Role of low-level thermodynamics on surface-convection interactions over West-Africa

This study aims to characterize the thermodynamics of the lower atmosphere across West Africa (WA), on the basis of observations from the AMMA project. A dataset made of several thousand high-resolution soundings is used for this purpose. Moist-convective related indexes, boundary layer (BL) parameters, and a combination of two indexes proposed by Findell and Eltahir (2003) (FE03) are computed for each sounding. These latter indexes are well suited for distinguishing between aspects of surface-atmosphere interactions involving atmospheric vertical structures. The variability of the low levels is found to be stronger in the Sahelian zone than in the Soudanian zone, from daily scale up to the monsoon season (JJAS). The FE03 indexes reveal that surface-atmosphere feedbacks are actually significant over West-Africa, but that the feedback loops change sign with latitude and season. Thus, the thermodynamic environment at low levels is broadly consistent with the idea of daytime convection being either suppressed or favoured over wet surface versus dry ones, depending on latitude and seasonal variations. This is further documented with offline land-surface modelling (ALMIP). Furthermore, variations of the lifting condensation level (LCL) and the level of free convection (LFC) are consistent with the previous analysis. For instance, in the Sahel, a strong diurnal cycle of the LCL is found, especially early in the season, while the LFC does not fluctuate much during the day, but shifts downwards from June to August. The LCL is however not such a good indicator of BL height when the low levels are the driest. This is particularly true prior to the monsoon onset, on days when none of the daytime BL convective plumes reach their LCL. In that case, moist convective development is limited by high convective inhibition (CIN) rather than convective available potential energy (CAPE). This finding is consistent with observations that daytime convection is not always favoured over wet land surfaces.