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Journal Article: BibTeX citation key:  Schuster2013a
Schuster, R., Fink, A. H. & Knippertz, P. (2013) Formation and maintenance of nocturnal low-level stratus over the southern West African monsoon region during AMMA 2006. IN Journal of Atmospheric Sciences, 70. 2337–2350.
Added by: Andreas Fink 2013-05-16 15:05:36    Last Edited by: Andreas Fink 2013-08-07 08:00:11
 B  
Categories: Monsoon system and its variability
Keywords: Atmospheric Boundary Layer, Diurnal cycle, Dynamics, Modelling, Soudanian zone
Creators: Fink, Knippertz, Schuster
Collection: Journal of Atmospheric Sciences

Peer reviewed
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Abstract
The southern parts of West Africa are frequently covered by an extensive deck of
shallow, low (200 - 400 m above ground) stratus or stratocumulus clouds during the
summer monsoon season. 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 state-of-the-art climate models. Here we use high-resolution regional
simulations with the Weather Research and Forecast (WRF) model and observations
from the African Monsoon Multidisciplinary Analysis (AMMA) 2006 campaign to
investigate (a) the spatiotemporal distribution, (b) the influence on the short-wave
radiation balance, and (c) the detailed formation and maintenance mechanisms of the
stratiform clouds. At least some configurations of WRF satisfactorily reproduce the
diurnal cycle of the low cloud evolution yielding the following main conclusions: (a) The
simulated stratus deck forms after sunset along the coast, spreads inland in the course
of the night, and dissipates in the early afternoon. (b) The average surface net shortwave
radiation balance in stratus-dominated regions is about 35 W m-2 lower than in
those with less clouds. (c) The cloud formation is related to a subtle balance between
"stratogenic" upward (downward) fluxes of latent (sensible) heat caused by sheardriven
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.
Added by: Andreas Fink    Last Edited by: Andreas Fink