Asymmetric response of maximum and minimum temperatures to soil emissivity change over the Northern African Sahel in a GCM

Pronounced changes in land use and surface properties like those that have occurred in the Sahel could lead to reduced land surface emissivity and thus might have an asymmetric impact on daytime and nighttime surface air temperature. This paper analyzes the sensitivity of simulated climate and energy balance to changes in soil emissivity over the Sahel using the recently developed Community Land Model/Community Atmosphere Model. Model simulations indicate that the reduction of soil emissivity warms minimum temperature (T(min)) much faster than maximum temperature (T(max)) and thus decreases the diurnal temperature range (DTR) significantly. Lower emissivity reduces the outgoing longwave radiation and thus provides more energy to the atmosphere through increasing of sensible heat flux, ground and surface air temperatures. Statistical results show that, on average, a decrease of soil emissivity of 0.1 will increase Tmin by 0.55 degrees C and 0.41 degrees C, and decrease DTR by 0.59 degrees C and 0.46 degrees C under clear-sky and all-sky conditions, respectively, while Tmax changes little. The warming in Tmin and the decrease in DTR are much stronger during the dry season than the wet season and are higher in clear-sky conditions than all-sky conditions. These results suggest that changes in land surface emissivity over some particular regions might explain part of the observed decrease in DTR, especially over semi-arid regions. Citation: Zhou, L., R. Dickinson, P. Dirmeyer, H. Chen, Y. Dai, and Y. Tian ( 2008), Asymmetric response of maximum and minimum temperatures to soil emissivity change over the Northern African Sahel in a GCM.