Parametrization of orographic thermal effect on the deep convection triggering in Global Model

The work is based on the hypothesis that anabatic winds (or valley breeze) is an important mechanism of deep convection triggering. Induced by the temperature difference between the mountain surface and the environmental air, anabatic winds own a kinetic energy which may eventually overcome the Planet Boundary Layer inhibition (CIN, Convective Inhibition) and allows the associated convection to develop into the free troposphere. This sub-grid scale phenomenon needs a special parametrization in general circulation models (GCMs). Its lack of representation in present GCM versions is thought of being the cause of the deficit of deep convection systems genesis observed in certain orographical zones, as Mount Cameroun in West Africa for example. A valley breeze parametrization has been designed and built in a GCM (LMDZ). The model computes kinetic energy of the valley breeze in relation to the sub-grid scale orographical characteristics (elevation, slope, orientation). It consists of a grid slim layer along the mountain surface. It is coupled with a multi-layers conductive-capacitive soil model. The coupling is accomplished by using the energy budget at the surface of the mountain. The model was tested in the dynamical mode by systematic sensitivity analysis to the principal parameters and to the environmental conditions. It has then been implemented in the 1D version of the GCM (SCM, Single Column Model), coupled with the Emanuel deep convection scheme, and tested against a radiative-convective equilibrium case and the Hapex campaign case. The stationnary solution of the aeraulic part of the model has been adopted for the GCM. The parametrization finally has been introduced in the 3D version of the GCM, in the diagnostic mode (without coupling to the convection process). It gives a spatial distribution of the triggering frequency of deep convection in coherence with that of the satellite image observation in the West Africa region, during the West African Monsoon season. The effect of soil humidity on triggering has in particular been brough to light.