The ERA5’s Diurnal Cycle of Low-Level Clouds Over Western Central Africa During June-September: Dynamic and Thermodynamic Processes.

Abstract

This paper analyzes the diurnal cycle of low cloud cover (LCC) and the atmospheric conditions under which it grows over Western Central Africa during the cloudiest season (June-September). Moderate Resolution Imaging Spectroradiometer (MODIS) observations, Extended Edited Clouds Reports Archive (EECRA) and the fifth generation of reanalysis of the European Centre for Medium Range Weather Forecasts (ECMWF), i.e., ERA5 are used. LCC peaks between 04LT and 07LT and tends to be less dense during the afternoon. The associated dynamic and thermodynamic ERA5 conditions reveal different processes. The strong low level (below 1000 m) southwesterly flow in the evening supplies the region with humidity from the ocean and leads to cloud formation. Relative humidity (RH) tendencies show that temperature contributes to 100% of RH changes : the strong cooling observed after sunset at 19LT increases RH in the area of about 8%/h in the lower layer (below 1000m). The nighttime cooling shows strong cooling rates of about -1.4K/h after sunset till 22 LT, then rates decrease during the night to reach a value of about -0.3K/h between 22LT and 07LT. The cloud formation is mostly related to horizontal air advection, strong convergence in the lower layer and turbulent upwards mixing of moisture, while cooling at the cloud-top helps to maintain the cloud deck once it has formed. During daytime, solar radiation suppressed cooling at the cloud-top, thereafter strong turbulent kinetic energy acts to partly destroy the cloud deck and cloud fraction.