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Amélioration de la représentation des nuages bas dans le modèle de circulation générale LMDz

Abstract : In this thesis we intent to reduce the "Too Few, Too Bright" bias shared by most GCMs, that tend to underestimate the low-cloud cover and overestimate its optical depth. We work on two aspects of the low-clouds representation in LMDz6. Sub-grid Scale: We introduce a parametrisation to distinguish cloud fractions by volume CFvol and by surface CFsurf inside GCM grid boxes, and we recalibrate the calculation of condensed water by taking into account the depth of the grid boxes that use to be neglected. This step allows: -to reduce the optical depth of clouds by taking into account the depth of grid boxes in the calculation of qc -to increase the cloud fraction seen by the radiation scheme, that receive CFsurf instead of CFvol Inter-grid Scale: We propose a new parametrisation of the decorrelation length of the exponential-random overlap scheme, that allows evaluating the overlap of clouds independently in each atmospheric column, while taking into account wind shear and being adaptive to modelling choices such as the vertical resolution of the GCM. We evaluate the radiative impacts of these parametrisations with the COSP2 simulator package in which we implement the cloud-generator of the radiative code ECRad. Our diagnostics show: -a reduction of the low-covered and highly-reflective areas -an increase of the low-covered and low-reflective areas -and increase of the mid-covered and mid-reflective areas These new parametrisations then tend to reduce the "Too Few, Too Bright" bias over tropical ocean in LMDz6.
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https://tel.archives-ouvertes.fr/tel-02066819
Contributor : Jean Jouhaud <>
Submitted on : Wednesday, March 13, 2019 - 4:47:05 PM
Last modification on : Tuesday, September 22, 2020 - 3:50:47 AM
Long-term archiving on: : Friday, June 14, 2019 - 5:36:15 PM

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  • HAL Id : tel-02066819, version 1

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Jean Jouhaud. Amélioration de la représentation des nuages bas dans le modèle de circulation générale LMDz. Modélisation et simulation. Sorbonne University UPMC, 2018. Français. ⟨tel-02066819⟩

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