Surface thermodynamics and radiative budget in the Sahelian Gourma: seasonal and diurnal cycles
Résumé
Our understanding of the role of surface-atmosphere interactions in the West African monsoon has been particularly limited by the scarcity of measurements. The present study provides a quantitative analysis of the very pronounced seasonal and diurnal cycles of surface thermodynamics and radiative fluxes in the Central Sahel. It makes use of data collected from 2002 to 2007 in the Malian Gourma, close to Agoufou, at 1.5°W-15.3°N and sounding data collected during the AMMA field campaign. The seasonal cycle is characterized by a broad maximum of temperature in May, following the first minimum of the solar zenithal angle (SZA) by a few weeks, when Agoufou lies within the West African Heat-Low, and a late summer maximum of equivalent potential temperature (thetae) within the core of the monsoon season, around the second yearly maximum of SZA. Distinct temperature and moisture seasonal and diurnal dynamics lead to a sharpening of the early (late) monsoon thetae increase (decrease), more steadiness of thetae and larger changes of relative humidity in between. Rainfall starts after the establishment of the monsoon flow, once temperature already started to decrease slowly, typically during June. Specific humidity increases progressively from May until August, while the monsoon flow weakens during the same period. Surface net radiation (Rnet) increases from around 10-day mean values of 20W.m-2 in Winter to 120-160 W.m-2 in late Summer, The increase is sharper during the monsoon than before, and the decrease fast. The seasonal cycle of Rnet arises from distinct shortwave and longwave fluctuations that are both strongly shaped by modifications of surface properties related to rainfall events and vegetation phenology (decrease of both surface longwave emission and albedo). During the monsoon, clouds and aerosols reduce the incoming solar radiation by 20-25% (about 70W.m-2). They also significantly enhance the day-to-day variability of Rnet. Nevertheless, the surface incoming longwave radiative flux (LWin) is observed to decrease from June to September. As higher clouds covers and larger precipitable water amounts are typically expected to enhance LWin, this feature points to the significance of changes in atmospheric temperature and aerosols along the monsoon season. The strong dynamics associated with the transition from a drier hot Spring to a brief cooler moist tropical summer climate involves large transformations of the diurnal cycle, even within the monsoon season, which significantly affect both thermodynamical, dynamical and radiative fields (and low-level dynamics). In agreement with some previous studies, strong links are found between moisture and LWnet all year long and a positive correlation is identified between Rnet and thetae. The observational results presented in this study further provide valuable ground truth for assessing models over an area displaying a rich variety of surface-atmosphere regimes.