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Paramétrisation de la couche limite atmosphérique convective et représentation du cycle diurne des nuages dans un modèle de climat.

Abstract : The main objective of this work is to improve the representation of the diurnal cycle of convective clouds in General Circulation Models. Some biases in the representation of this diurnal cycle are com- monly found in GCMs : under-estimation of low clouds, precipitations in phase with solar activity while the maximum of precipitations is observed in the late afternoon. A better representation of convective processes in the boundary layer is one of the key issues to improve those various aspects in GCMs. Processes involved in boundary layer turbulence, dry and moist convection, either shallow or deep, are first studied using observations and high resolution simulations. This leads to various hypothesis at the basis of the parameterization we want to develop. To try to take into account all turbulent scales of the convective boundary layer, a classical diffusive scheme is combined with a mass-flux scheme represen- ting boundary layer coherent structures : the cloudy thermal plume model. The scheme is implemented in an 1D configuration of the GCM LMDZ, developed in the Laboratoire de Météorologie Dynamique, and evaluated against observations, Large Eddy Simulations and Cloud Resolving Models on cases of dry, shallow and deep convection over land. The explicit representation of a mean ascending plume, entrai- ning and detraining, and of a compensating subsidence in the environment, improves the diurnal cycle of the boundary layer and shallow cumulus clouds. This better representation of boundary layer clouds has an impact on the diurnal cycle of deep convection associated with precipitations and parameterized independently. By controling deep convection by so-called "underlying processes", like boundary layer convection or wakes generated by evaporation of precipitations under the convective system, it is pos- sible to delay the onset of precipitations by several hours and to maintain them after sunset. Finally, developments are adapted to the representation of convection induced by biomass burning in the Tropics. Initialized by fires characteristics, the thermal plume model becomes the pyro-thermal plume model, generating plumes in the boundary layer which can also reach the mid-troposphere. The various parameterizations used in this study will be part of the new 3D version of LMDZ, in order to run the simulations required for the next IPPC (Intergovernmental Panel on Climate Change) report.
keyword : Modèle de climat
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Catherine Rio. Paramétrisation de la couche limite atmosphérique convective et représentation du cycle diurne des nuages dans un modèle de climat.. Sciences de l'ingénieur [physics]. Ecole Polytechnique X, 2007. Français. ⟨pastel-00003728⟩

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