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La microphysique des cirrus a l'echelle du globe : Correlation avec les proprietes atmospheriques et meilleure representation dans les modeles de climat.

Abstract : Clouds play a crucial role in modulating energy trading system Earth-atmosphere. Indeed, they reflect some solar radiation contributing thus cooling the atmosphere, they trap some of the infrared radiation LAND contributing to global warming. The competition between these two effects opposite is governed by the physical and microphysical properties of clouds. In general low clouds with their low altitudes and high optical depths reflect more solar radiation and thus have a cooling trend. For cons, the cirrus clouds because of their low optical depths are relatively transparent to solar radiation and simultaneously capture a portion of long wave radiation re-emit infrared and to the ground contributing to the warming of the atmosphere and Earth's surface. The study of climate and its evolution, climate models, requires a good handling account the radiative effects of clouds. For now, the determination of the radiative effects has been identified by the IPCC, "Intergovernmental Panel on Climate Change" as one of the main sources of uncertainty in climate prediction. The radiative impact of cirrus clouds (clouds of high altitudes) is particularly poorly taken into account in some models because of the extreme variability of sizes and shapes of crystals ice clouds. Indeed, in some models, the size distribution and shape these crystals is replaced by an effective diameter assuming the crystals form spherical. Studies of Kristjansson et al. (2000) on the sensitivity of two models environment to assumptions about the microphysical properties of cirrus clouds have shown that resulting uncertainty on the radiative effect associated with a doubling of the concentration Atmospheric carbon dioxide is about 50%. The results of such studies explain the motivation of the scientific community to deepen our knowledge about the microphysics of clouds. Only observation of the properties of cirrus clouds at different scales to understand, and therefore better represent their impact on climate. Satellite observations offer opportunity to study the properties of cirrus clouds on a global scale and over long periods time. Pollsters TOVS (TIROS Operational Vertical Sounder-N) aboard platforms of NOAA have already demonstrated their ability to return properties natural (altitude, temperature and emissivity) and microphysical (size of the crystals and thick ice) cirrus thanks to their good spectral resolution in heat. The work of my thesis took place in the European project CIRAMOSA (Cirrus microphysical properties and Their Effect on Radiation: survey and integration Into climate Model From a combination of satellite observations), a collaboration between the Laboratory Dynamic Meteorology, the MetOffice at Exeter in Great Britain, the Institute for Marine Research in Kiel, Germany and the Laboratory of Atmospheric Optics in Lille. This project aimed to study the physical and microphysical properties of cirrus clouds to from satellite observations of different instruments as well as in situ measurements. The correlations between the microphysical properties of cirrus and the state of the atmosphere have been Demonstrated global scale, which will contribute to improved parameterizations microphysical properties of cirrus in climate models to better evaluation of their radiative effects. The LMD has participated in this project by developing a database of properties climate physical and microphysical cirrus clouds from measurements of vertical TOVS sounders. This climatology is the first of its kind. The first step of my thesis (Chapter 2) is devoted to the evaluation of the altitude of cloud base climate TOVS Path-B, a very important variable for calculation of radiative fluxes. For this evaluation I combined the data with TOVS Path-B LITE lidar observations close in time and space. This analysis showed that General altitude cloud TOVS Path-B corresponds to the middle of a cloud. The agreement best for low clouds than for high clouds. These are usually the one hand more heterogeneous and on the other hand, in the case of a cloud underlying the vertical structure of cloud top appears to be different (with a maximum signal backscatter above). In the latter case, on average, the altitude of a cloud of TOVS Path-B is underestimated by about 700m. In a second step (Chapter 3), I am interested in studying correlations between microphysical properties of cirrus and the state of the atmosphere. The spatio-temporal brought into play by the physical processes of cloud formation are characterized by a wide spectrum of variability. However, the spatial and temporal resolution of climate models is reduced and does not take into account such variability. Therefore it is necessary to develop parameterizations for representing these microphysical properties in climate models. Design and development of these parameterizations require an understanding of interrelationships between microphysics clouds and the generation process of these clouds. So far the correlation between microphysical properties of clouds and the state of the atmosphere have been studied in campaigns in-situ measurements and hence in areas and for limited periods. The database TOVS Path-B microphysical properties of cirrus clouds, with its cover comprehensive and lasting four years, has allowed us to study across the globe consistency of these correlations identified in regional campaigns. In some number of climate models, the effective size of ice crystals or ice thickness cirrus clouds are parameterized according to the temperature of cloud. Our analysis with TOVS Path-B data have shown that this correlation does not exist globally. For go further, I sought to study the effect of the dynamics and thermodynamics of the atmosphere on the microphysical properties of cirrus clouds. For this study, I combined the properties of cirrus TOVS Path-B with the parameters of atmospheric re-analysis of ECMWF. Our studies have demonstrated the important role played by the wind (horizontal and vertical) and the humidity in the modulation of the microphysical properties of cirrus clouds. In general, in situations of strong vertical winds, horizontal cirrus have a size actual ice crystals rather low. Similarly, the thickness of cirrus ice bathing in a humid atmosphere containing strong updrafts are rather large. The last part of my thesis (Chapter 4) is dedicated to the study of coherence parameterizations of scattering properties of ice crystals between the thermal and the solar field. These scattering properties depend on the actual size of the crystals ice, the shape of the crystals and the wavelength. The non-regular forms of ice crystals of cirrus clouds that treatment of the diffusion and absorption radiation by these crystals can be done by approximate methods. We determined the radiative fluxes at the top of the atmosphere in the presence of cirrus clouds from radiative transfer model used in the model of the British climate MetOffice. The physical properties of cirrus clouds from TOVS Path-B, and the properties microphysics derived from measurements in the thermal field, were used to calculate albedos, which then were compared with observed albedo of cirrus clouds. These latter were determined by the radiometer ScaRaB. Three parameterizations of properties Dissemination of ice crystals were tested. These parameterizations have been developed from different physical principles and different assumptions about the shape of ice crystals. Assuming shaped crystals of hexagonal columns (Fu, 1998) observations are in agreement with the calculations for thin cirrus clouds in ice. The albedos calculated from the parameterization of Baran (2003) for crystals in aggregates are much better agreement with the observed albedos provided that the effective diameter of the crystals increases with the thickness of ice in cirrus clouds.
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Fadoua Eddounia. La microphysique des cirrus a l'echelle du globe : Correlation avec les proprietes atmospheriques et meilleure representation dans les modeles de climat.. Géophysique [physics.geo-ph]. Ecole Polytechnique X, 2004. Français. ⟨pastel-00001049⟩



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