Nocturnal low-level clouds over southern West Africa analysed using high-resolution simulations

Abstract

Abstract. We performed a high-resolution numerical simulation to study the development of extensive low-level clouds that frequently form over southern West Africa during the monsoon season. This study was made in preparation for a field campaign in 2016 within the Dynamicsaerosol-chemistry-cloud interactions in West Africa (DAC-CIWA) project and focuses on an area around the city of Savè in southern Benin. Nocturnal low-level clouds evolve a few hundred metres above the ground around the same level as a distinct low-level jet. Several processes are found to determine the spatio-temporal evolution of these clouds including (i) significant cooling of the nocturnal atmosphere caused by horizontal advection with the south-westerly monsoon flow during the first half of the night, (ii) vertical cold air advection due to gravity waves leading to clouds in the wave crests and (iii) enhanced convergence and upward motion upstream of existing clouds that trigger new clouds. The latter is caused by an upward shift of the low-level jet in cloudy areas leading to horizontal convergence in the lower part and to horizontal divergence in the upper part of the cloud layer. Although this single case study hardly allows for a generalisation of the processes found, the results added to the optimisation of the measurements strategy for the field campaign and the observations will be used to test the hypotheses for cloud formation resulting from this study.

Cloud and aerosol radiative effects as key players for anthropogenic changes in atmospheric dynamics over southernWest Africa

“…2c). Adler et al (2017) identify a regular occurrence of a stationary coast-parallel front over SWA about 30 km inland that propagates northwards during undisturbed monsoon conditions after about 16 UTC. Similar characteristics were described in Grams et al (2010) for Mauritania.…”

“…The study of Adler et al (2017) reveals that the AI frequently occurs under undisturbed monsoon conditions over SWA, 20 reaching Savè around 21 UTC. This agrees well with the latitudinal AI front evolution in this study (not shown).…”

“…With the reduction in turbulence in the afternoon the front begins propagating inland. The studies of Grams et al (2010) and Adler et al (2017), both based on modeling studies using the COSMO model, highlight the need to distinguish this feature from the land-sea breeze and the sea breeze front, since the dominating monsoon flow suppresses the formation of a land wind during night. In the following, we use the term Atlantic Inflow (AI) as proposed by 10 Grams et al (2010), which is connected with an AI front and an AI airmass located behind the front.…”

“…5b) in the reference case. Although the focus is on 2 July, 3 July is added to underline that the AI is a robust feature occurring frequently over SWA, which is also indicated by the results of Adler et al (2017). The θ s method for the AI front location is only an estimation, since the potential temperature is also altered by surface conditions and diabatic effects.…”

The observed diurnal cycle of nocturnal low-level stratus clouds over southern West Africa: a case study

“…The Savè supersite (8.00 o N, 2.43 o E, 166 m above sea level (a.s.l.)) is located approximately 185 km inland from the coast in a moderately hilly region which is favorable for LLC formation (van der Linden et al, 2015;Adler et al, 2017;Kalthoff et al, 2018). A comprehensive mea-surement set-up at this supersite allows the detailed investigation of atmospheric conditions and identification of atmospheric processes relevant to the observed life-cycle of LLC.…”