The Role of Wind Shear in Organised Deep Moist Convection in the West African Monsoon

The West African Monsoon is characterised by mesoscale convective systems (MCSs) and a strong diurnal cycle in convection and wind shear. However, weather models struggle to provide skilful rainfall forecasts for the region, and climate projections are uncertain. Observational studies have linked recent decadal increases in extreme rainfall to stronger wind shear but this has not been reproduced in numerical models. Indeed, while wind shear is accepted to support organised convection, the underlying mechanisms are an active area of research. This thesis investigates the role of shear in diurnal and decadal trends in MCSs including examining the underlying mechanisms and theory.

The impact of environmental changes are investigated using an idealised model which allows thermodynamic effects to be isolated from those of shear. Greater shear led to increased MCS intensity and rainfall, but the effects of the thermodynamic changes dominated on both diurnal and decadal time scales. Our results explain the observed diurnal cycle, with the evening and night most favourable for MCSs, and are consistent with the decadal trend of increasing shear having enhanced storm severity.

A layer-lifting model correlated well with bulk ascent and rainfall rates of simulated MCSs. However, differences in microphysics and storm organisation are not accounted for and this is particularly clear when the thermodynamic profile is varied. Passive tracers reveal that inflowing air either descends in layers or is transported upward. Shear is shown to reduce entrainment dilution, but the effect is not captured well at coarse grid-spacings of 4 km.

This thesis begins to reconcile different theories for the role of wind shear in organised convection where system-relative inflows explain bulk properties while the cold pool to shear balance indicates storm organisation. Thus, to correctly capture MCSs in global weather and climate models, they must include the diverse impacts of shear across scales.