Etude du couplage radar-lidar sur plates-formes spatiales et aeroportees. Application a l'etude des nuages, des aerosols et de leurs interactions

摘要

The aerosol and cloud effect on climate is the main uncertainty on global warming prediction through their influence on the solar radiative forcing and their interactions. Using simultaneous lidar, radar and radiometry measurements is one of the path explored by the scientic community to reduce this uncertainty. It is the reason for the development of new instruments onboard the A-Train platforms and the associated development of operational algorithms. Those algorithms possess some intrisic limitation which lead us to revisit the data analysis procedure of the A-Train spaceborne platform (CALIPSO, CLOUDSAT) and to develop our own algorithms through the analysis of sea surface echo, and to identify theoretical multi-wavelength oceanic surface model with self-consistent scattering properties observed by different instruments. Using this model and the observations of the micro-wave radiometer AMSR-E, allowed to improve the calibration procedure of both lidar and radar instruments with the identication of a systematic and high signal to noise ratio calibration reference. This calibration increases the accuracy of physical parameters retrieved on the research operational products and give access to a higher number of derived products. Using the sea surface reference provided by the active (radar) and passive (radiometer) microwave sensors also allows to measure the aerosol optical thickness at the lidar wavelength. This measurement does not use any assumptions on the scatterers microphysical properties, is usable day and night, offers the highest available signal to noise ratio and allows a good aerosol-cloud discrimination with the lidar multispectral vertical information. The comparison with MODIS shows a good statistical agreement. The new methodologies developped for the A-Train offer a complete tool to analyse both vertical structure of aerosols and clouds as well as the aerosol optical thickness over the ocean and liquid water clouds. This opens a new way for aerosol direct radiative forcing quantication, which is out of range of present radiometric measurements. The preliminary studies we conducted confirm the signicant positive radiative forcing on the Gulf of Guinea area in presence of the biomass burning aerosol plumes observed during the AMMA campaign. The negative forcing over the ocean is an order of magnitude lower than the positive forcing over the cloud layers. The positive radiative forcing we observed on the Gulf of Guinea area (between +5 and +10 W/m2 ) is strongly dependent of cloud cover which can be better characterized by the small scale A-Train measurements, and the warming induced by aerosol direct effect must be better parameterized in the climate models. The emission of absorbing aerosols (fire, pollution...) and their long range transport at elevated altitudes when they can stay over clouds, represent a critical burden on the climate system. The present estimation of a global negative forcing of -0. 5 W/m2 to characterize the aerosol radiative effect needs to be carefully examined to the light of this impact.