Impact of burned areas on the northern African seasonal climate from the perspective of regional modeling

Abstract:

This study presents an investigation of the impact of burned areas on the surface energy balance and monthly precipitation in the northern Africa as simulated by a state-of-the-art regional model. Mean burned area fraction derived from MODIS approximate date of burning product were implemented in a set of 1-year long WRF/NMM/SSiB2 model simulations. Vegetation cover fraction and LAI were degraded daily based on mean burned area fraction and on the survival rate for each vegetation land cover type. Additionally, ground darkening associated with wildfire-induced ash and charcoal deposition was temporarily imposed through lower ground albedo for a period of 10 days after burning.

In general, wildfire-induced vegetation and ground degradation increased surface albedo by exposing the brighter bare ground of the region, which in turn caused a decrease in surface net radiation and evapotranspiration in northern sub-saharan Africa. A decrease in atmospheric moisture flux convergence was simulated in the burned area experiments, which plays a dominant role in reducing precipitation over the area, especially in the months preceding the West African monsoon onset. The areas with largest impacts were those covered by forests and savanna, where annual precipitation decreased by 4.2% and 3.6%, respectively.

This study suggests the cooling and drying of atmosphere induced by burned areas led to strengthening of subsidence during pre-onset and weakening of upward motion during onset and mature stages of the monsoon leading to a waning of convective instability and precipitation. Monthly vertical wind over the area showed a strengthening of downward motion in winter and spring seasons, and weakening of upward movement during the rainy months. Furthermore, precipitation energy analysis revealed that most of precipitation decrease originated from convective events, especially for those with daily precipitation rates above 2.0 mm day^-1, which substantiates the hypothesis of convective instability decreasing resultant from burned-area-induced land degradation.