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PhD Defense

Jean Jouhaud (LMD)

Title : Amélioration de la représentation des nuages bas dans le modèle de circulation général LMDz

Date and time : The 14-12-2018 at 14h00

Type : thèse

Université qui délivre le diplôme : Sorbonne Université

Location : Amphi Herpin, Bâtiment Esclangon, Sorbonne Université, Campus de Jussieu, Paris 75005
Members of jury :

Mme. Hélène CHEPFER (Sorbonne Université, Paris), Examinatrice

Mme. Dominique BOUNIOL (CNRM, Toulouse), Rapporteure

M. Johannes QUAAS (University of Leipzig), Rapporteur

Mme. Céline CORNET (Université de Lille), Examinatrice

Mme. Catherine RIO (CNRM, Toulouse), Examinatrice

M. Romain ROEHRIG (CNRM, Toulouse), Examinateur

M. Jean-Louis DUFRESNE (Sorbonne Université, Paris), Directeur de Thèse

M. Jean-Baptiste MADELEINE (Sorbonne Université, Paris), Co-Directeur de Thèse

Summary :

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 parametrization 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 parametrization 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 modeling choices such as the vertical resolution of the GCM.

We evaluate the radiative impacts of these parameterizations 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 parameterizations then tend to reduce the "Too Few, Too Bright" bias over tropical ocean in LMDz6.

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