Parametrization of dust storms in the Sahel by cold pools

The cold pools, created below cumulonimbus from the evaporation of precipitation, generate the strong winds responsible for the large dust storms called “haboobs” which appear in the Sahel in summer. Most global climate models do not take into account these types of dust emissions due to lack of parameterization of cold pools and associated gusts (Marsham et al. 2011 ; Pantillon et al. 2015).

The introduction of a parameterization of cold pools in the LMDZ climate model has improved the representation of convection, and in particular of the diurnal cycle of continental precipitation in the tropics (Rio et al. 2009). The aim of this work is to develop a parameterization of gusts related to cold pools in order to take into account ‘‘hoobobs’’ in LMDZ model. To do this, we use Large Eddy Simulations (LES) performed on an oceanic domain and in Radiative-Convective Equilibrium (RCE) mode. We use a LES of an oceanic RCE case, easier to analyze because the temperatures are uniform on the surface and therefore the cold pools easier to detect. Before developing a gust parameterization, we evaluate the cold pools parameterization in LMDZ on this RCE case, which has never been done so far. If the comparison confirms the relevance of the scheme and its qualitative match to the LES behavior, is also led to substantial improvements and adjustments to this scheme. Next, we analyze the wind distributions in the LES in order to construct a parametrization based on a probability distibution function of the subgrid scale distribution of the wind which will allow us to take into account the effect of gusts on dust storms. The parametrization relates the moments of the distribution to large-scale wind speed, the spreading speed of the cold pools and the surface fraction covered by the latter. In the following, we will test the parametrization on the LMDZ model by focusing on dust storms in the Sahel during the rainy season.