Convective Squalls over the Eastern Equatorial Atlantic

Abstract

Abstract The Congo Basin and the adjacent equatorial eastern Atlantic are among the most active regions of the world in terms of intense deep moist convection, leading to frequent lightning and severe squalls. Studying the dynamics and climatology of this convection is difficult due to a very sparse operational network of ground-based observations. Here, a detailed analysis of recently available high temporal resolution meteorological observations from three oil platforms off the coast of Angola spanning the three wet seasons from 2006/07 to 2008/09 is presented. The annual cycle of squall days as identified from wind data closely follows that of convective available potential energy (CAPE) and therefore mirrors the cycle of wet and dry seasons. The diurnal cycle of squall occurrence varies from station to station, most likely related to local features such as coastlines and orography, which control the initiation of storms. An attempt to classify squalls based on the time evolution of the station meteorology and satellite imagery suggests that microbursts account for at least one-third of the strong gusts, while mesoscale squall lines appear to be quite rare. On a daily basis the probability of squall occurrence increases with increasing values of CAPE, downdraft CAPE, and 925–700-hPa wind shear, and decreases for high convective inhibition, all calculated from vertical profiles of temperature and humidity at the nearest grid point in the NCEP–NCAR and ECMWF reanalysis datasets. Both the climatological results and the stability indices can be used for local forecasting to avoid squalls impacting on operations on the offshore platforms.