Inferring regional surface fluxes from convective boundary layer characteristics in a Sahelian environment

Regional scale fluxes of sensible heat, latent heat, and CO2 are estimated by means of a method which involves the heat and mass budget of the convective boundary layer (CBL). This method is based upon an integrated form of the conservation equation for a given scalar and is used with radiosounding data collected during several clear days of measurements of the Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) at the end of the rainy season and the beginning of the dry season in 1992. The regional estimates of sensible heat flux provide plausible results, For latent heat flux, only 4 days out of 8 give satisfactory agreement, and the introduction of a correction for advection does not improve the agreement for the rest of the days. For CO2 flux calculation the method used links regional fluxes to concentrations measured within the surface layer. The estimates obtained for the CO2 uptake at a regional scale differ from those measured on a fallow savannah, but the overall trend (a decrease of the CO2 uptake due to soil water depletion and vegetation drying out) is respected, Our general conclusion is that the CBL budget method can work to estimate sensible heat flux, but it is very hazardous to estimate latent heat flux and CO2 uptake.