The sensitvity of West African squall line development to land cover changes

The West African Sahel has significant heterogeneity in precipitation and land cover, making it an excellent location to test hypotheses on land/atmosphere interaction. The majority of wet season precipitation in the Sahel is contributed by squall lines that can extend 1000 km in length and travel westward for 12 hours or more. This study uses a coupled numerical land/atmosphere (cloud-resolving) model to test the sensitivity of squall line development to land surface conditions. Simulations were conducted over a 1000 km zonal transect with an inner grid (750 km) at 1.5-km resolution and stretched on either side. Sensitivity to savanna, semi-desert, and broadleaf forest vegetation was assessed. Although large-scale conditions (e.g., easterly waves) determined potential for system initiation, local conditions affected individual cell development. Vegetation density was a significant control on the moist static energy per unit area in the boundary layer. Integrated over time, small changes in moist static energy affected system size, longevity, and rainfall amounts. Squall lines in simulations with semi-desert cover had the highest inner core reflectivities and average and maximum rain rates for the first 4-6 hours of their lifetimes. However, since savanna and forest squall lines tended to be larger and had longer lifetimes, they produced significantly more total rainfall (50% or more). The implication of this study is that land surface conditions have a non-negligible effect on the characteristics of these important precipitating systems and thus the Sahelian hydrologic cycle.