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D. Ramond

Abstract

The pressure perturbation associated with a convective cell has been measured with an aircraft, at one level about 1500 m below cloud top. Agreement is found with Barnes' (1969) experimental data and with the results of the Wilhelmson's (1974) three-dimensional model of a convective cloud in a shared environment. The efficiency of the “pressure shield” against entrainment is examined.

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B. Pinty and D. Ramond

Abstract

Surface albedo can be inferred from geostationary satellite measurements as long as the effects due to the atmosphere, the spectral response of the sensor, and the angular anisotropy of the reflected field are corrected. In this paper, we developed a method which includes ad hoc correction procedures for the three categories of effects. An application of the method is conducted over the Sahara and the African Sahel using METEOSAT radiances together with auxiliary data derived from other satellites (Tiros-N and Nimbus-7) and standard meteorological observations. The surface albedo maps are estimated over this region, at a spatial resolution compatible with one used in climate models, for 2 days representative of the dry and the wet seasons, respectively. The observed seasonal surface albedo change and the relationships between the surface and the planetary albedos are discussed in order to examine the validity of the method and the correction procedures.

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J. Stum, B. Pinty, and D. Ramond

Abstract

The conversion of radiances measured by the METEOSAT visible channel into broadband radiances can be performed as long as the appropriate conversion factors are known. A simple model allowing a spectral description of the optical properties of cloud free atmospheres and land surfaces is used to estimate these conversion factors. A sensitivity study of these factors indicates that a knowledge of the optical properties of the surfaces (described through spectrally averaged albedos and spectral band ratios) is decisive for retrieving broadband conversion factors. A parameterization is proposed which permits estimation of METEOSAT conversion factors for radiation budget calculations.

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D. Ramond, H. Corbin, M. Des̄bois, G. Szejwach, and P. Waldteufel

Abstract

A polar jet stream flowing over northern Europe is investigated using both the radiosounding network and the METEOSAT water vapor (WV) channel imagery. A maximum radiance band associated with the polar jet stream is shown to accurately delineate the tropopause break; a major difference in the subtropical jet stream radiative signature is noted, however. An analytic approximation of the radiative transfer equation is established for the METEOSAT WV channel and leads to the conclusion that the radiance field, away from the polar frontal zones, is representative of the temperature field on a water vapor isosteric surface.

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