Humidite de la troposphere libre africaine: Elaboration d'une archive METEOSAT, Analyse climatique et Evaluation de modeles

Abstract : The hydrological cycle is a key component of the Earth climate and its knowledge lies on both observations and on simulation of its aspects, such as transport, phase transitions or radiative interactions. METEOSAT geostationary satellites have, onboard, radiometer that observes, in the infrared, the water vapor of a broad layer of the troposphere over tropical and subtropical zones of the Africa / Atlantic Ocean area. Available since the 80s, these so-called "Water Vapor" observations authorize to study the water vapor distribution in the tropical free troposphere and its variability, from daily to decennial scales. In this thesis, a free tropospheric humidity archive (FTH) with high spatial (0,625°x0,625°) and temporal (3h) resolutions is built using IR observations of the successive METEOSAT satellites (MET-2 to MET-5), whose calibration is adjusted on the one of the HIRS-12/NOAA-12 sounder, taken as a reference. Under clear sky conditions, the "WV" measurements are shown to be a surrogate of the mean relative humidity of a variable layer of the atmosphere, the latter being defined precisely thanks to the relative humidity jacobian, describing the sensitivity of the measure to local relative perturbations in the relative humidity profile. Low level clouds having a small radiative impact in this spectral band, they are kept in the database in addition to the clear sky scenes, and selected using cloudiness information from the ISCCP project. This low level cloud sampling gives a better representation of subtropical areas and of the nearby convective areas. Analyses of the tropospheric water vapor distribution reveal the weight of the relative interannual variability of FTH in the dry subtropical zones, notably in the Eastern Mediterranean region. At an intraseasonal, a lagrangian back-trajectory model shows that the relative humidity at 500hPa in this subsiding area is characterized by a mixing of tropical and extra-tropical air masses, coming from more or less cold areas of the troposphere. The FTH archive is also used to evaluate climate models simulations, in the frame of an international intercomparison project, and highlights the difficulty of the models to reproduce the interannual variability of FTH over the dry subtropical areas.
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Submitted on : Thursday, January 13, 2005 - 9:37:59 PM
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  • HAL Id : tel-00008052, version 1

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Helene Brogniez. Humidite de la troposphere libre africaine: Elaboration d'une archive METEOSAT, Analyse climatique et Evaluation de modeles. Physique [physics]. Université Pierre et Marie Curie - Paris VI, 2004. Français. ⟨tel-00008052⟩

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