Insights from the first convection-permitting simulations of climate change over Africa

Abstract:

Deep moist convection is a basic building block of tropical climate, yet is poorly represented in global models. In recent years convection-permitting simulations, run over large domains for many years, have shown how this biases projections of changes in extremes in the mid-latitudes. Here, we will present some of the first results from the “CP4A simulations”: convection-permitting Unified Model simulations run for 10 years current climate and 10 years future climate over the whole of Africa, some of the first simulations of this type to be run in the tropics, and the first for Africa. Results show that the explicit representation of convection gives a larger change in extreme rainfall, and dry spells, than in a parameterized model, and improves the representation of such events in current climate. In addition, in this tropical environment effects of the improved convection also improves the large-scale mean state and changes the climate change in that state, in particular giving a greater slowdown in the modeled Hadley circulation. Analysis over West Africa shows the convection-permitting model responds to the climate change increase in vertical wind-shear, which results from the Sahara warming faster than the Gulf of Guinea, with the stronger shear giving more intense mesoscale convective systems. Analysis over East Africa reveals how explicitly modelling the convection fundamentally changes its couplings with mesoscale circulations, such as the sea-breeze, affecting projections of climate change in rainfall and extremes. The results demonstrate the value of such computationally expensive runs and the implications for future regional and global climate and impacts projections will be discussed.