlantic exhibits a very strong diurnal/nocturnal cycle, with maximum cover close to local sunrise. Computing the effect of the diurnal variation on the radiation balance of the area, we find a significant weakening of the cloud albedo erred. If daytime sun-synchronous satellite data alone are used to estimate cloud cover, bias as high as 25% (15–20 W m−2) can result in the calculation of radiation balance.

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C. J. Stubenrauch
,
J-P. H. Duvel
, and
R. S. Kandel

Abstract

The conversion of measured radiances into radiative fluxes requires application of angular corrections; in the Earth Radiation Budget Experiment (ERBE), the longwave anisotropic emission factors (AEFS) were tabulated for different viewing zenith angles, seasons, latitude bands, and scene types, including four differment cloud-cover classes. An alternative approach is investigated using simultaneous infrared atmospheric window (10.5-12.5 µm) and broadband longwave (LW) measurements. Such measurements will be available from the ScaRaB (Scanner for Radiation Balance) instrument whose launch is planned to occur in 1993.

Using a radiative transfer model to simulate the combined measurements, the AEF is parameterized as a function of viewing zenith angle and a single other variable—atmospheric pseudoabsorptance—defined as the normalized difference between the broadband LW radiance and the integrated Planck emission at the 11.5-µm brightness temperature. For validation of the parameterization with existing satellite data, simultaneous collocated NOAA-9 ERBE Advanced Very High Resolution Radiometer data were used for broad- and narrowband radiances. The comparison between fluxes corrected with the parameterized AEF and those corrected with the ERRE AEF shows that the parameterization 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 this parameterization. Enhanced anisotropy of semitransparant cirrus was also considered. Model and data show that although not explicitly treated in this procedure, the parameterization gives good results. This parameterization may also be adapted for somewhat different wavelength bands as in the NASA CERES (Clouds and the Earth's Radiant Energy System) project.

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C. J. Stubenrauch
,
G. Seze
,
N. A. Scott
,
A. Chedin
,
M. Desbois
, and
R. S. Kandel

Abstract

Gaining a better understanding of the influence of clouds on the earth's energy budget requires a cloud classification that takes into account cloud height, thickness, and cloud cover. The radiometer ScaRaB (scanner for radiation balance), which was launched in January 1994, has two narrowband channels (0.5–0.7 and 10.5–12.5 µm) in addition to the two broadband channels (0.2–4 and 0.2–50 µm) necessary for earth radiation budget (ERB) measurements in order to improve cloud detection. Most automatic cloud classifications were developed with measurements of very good spatial resolution (200 m to 5 km). Earth radiation budget experiments (ERBE), on the hand, work at a spatial resolution of about 50 km (at nadir), and therefore a cloud field classification adapted to this scale must be investigated. For this study, ScaRaB measurements are simulated by collocated Advanced Very High Resolution Radiometer (AVHRR) ERBE data. The best-suited variables for a global cloud classification are chosen using as a reference cloud types determined by an operationally working threshold algorithm applied to AVHRR measurements at a reduced spatial resolution of 4 km over the North Atlantic. Cloud field types are then classified by an algorithm based on the dynamic clustering method. More recently, the authors have carried out a global cloud field identification using cloud parameters extracted by the 3I (improved initialization inversion) algorithm, from High-Resolution Infrared Sounder (HIRS)-Microwave Sounding Unit (MSU) data. This enables the authors first to determine mean values of the variables best suited for cloud field classification and then to use a maximum-likelihood method for the classification. The authors find that a classification of cloud fields is still possible at a spatial resolution of ERB measurements. Roughly, one can distinguish three cloud heights and two effective cloud amounts (combination of cloud emissivity and cloud cover). However, only by combining flux measurements (ERBE) with cloud field classifications from sounding instruments (HIRS/MSU) can differences in radiative behavior of specific cloud fields be evaluated accurately.

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J.-Ph. Duvel
,
M. Viollier
,
P. Raberanto
,
R. Kandel
,
M. Haeffelin
,
L. A. Pakhomov
,
V. A. Golovko
,
J. Mueller
,
R. Stuhlmann
, and
the International ScaRaB Scientific Working Group

Measurements made by the second flight model of the Scanner for Radiation Budget (ScaRaB) instrument have been processed and are now available for the scientific community. Although this set of data is relatively short and sparse, it is of excellent quality and is the only global broadband scanner radiance information for the period between October 1998 and April 1999. This second flight model marks the conclusion of the ScaRaB cooperative program of France, Russia, and Germany. The two flight models of the ScaRaB instrument gave broadband radiance measurements comparable in quality to those made by the Earth Radiation Budget Experiment and the Clouds and Earth Radiant Energy System scanning instruments. In addition, the ScaRaB instrument gave unique results for the comparison between narrowband (visible and infrared atmospheric window) and broadband radiance measurements. These measurements were mostly used to improve the broadband data processing and to study the error budget resulting when narrowband channel data are used to estimate the earth radiation budget. These concomitant narrow- and broadband measurements made by the two flight models of ScaRaB contain original information of considerable interest for further scientific use.

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R. Kandel
,
M. Viollier
,
P. Raberanto
,
J. Ph. Duvel
,
L. A. Pakhomov
,
V. A. Golovko
,
A. P. Trishchenko
,
J. Mueller
,
E. Raschke
,
R. Stuhlmann
, and
the International ScaRaB Scientific Working Group (ISSWG)

Following an overview of the scientific objectives and organization of the French–Russian–German Scanner for Radiation Budget (ScaRaB) project, brief descriptions of the instrument, its ground calibration, and in-flight operating and calibration procedures are given. During the year (24 February 1994–6 March 1995) of ScaRaB Flight Model 1 operation on board Meteor-317, radiometer performance was generally good and well understood. Accuracy of the radiances is estimated to be better than 1% in the longwave and 2% in the shortwave domains. Data processing procedures are described and shown to be compatible with those used for the National Aeronautics and Space Administration's (NASA) Earth Radiation Budget Experiment (ERBE) scanner data, even though time sampling properties of the Meteor-3 orbit differ considerably from the ERBE system orbits. The resulting monthly mean earth radiation budget distributions exhibit no global bias when compared to ERBE results, but they do reveal interesting strong regional differences. The “ERBE-like” scientific data products are now available to the general scientific research community. Prospects for combining data from ScaRaB Flight Model 2 (to fly on board Ressurs-1 beginning in spring 1998) with data from the NASA Clouds and the Earth's Radiant Energy System (CERES) instrument on board the Tropical Rainfall Measurement Mission (TRMM) are briefly discussed.

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