Nocturnal Continental Low-Level Stratus over Tropical West Africa: Observations and Possible Mechanisms Controlling Its Onset

Abstract

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 nonprecipitating 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 0236 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 h 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.

West African monsoon decadal variability and surface-related forcings: second West African Monsoon Modeling and Evaluation Project Experiment (WAMME II)

“…About 90 % of Sahel rainfall comes from intense, organized convective activity (Mathon et al 2002). In addition, low-level continental stratus clouds frequently occur over large regions of southern West Africa during the WAM wet season (Stein et al 2011;Schrage and Fink 2012). Associated with the widespread relative sinking motion (Fig.…”

Investigating the frequency and interannual variability in global above-cloud aerosol characteristics with CALIOP and OMI

“…Nighttime ACA events are also observed over the east coast of Asia year-round. One reason for differences in spatial coverage between daytime and nighttime ACA events is plausibly linked to a lower planetary boundary layer that affects the formation of low clouds (e.g., Schrage and Fink, 2012). Still, the discrepancy between nighttime and daytime ACA events can be partially attributed to the potential detection of relatively diffuse ACA plumes that are more detectable during nighttime compared with day as a result of the higher signal to noise ratio for CALIOP nighttime data (e.g., Kacenelenbogen et al, 2014).…”

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.…”

“…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. Schrage and Fink (2012) investigated the nighttime cloud formation. They stated that the presence of a nighttime low-level jet induces a shear-driven vertical mixing of moisture accumulated near the surface.…”

“…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.…”