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Journal Article: BibTeX citation key:  Schragea
Schrage, J. M. & Fink, A. H. Nocturnal continental low-level stratus over tropical West Africa: Observations and possible mechanisms controlling its onset. IN Monthly Weather Review, 140. 1794–1806.
Added by: Andreas Fink 2012-01-26 12:54:44    Last Edited by: Andreas Fink 2012-05-24 08:30:32
Categories: Monsoon system and its variability
Keywords: Atmospheric Boundary Layer, Diurnal cycle, Dynamics, Soudanian zone
Creators: Fink, Schrage
Collection: Monthly Weather Review

Peer reviewed
Number of views:  775
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Some spatiotemporal characteristics and possible mechanisms controlling the onset of the widespread, low-level nocturnal stratiform clouds that formed during May–October 2006 over southern tropical West Africa are investigated using cloudiness observations from surface weather stations, data from various satellite platforms, and surface-based remote sensing profiles at Nangatchori in central Benin. It is found that the continental stratus is lower than the maritime stratus over the Gulf of Guinea and persists well into the noon hours. For the study period, a clear seasonal cycle was documented, as well as a dependence on latitude with the cloudiest zone north of the coastal zone and south of approximately 9°N. It is also shown that non-precipitating clear and cloudy nights observed at Nangatchori in central Benin often reflect clearer and cloudier than normal conditions over a wide region of southern West Africa. At Nangatchori, on average the stratus developed at 0226 UTC (about local time) with an extremely low cloud base at 172 m (above ground level) when averaged over all cloudy nights. About 2-3 hours before cloudiness onset, a distinct nighttime low-level jet formed that promoted static destabilization and a low Richardson number flow underneath it. The ensuing vertical upward mixing of moisture that accumulated under the near-surface inversion after sunset caused the cloud formation. It is argued that a strong shear underneath the nighttime low-level jet is the major process for cloud formation, but the low-level static stability and the time scale of the shear-driven mixing are other potential factors.
Added by: Andreas Fink    Last Edited by: Andreas Fink