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  • Author or Editor: J. Ph. Duvel x
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J. Ph Duvel
and
F. M. Bréon

Abstract

The clear-sky greenhouse effect response to a sea surface temperature (SST or Ts ) change is studied using outgoing clear-sky longwave radiation measurements from the Earth Radiation Budget Experiment (ERBE). Considering geographical distributions for July 1987, the relation between the SST, the greenhouse efect G (defined as the outgoing infrared flux trapped by atmospheric gases), and the precipitable water vapor content (W), estimated by the Special Sensor Microwave Imager (SSM-I), are analyzed first. A fairly linear relation between Wand the normalized greenhouse effect g, defined as GTs 4, is found. On the contrary, the SST dependence of both W and g exhibits nonlinearities with, especially, a large increase for SST above 25°C. This enhanced sensitivity of g and W can be interpreted in part by a corresponding large increase of atmospheric water vapor content related to the transition from subtropical dry regions to equatorial moist regions.

Using two years of data (1985 and 1986), the normalized greenhouse effect sensitivity to the sea surface temperature is computed store the interannual variation of monthly mean values. Although subject to uncertainties, results show a smooth variation over the 0°−32°C temperature range. A maximnal sensitivity of g(∼10 × 10−3 K−1) is found for both extreme temperature ranges (0°−4° and 28°–32°C), while a minimal sensitivity (∼6 × 10−1 K−1) is found in the 12°−16°C temperature range. The enhanced greenhouse effect sensitivity in the warmest temperature intervals is tentatively explained by increased convection that injects water vapor into the middle and upper atmosphere. In the coldest temperature ranges, the atmosphere is dry and implies more nonsaturated absorption bands and, therefore, higher sensitivity to water vapor content. These values are related to a small interannual variation of outgoing longwave flux with SST (∼1 W m−1 K−1), while without water vapor feedback, this sensitivity would be on the order of 4 W m−2 K−1.

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L. A. Toledo Machado
,
M. Desbois
, and
J-Ph Duvel

Abstract

The structural properties of convective cloud clusters of tropical Africa and the Atlantic Ocean are studied using six summers of Meteosat satellite data in the atmospheric infrared window. A cluster at a given brightness temperature threshold is defined as the area covered by adjacent cloud cells with brightness temperature lower than the threshold. The clusters are classified according to the area they cover and the position of their center of mass. Results show that the convective cluster number can be approximated by a power law of the radius with an exponent around −2. This gives a nearly equal contribution of each cluster size to the mean high cloud cover for a given brightness temperature threshold. Using the visible channel (0.4–1.1 μm) of Meteosat, we show that the part of the clusters with reflectance larger than 0.7 also follows a power law.

The cluster-size distributions remain similar for different subregions and seasons, even if they are subject to large variations in the mean cloudiness. We further inspect the relatively large diurnal and interdiurnal variations of the cluster-size distribution. We also look at the variations of the cluster-size distribution as a function of a vertical extension, defined as the lowest brightness temperature reached by the cluster. We find preferential sizes that increase as the vertical extension increases. We also show that the distance between clusters, defined as the minimum distance between clouds of the same size, also follows a well-defined distribution.

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L. A. Toledo Machado
,
J-Ph Duvel
, and
M. Desbois

Abstract

Using Metecosat satellite data in the atmospheric infrared window, the authors study short time-scale fluctuations of the size distribution of tropical convective cloud clusters for July to September 1989. A cluster at a given brightness-temperature threshold (T IR) is defined as the area covered by adjacent cloudy pixels with brightness temperature lower than the threshold. The clusters are classified according to the area they cover and the position of their center of mass.

Over land regions of West Africa, the size distribution undergoes a very coherent diurnal behavior with development of small cells between noon and 1500 LST that later grow or merge into larger clusters. Over the Atlantic Ocean, the highest cloudiness has a weak maximum extent in early morning, while cloudiness at lower levels (but with infrared brightness temperature T IR < 253 K) is more extended in the afternoon. This diurnal behavior is primarily due to large cloud clusters (r > 100 km at T IR = 218 K), suggesting that the diurnal variation over the ocean results mainly from internal variations of large convective systems and not from the initiation of convection at a given hour of the day. This is confirmed by the analysis of 15 large convective systems propagating over the ocean.

In agreement with previous studies, we find that the high cloud cover is maximum within the trough of easterly waves. At midlevel (T IR = 253 K), these waves modulate mostly the number of clusters with radii larger than 200 km. At colder levels (T IR = 218 K), while the wave modulates the number of clusters at all sizes, the clusters are organized at larger scale within the trough. The cluster size also depends on the wave amplitude with larger mean cluster size when the amplitude is larger. These results show that over the ITCZ, the trough phase of the wave more promotes the development of large clusters than it favors the initial stage of the convection.

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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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