The Influence of the Diurnal Cycle in Wind Shear and Thermodynamics on Squall Lines in the West African Monsoon

The West African Monsoon has a clear diurnal cycle in boundary-layer properties, synoptic flow and moist convection. A nocturnal low-level jet (LLJ) brings cool, moist air into the continent and we hypothesise that it may support storms by providing vertical wind shear and a source of moisture. We use idealised simulations to investigate how the mean diurnal cycle in temperature and humidity compared with that of the wind shear impacts on mature squall lines. Thermodynamic diurnal changes dominate those of the winds, although when isolated the LLJ wind is favourable for more intense systems. Bulk characteristics of the storms, including in-cloud upward mass flux and — if precipitation evaporation is accounted for — total surface rain-rates, correlate well with the system-relative inflow of convectively unstable air and moisture into the storms. Mean updraft speeds and mean rainfall rates over the storms do not correlate as well with system-relative inflows due to variations in storm morphology such as cold-pool intensity. We note that storms tend to move near the speed of the African Easterly Jet and so maximise the inflow of convectively unstable air. Our results explain the observed diurnal cycle in organised moist convection, with the hours from 1800 to 0000 UTC being the most favourable. Storms are more likely to die after this, despite the LLJ supporting them, with the environment becoming more favourable again by midday.