Formation and Maintenance of Nocturnal Low-Level Stratus over the Southern West African Monsoon Region during AMMA 2006

The southern parts of West Africa are frequently covered by an extensive deck of shallow, low (200-400m AGL) stratus or stratocumulus clouds during the summer monsoon. These clouds usually form at night in association with a nocturnal low-level jet (NLLJ) and can persist into the early afternoon hours. Recent work suggests that the stratus deck is unsatisfactorily represented in standard satellite retrievals and stateof- the-art climate models. Here the authors use high-resolution regional simulations with the Weather Research and Forecasting Model (WRF) and observations from the African Monsoon Multidisciplinary Analysis (AMMA) 2006 campaign to investigate (i) the spatiotemporal distribution, (ii) the influence on the shortwave radiation balance, and (iii) the detailed formation and maintenance mechanisms of the stratiform clouds. At least some configurations of WRF satisfactorily reproduce the diurnal cycle of the lowcloud evolution, yielding the following main conclusions: (i) The simulated stratus deck forms after sunset along the coast, spreads inland during the course of the night, and dissipates in the early afternoon. (ii) The average surface net shortwave radiation balance in stratus-dominated regions is about 35Wm22 lower than in those with fewer clouds. (iii) The cloud formation is related to a subtle balance between ''stratogenic'' upward (downward) fluxes of latent (sensible) heat caused by shear-driven turbulence below the NLLJ, cold advection, orographic lifting, and radiative cooling on one hand, and ''stratolytic'' dry advection and latent heating on the other hand.