Ultra-low clouds over the southern West African monsoon region

Abstract

[1] New ground-and space-based observations show that summertime southern West Africa is frequently affected by an extended cover of shallow, non-precipitating clouds only few hundred meters above the ground. These clouds are associated with nocturnal low-level wind speed maxima and frequently persist into the day, considerably reducing surface solar radiation. While the involved phenomena are well represented in re-analysis data, climate models show large errors in low-level wind, cloudiness, and solar radiation of up to 90 W m −2 . Errors of such a magnitude could strongly affect the regional energy and moisture budgets, which might help to explain the notorious difficulties of many models to simulate the West African climate. More effort is needed in the future to improve the monitoring, modeling, and physical understanding of these ultra-low clouds and their importance for the West African monsoon system. Citation: Knippertz, P., A.

Formation and Maintenance of Nocturnal Low-Level Stratus over the Southern West African Monsoon Region during AMMA 2006

“…Recent work (Schrage et al 2007;Knippertz et al 2011;Schrage and Fink 2012) involving authors from the present paper has drawn attention to a previously understudied source of uncertainty: the formation of lowlevel continental stratus and stratocumulus cloud decks over moist southern West Africa between the Sahel and Guinea Coast, and the associated error in cloud radiative forcing. Knippertz et al (2011) documented errors of up to 90 W m 22 in the mean daily surface solar irradiance in this region in global climate models used for phase 3 of the Coupled Model Intercomparison Project (CMIP3) with individual models' errors closely related to their low-level stratus cover.…”

“…Knippertz et al (2011) documented errors of up to 90 W m 22 in the mean daily surface solar irradiance in this region in global climate models used for phase 3 of the Coupled Model Intercomparison Project (CMIP3) with individual models' errors closely related to their low-level stratus cover. The stratus decks frequently cover an extensive region stretching from the Guinea Coast (;58-68N) to about 98-108N during the night and morning hours (Schrage and Fink 2012).…”

“…The widely used International Satellite Cloud Climatology Project (ISCCP) dataset dramatically underestimates the extensive coverage of very low clouds owing to the small contrast in infrared radiation with the underlying surface . To circumvent this problem, Knippertz et al (2011) and Schrage and Fink (2012) used a slightly modified version of the ''night microphysical'' scheme from Lensky and Rosenfeld (2008) to visualize the stratus deck at night, based on three infrared channels of Meteosat Spinning Enhanced Visible and Infrared Imager (SEVIRI). However, Knippertz et al (2011) also documented problems in low-cloud detection due to mid-or highlevel clouds in SEVIRI and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data and due to ground clutter in CloudSat data.…”

“…To circumvent this problem, Knippertz et al (2011) and Schrage and Fink (2012) used a slightly modified version of the ''night microphysical'' scheme from Lensky and Rosenfeld (2008) to visualize the stratus deck at night, based on three infrared channels of Meteosat Spinning Enhanced Visible and Infrared Imager (SEVIRI). However, Knippertz et al (2011) also documented problems in low-cloud detection due to mid-or highlevel clouds in SEVIRI and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data and due to ground clutter in CloudSat data. Stein et al (2011) used a combined CloudSat-CALIPSO product to create a vertical cloud climatology and found nocturnal low-level clouds to occur during 50% of the nights between June and September, which appears to be biased low compared to synoptic reports analyzed by Schrage and Fink (2012).…”

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.…”