Extended Abstract (204K)3B.7
An Idealized Approach of the West African Monsoon
Philippe Peyrillé, CNRM, Météo-France, and CNRS, Toulouse, France; and C. Piriou, J. P. Lafore, and J. L. Redelsperger
The non-hydrostatic meso-scale model Méso-NH (Lafore et al., 1998) is used here to perform simulations of the West African Monsoon (WAM) in an idealized two-dimensional framework. We aim to represent the mean atmospheric circulation between 10°W and 10°E on a vertical-meridional plane for a typical regime of July-August. Two sets of simulations are performed using the scheme of Bechtold et al. (2000) to parameterize convection with a 70 km horizontal resolution.
The first one can be called the "simplest model" version where idealized surface conditions are prescribed at the beginning of the simulation. There is no relief and no external forcing. In this configuration, the atmospheric system is only a response to the surface conditions (continental and oceanic), and so doesn't represent large scales effects. After a transition period of 15 days, this version of the model is able to recreate a mean monsoon circulation, but prescribed SST values 1.5°C hottest than July's observations need to be prescribed to get a realistic monsoon regime. Sensitivity tests to different idealized SST profiles (May, June, July) confirm the critical role played by the seasonal evolution of the SST and its impact on the location of the convection zone.
The second set of simulations is a "forced version". It corresponds to the simplest model with a continental plateau and external forcings that take into account the large-scale fields effects and their zonal variation. The forcings are calculated in term of advection of temperature and humidity from the ECMWF analyses. They are able to stop the northward propagation of the monsoon and generate an harmattan in our simulation (absent in the simplest model simulations). They appear thus to be necessary to get a realistic WAM with July SST. Sensitivity tests to these forcings reveal that they have a strong impact on our simulations and stress the important role played by the heat low dynamic on the equilibrium of the WAM system.
The next step will be to confirm these results in performing high-resolution simulations (~2km). We will then use the grid-nesting method to study the effect of the representation of convection in this system (explicit vs parameterized).
Session 3B, Monsoon I
Monday, 3 May 2004, 1:30 PM-3:30 PM, Napoleon I Room
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