African Monsoon Precipitating Systems During a Dust Outbreak

African monsoon precipitating systems are analyzed during a dust outbreak using satellite observations combined with ECMWF analysis and a set of high- and low-resolution Méso-NH simulations (grid spacing of 2.5 and 20 km). A common analysis is applied to the observations and the simulations. The precipitating systems are identified as individual cloud objects and classified as Deep Convective Clouds (DCCs), Convective Anvil Clouds (CACs) or other clouds according to their infrared signature. DCCs are tracked and classified as short- or long-lived whether their life time overpasses 6 h or not. Their properties are characterized in terms of wind, temperature, humidity and precipitation and analyzed in Southern West Africa (SWA), Central Africa (CAF) and Ethiopia (ETH).

Satellite observations show that DCCs account for 71% of precipitation and CACs for 19%. Precipitation shows a marked diurnal cycle with a peak in the evening as previously seen over longer periods. Our analysis further reveals that long-lived DCCs produce more than half of precipitation making them the main contributor to the evening peak. Their size, zonal speed and duration increase from east to west. They are the largest, the fastest and the longest-lived over SWA. This is explained by enhanced African Easterly Jet (AEJ) and monsoon flows leading to a stronger shear and a larger conditionally-unstable atmosphere.

The convection-permitting simulation captures most of the observed precipitation features: its distribution according the cloud types, the diurnal cycle and the contribution of DCCs as well as the size, zonal speed and duration of the long-lived DCCs over CAF and ETH. The simulation failed to represent the characteristics of long-lived DCCs over SWA. Their size, zonal speed and duration are underestimated. The too strong moisture in a too zonal AEJ flow suggests that the long-lived DCCs in SWA are poorly located with respect to African Easterly Waves (AEWs), which do not favor their mesoscale organization. The convection-parametrization simulation distributes precipitation mainly to other clouds than DCCs while long-lived DCCs have a too low size, zonal speed and duration. The contrasting results between the two simulations highlight the importance of convective scale needed for representing the DCCs correctly. Further work is needed for better understanding the failures of the simulation to represent the role of AEWs in organizing and maintaining long-lived DCCs.

This study was supported by the DACCIWA project.