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-level cloud cover (LLC) and its atmospheric drivers over Western Central Africa (WCA) 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 (ERA5) are used. LLC peaks between 04 LT and 07 LT and tends to be less extensive during the afternoon. 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 during the night: the strong cooling observed after sunset increases RH in WCA by about 8%/h in the layer below 1000 m. Strong cooling rates of about −1.4 K/h occur indeed from sunset until 22 LT, then decrease then after to reach a value of about −0.3 K/h at 07 LT. Cloud formation is mostly related to horizontal moisture flux, strong convergence in the lower layer and turbulent upward mixing of moisture. The reinforcement of the radiative cooling at the cloud top helps to maintain the cloud deck once it has formed. During daytime, latent and sensible heat flux from the surface, due to downward shortwave radiation, contributes to increase the vertical turbulence and deepen the boundary layer and thus, combined with decreasing RH in low levels ∼1000 m mostly due to the diurnal warming, act to partly destroy the cloud deck and decrease the cloud fraction.