The effect of vegetation changes on precipitation and Mesoscale Convective Systems in the Sahel.

A number of general circulation model studies have assessed the impact of degradation of the land surface in the Sahel, mostly with idealized degradation scenarios. This paper builds on the previous research by testing the sensitivity of Mesoscale Convective Systems (MCS) and associated rainfall amounts to observed vegetation changes using a regional atmospheric model. Over the last 20 years, the vegetation in the Sahel has recovered from the drought in the 1980s and vegetation cover values have increased up to 20%. The sensitivity for both a vegetation increase and a decrease by these realistic amounts is investigated. The model simulations span 42 days of the rainy season and are centred over the region of the Hydrological and Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel), of which the data are used to evaluate model results. The model is able to correctly reproduce rainfall amounts and atmospheric profiles. Total precipitation is found to be insensitive to the applied vegetation changes, but the latter do have an impact on the rainfall patterns and the location of MCS. The model results indicate that the change in vegetation cover influences the MCS in two different ways: Firstly, the vegetation change is found to affect the surface fluxes and this in turn is found to affect the Convective Available Potential Energy (CAPE) and thereby the strength of the convective systems. The relation between vegetation cover and CAPE turns out to be affected by the time in-between precipitation events. Secondly, a change in atmospheric dynamics, especially the mid-tropospheric zonal flow, is modelled as response to a change in the spatial temperature and humidity distribution. Both mechanisms are likely to play a role in determining the characteristics of the rainfall pattern. [ABSTRACT FROM AUTHOR]