A possible mechanism regulating nocturnal stratocumulus decks in West Africa

On many nights during the summer monsoon, the Soudanian (9°N-12°N) and Guinea Coast (south of 9°N) climate zones of sub-Saharan West Africa are mantled in a shallow layer of stratocumulus clouds. In the present study, it is shown that the formation of nocturnal stratiform cloud decks is preceded by a period of destabilization in the near-surface layer. This destabilization seems to occur on most nights in which a nocturnal low level jet forms in the lowest 200 to 400 m of the boundary layer. This jet produces mechanical turbulence and vertical mixing that tends to reduce the static stability of the layer. This process naturally competes with the tendency of the near-surface layer to become increasingly stable over the course of the night by purely radiative mechanisms. The timing and magnitude of the jet and the radiative processes becomes critical for the formation of nocturnal stratocumulus clouds. The mixing and resulting destabilization caused by the low level jet typically lead the onset of stratocumulus clouds by more than one hour. Clear nights, on the other hand, typically exhibited a low level jet that either was too late in the night to overcome a well-developed nocturnal inversion or was too weak to produce sufficient mixing.