Abstract/Summary

The Eastern Africa precipitation seasonal cycle is of significant societal importance, and yet the current generation of coupled global climate models fails to correctly capture this seasonality. The use of convective parametrisation schemes is a known source of precipitation bias in such models. Recently, a high-resolution regional model was used to produce the first pan-African climate change simulation that explicitly models convection. Here, this is compared with a corresponding parametrised-convection simulation, to explore the effect of the parametrisation on representation of Eastern Africa precipitation seasonality. Both models capture current seasonality, although an overestimate in September-October in the parametrised simulation leads to an early bias in the onset of the boreal autumn short rains, associated with higher convective instability and near-surface moist static energy. This bias is removed in the explicit model. Under future climate change both models show the short rains getting later and wetter. For the boreal spring long rains, the explicit convection simulation shows the onset advancing but the parametrised simulation shows little change. Over Uganda and western Kenya both simulations show rainfall increases in the January-February dry season, and large increases in boreal summer and autumn rainfall, particularly in the explicit convection model, changing the shape of the seasonal cycle, with potential for pronounced socio-economic impacts. Interannual variability is similar in both models. Results imply that parameterisation of convection may be a source of uncertainty for projections of changes in seasonal timing from global models, and that potentially impactful changes in seasonality should be highlighted to users.