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Représentation de la convection par CNRM-CM6 dans le cadre de la campagne CINDY2011/DYNAMO

Abstract : The tropical climate and its variability at multiple timescales are dominated by interactions between moist convection and the large-scale atmospheric circulation. Small-scale processes associated with convective clouds such as condensation and evaporation, radiation, and vertical mixing all contribute to atmospheric temperature gradients which generate large-scale circulations. Such circulations exert a control on the spatio-temporal distribution of energy and humidity within the tropical atmosphere and, in turn, on moist convection. These twoway interactions represent one of the most difficult scientific challenge for global atmospheric modelling. The main objective of the present thesis is to analyse the representation of these interactions in Version 6 of the ARPEGE-Climat atmospheric general circulation model and to understand their possible contribution to the main model biases in the tropics, especially a cold bias in the mid and upper troposphere and a dry bias around 850 hPa. The large-scale impacts of moist convection are often characterized by two quantities, the apparent heat source, Q1, and the apparent moisture sink, Q2. Although difficult to observe, these two quantities can be estimated by deploying a sounding array of sufficient density to compute the different terms of the water and energy budgets over a selected domain. Such a strategy was implemented between October and December 2011 during the CINDY2011/DYNAMO field campaign in the middle of the tropical Indian Ocean. The collected observations and the derived Q1 and Q2 estimates are used in the present thesis to (i) characterize the life cycle of the tropical convection and (ii) set up a single column configuration of the ARPEGE-Climat model on the northern and southern domains of the campaign. Results show that the model is able to capture satisfactorily the transitions between different convective regimes, from shallow to deep and stratiform, despite a strong undestimation of surface evaporation and of the overall convective activity over the northern domain. The model however shows some difficulties at simulating the troposphere moistening during the shallow cumulus regime. The single column model results are then compared to 3D configurations of the ARPEGEClimat model, both in AMIP mode where the model is only driven by observed sea surface temperatures, and in " Transpose-AMIP " mode where the model is also initialized from realistic atmospheric conditions. Through the analysis of the systematic atmospheric drift across these Transpose-AMIP integrations, the dominant contribution of fast (within a few days) processes to the model biases found in AMIP mode is highlighted. Such biases also show some similarity with the errors simulated in the single-column framework. A more detailed analysis of the model systematic errors reveals a strong contribution of deep convection, especially in its stratiform regime, to the cold bias in the upper troposphere, and of deficiencies in the shallow cumulus regime to the moisture biases. These regimes will therefore deserve a particular attention during the next phase of development of the ARPEGE-Climat model.
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Ahmat Younous Abdel-Lathif. Représentation de la convection par CNRM-CM6 dans le cadre de la campagne CINDY2011/DYNAMO. Climatologie. Université Paul Sabatier - Toulouse III, 2018. Français. ⟨NNT : 2018TOU30025⟩. ⟨tel-02061600⟩

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