Nocturnal stratiform cloudiness during the West African monsoon

Abstract

SummaryAn analysis of METEOSAT satellite images and synoptic reports from Parakou, Benin suggests that those nights that lack deep convection during the West African monsoon are often either completely clear or completely overcast at low levels. Using radiosonde observations gathered at Parakou during the IMPETUS field campaign of the summer of 2002 and ECMWF operational analyses, the composite structure of the atmosphere for both cloudy and clear nights is presented. Cloudy nights are found to occur when turbulent processes result in large-scale speed convergence in the boundary layer and a positive net moisture flux convergence. Conversely, the clear conditions occur when a nocturnal inversion decouples the boundary layer from the surface. The cloudy and clear conditions are also shown to be related to large-scale changes in the configurations of the African Easterly Jet and the Tropical Easterly Jet, although the nature of this relationship remains unclear.

Ultra-low clouds over the southern West African monsoon region

“…Based on this the following mechanism is suggested [see also Bonner and Winninghoff, 1969;Schrage et al, 2007;Schrage and Fink, 2010]: (I) Around sunset, mixing in the planetary boundary layer breaks down followed by a minimum in cloudiness. (II) Radiative cooling stabilizes a shallow surface layer, where winds slacken and moisture accumulates through evapotranspiration.…”

“…Here we use new ground-and space-based observations to show that the frequent occurrence of extended, shallow, ultra-low, nonprecipitating stratiform clouds, which form in association with nocturnal low-level wind speed maxima, considerably reduce surface solar radiation over summertime southern West Africa. These clouds have so far received little attention [Schrage et al, 2007;Schrage and Fink, 2010] in contrast to their oceanic counterparts [Albrecht et al, 1995] and their role for the whole WAM system is unknown.…”

LES study of the impact of moist thermals on the oxidative capacity of the atmosphere in southern West Africa

“…When updrafts lead to cloud formation, Vilà-Guerau de Arellano et al (2005) found a decrease of 10 to 50% of tracer mixing ratios High resolution simulations which explicitly resolve the turbulent and convective advection terms were conducted (Vilà-Guerau de Arellano and Cuijpers, 2000; Vilà-Guerau de Arellano et al, 2005;Kim et al, 2012Kim et al, , 2016 The goal of this work is to evaluate the role of thermals on OH reactivity in the framework of a convective boundary layer with contrasted chemical environments in southern West Africa. This region is characterized by high occurrence of low-level stratus and stratus deck poorly represented in climate models, leading to errors in radiative forcing (Schrage et al, 2007;Knippertz et al, 2011;Schrage and Fink, 2012). Two chemical regimes represented by a detailed chemical scheme are contrasted in LES studies.…”

“…This could lead to too low simulated clouds and too high incoming radiation at the surface that imply too high diurnal cycles of temperature and relative humidity over this region. The nocturnal low-level stratus have been studied during the monsoon period at Parakou (Benin) by Schrage et al (2007) with radiosondes. The authors found that turbulent 30 processes are responsible for cloudy nights whereas clear nights are associated with a nocturnal inversion promoting the decoupling of surface and lower atmosphere.…”

“…This leads to cloud formation whose cover is likely to persist until the early afternoon (Schrage et al, 2007;Schrage and Fink, 2012). This feature is simulated in this work as low level stratus clouds occupy a large fraction of the simulated domain in the morning ( Fig.…”