Synoptic observations and various satellite products have been utilized for computing climatologies of low-level stratus over southern West Africa for the wet monsoon seasons July–September of 2006–2011. Previous studies found inconsistencies between satellite cloud products; climate models often fail to reproduce the extensive stratus decks. Therefore, a better observational reference and an understanding of its limitations are urgently needed to better validate models. Most detailed information of the spatiotemporal characteristics of low-level clouds was obtained from two Meteosat Second Generation (MSG) satellite-based data sets. However, CALIPSO and CloudSat cross sections of cloud occurrence frequency suggest that both MSG products underestimate the low-level cloudiness over Nigeria due to shielding by abundant upper level and midlevel clouds and reveal that the stratus is lower over the continent than over the ocean. The Terra Multiangle Imaging Spectroradiometer product appears to overestimate the morning extent of low-level clouds. The climatology presented here shows that the zone of abundant low-level stratiform clouds is at its diurnal minimum south of 6–7°N around sunset (~1800 UTC). Thereafter, it starts to spread inland and reaches its maximum northward extent of 10–11°N between 0900 and 1000 UTC. The maximum affected area is approximately 800,000 km2. After about 1000 UTC, the northern boundary gets fragmented due to the breakup of stratus decks into fair-weather cumuli. The stratus is most frequent around Cape Palmas, over Ivory Coast, and at the windward sides of the Mampong Range (Ghana) and Oshogbo Hills (Nigeria).

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JD022614

Satellite-based climatology of low-level continental clouds in southern West Africa during the summer monsoon season

A remote-sensing study of the inland penetration of sea-breeze fronts from the English Channel

. 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 Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on an area around the city of Save 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.

/pdf/nocturnal-low-level-clouds-over-southern-west-africa-14jw3897gd.pdf

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

[1] A statistical analysis is performed between the rainfall anomalies in Cote d'Ivoire (West Africa) and those of sea surface temperature and pseudo-wind stress in the tropical Atlantic. The correlation structures show that the coastal and northern regions of Cote d'Ivoire are differently related to the meteo-oceanic variables. While the main rainy seasons of these two Ivorian regions are both related to a negative SST/rain correlation pattern, itself strongly linked to the abnormal position of the intertropical convergence zone, the dry season of the littoral zone is related to a positive SST/rain correlation pattern which remains offshore of Cote d'Ivoire. Our study illustrates the inverse climatic behaviour observed between the northern and the littoral zones during the two “Atlantic's El Ninos” of 1968 and 1984.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL015290

Tropical Atlantic and rainfall variability in Côte d'Ivoire

5-year 3-hourly wind speed and direction, air temperature over land and sea surface temperature data are analysed in order to characterize sea breezes at Cotonou (Republic of Benin, West Africa). A set of criteria based on the diurnal reversal of wind direction, the thermal gradient and the number of sunshine hours is used to identify sea breeze days. Statistics are presented that describe the occurrence, duration and strength of the sea breezes. It is found that the sea breeze occurs during all seasons in this region.



Sea breeze occurrence is found to be strongly influenced by the West African monsoon. The frequency of occurrence shows clearly two regimes characterized by two maxima (December and May). Sea breezes occur about half of the days during non-monsoon season (winter). Land/sea breezes onset and cessation times show a pronounced seasonal variation. Sea breeze strength is greater during the West African monsoon season.



A hodograph analysis reveals the existence of both clockwise and anticlockwise rotation. Average hodographs show clearly the onshore–offshore nature of the sea breeze circulation, but have indeterminate rotation. There is a significant seasonal effect of large-scale flows on sea breeze hodograph patterns.

Sea breezes at Cotonou and their interaction with the West African monsoon

The airflow over Cape of Three Points (Gulf of Guinea: 4.5°N, 2°W) has been simulated using a three-dimensional mesoscale model in order to investigate the sea breeze developing in synoptic vertical wind shears during the 1979 dry season. Two different meteorological situations, characterized by two contrasted wind profiles between 500 and 2000 m have been studied, with two types of transitions between the lower circulation (SW monsoon) and the upper African easterly jet (AEJ). The first one is a veering case (6 January) and the second is a backing case (23 January). Calculations of CAPE (convective available potential energy) show that whereas instability is a maximum at both sides of the cape, the site of enhanced convection is determined by the wind shear in the 500–2000 m layer. Numerical results confirm satellite observations.

S. Cautenet

Papers

Numerical Simulation Of Sea Breezes with Vertical Wind Shear during Dry Season at Cape of Three Points, West Africa