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M. Rieland and R. Stuhlmann

Abstract

The purpose of this paper is to investigate the influence of cloudiness on the shortwave radiation budget at the top of the atmosphere, at the surface, and, as a residual, for the atmosphere itself. The data used for this study are derived exclusively from satellite measurements. Calculations for the top of the atmosphere are based entirely on measurements of the Earth Radiation Budget Experiment (ERBE). For the solar radiation budget at the surface, the incoming surface solar radiation is derived from Meteosat data and the surface albedo is calculated from ERBE clear-sky planetary albedo measurements by applying an atmospheric correction scheme. As results, maps of absorbed solar radiation for the total earth–atmosphere system, the surface, and for the atmosphere are presented for the area of investigation, ±60° longitude and latitude. To infer the contribution of clouds, the concept of cloud radiative forcing is applied to these different datasets. It is shown that the solar cloud forcing at the top of the atmosphere (CFTOA), and at the surface (CFSUR), are of the same order of magnitude and well correlated with cloud cover (R = 0.83). On the contrary, the solar cloud forcing of the atmosphere itself, CFATM, is about one order of magnitude less and not very highly correlated with cloud cover (R = 0.37). The mean value of the annual averaged solar cloud forcing for the area of investigation is calculated for the top of the atmosphere to be CFTOA = 50 ± 4 W m−2, for the surface to be CFSUR = 55 ± 6 W m−2, and for the atmosphere to be CFATM = − 5 ± 10 W m−2. Related to the annual mean solar insulation, the CFATM corresponds to an additional contribution of the clouds to atmospheric solar absorption of 1.4%. The uncertainty range for this additional absorption is calculated to be − 1.4% to + 4.2%.

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R. Stuhlmann and G. L. Smith

Abstract

Cloud-generated radiative heating is computed for January zonal mean conditions for low and midclouds. For both cases, the strongest influence was found to be in the low troposphere, with marked differences in signs and magnitudes. Optically thin low clouds generate net radiation cooling in the atmosphere at all latitudes. As cloud optical thickness increases, the shortwave absorption becomes more important. Midlevel clouds generate a net radiative heating for the low troposphere at latitudes 40°S to 40°N. Poleward of 40°, midlevel clouds generate a net cooling for the lower troposphere.

At extratropical latitudes, both cloud classes generate net radiative cooling. In the tropics, the effect of low clouds changes from net cooling to net heating as the optical thickness increases, and midclouds cause net heating. A mechanism is described whereby this dependence produces a strong positive feedback effect on the development of sea surface temperature anomalies in the tropical oceans, as during an El Niño event. The cloud pattern changes over the East Pacific Ocean during the onset of an El Niño result in a strong increase in radiative heating which is at least as large as the latent heat release.

The generation of zonal available potential energy (ZAPE) by net radiative heating due to clouds is computed for January mean conditions. In terms of the effects of clouds on the general circulation, the globe can be divided into two regimes. The first regime consists of the portions of the Earth south of 40°S and north of 30°N, i.e., the extratropics. In this regime, both low and midclouds generate ZAPE. Their effects increase with latitude, independent of optical thickness. The other regime is the belt between 40°S and 30°N, i.e., the tropics and subtropics. Here, the effect of clouds on ZAPE is less than half that in the extratropics. However, a change in optical thickness and cloud class will significantly contribute to ZAPE. Any increase in ZAPE will intensify the Hadley circulation, linking tropical sea surface temperature changes to the extratropical circulation.

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R. Stuhlmann and G. L. Smith

Abstract

The theory is presented of the effect of radiative heating and cooling by clouds on the available potential energy (APE). This provides a measure of the influence of clouds on the general circulation. Absorption and scattering of solar radiation and absorption and emission of longwave radiation are considered. It is shown that the cloud radiative contribution to the generation of APE is determined by the net cloud radiative heating and the efficiency factor, which is a function of the temperature distribution of the atmosphere.

Cloud classes are defined in terms of cloud top heights and optical thickness. Within each class, the microphysical and macrophysical properties are used in a two-stream radiation computation with 37 spectral intervals in the shortwave and 50 in the longwave ranges. The probability of occurrence of each cloud class is used in the computations to account for nonlinearities between cloud parameters and the radiation field.

Results are presented for low and middle clouds effect on each of three atmospheric layers: 1000–500 mb, 500–100 mb, and 100–1 mb. The cloud radiative heating is found to be a single function of cloud optical thickness for all classes. It is shown that low clouds cool the lower layer and, to a smaller degree, the middle layer. Midclouds coal the middle layer more strongly and heat the low layer. Thus, low clouds at low latitudes destroy APE and midclouds generate APE. The clear sky state and surface properties are found to have only secondary influence on the results.

A concept is developed to relate the cloud radiative heating to cloud heights and optical depths; this can be estimated from satellite measurements, such as those which will be produced by the International Satellite Cloud Climatology Project. Thus, given such measurements, the impact of cloud radiative heating on the general circulation can be inferred.

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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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K. Holmlund, J. Grandell, J. Schmetz, R. Stuhlmann, B. Bojkov, R. Munro, M. Lekouara, D. Coppens, B. Viticchie, T. August, B. Theodore, P. Watts, M. Dobber, G. Fowler, S. Bojinski, A. Schmid, K. Salonen, S. Tjemkes, D. Aminou, and P. Blythe

Abstract

Within the next couple of years, the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) will start the deployment of its next-generation geostationary meteorological satellites. The Meteosat Third Generation (MTG) is composed of four imaging (MTG-I) and two sounding (MTG-S) platforms. The satellites are three-axis stabilized, unlike the two previous generations of Meteosat that were spin stabilized, and carry two sets of remote sensing instruments each. Hence, in addition to providing continuity, the new system will provide an unprecedented capability from geostationary orbit. The payload on the MTG-I satellites are the 16-channel Flexible Combined Imager (FCI) and the Lightning Imager (LI). The payloads on the MTG-S satellites are the hyperspectral Infrared Sounder (IRS) and a high-resolution Ultraviolet–Visible–Near-Infrared (UVN) sounder Sentinel-4/UVN, provided by the European Commission. Today, hyperspectral sounding from geostationary orbit is provided by the Chinese Fengyun-4A (FY-4A) satellite Geostationary Interferometric Infrared Sounder (GIIRS) instrument, and lightning mappers are available on FY-4A and on the National Oceanic and Atmospheric Administration (NOAA) GOES-16 and GOES-17 satellites. Consequently, the development of science and applications for these types of instruments have a solid foundation. However, the IRS, LI, and Sentinel-4/UVN are a challenging first for Europe in a geostationary orbit. The four MTG-I and two MTG-S satellites are designed to provide 20 and 15.5 years of operational service, respectively. The launch of the first MTG-I is expected at the end of 2022 and the first MTG-S roughly a year later. This article describes the four instruments, outlines products and services, and addresses the evolution of the further applications.

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