Regional Climate Model Simulations Of Present-Day And Future Climates Of Southern Africa

The Hadley Centre Regional Climate Model (RCM), HadRM3H, is run over southern Africa with the aim of examining climate change scenarios for the future. The model has a ~50 km resolution and is forced at its lateral boundaries by a high resolution (~150 km) atmosphere-only GCM, HadAM3H. The present-day simulation with the RCM (1961-1990) is evaluated, including an examination of the impact of enhanced resolution and an identification of biases in the RCM climate. The RCM is able to resolve features on finer scales than those resolved by the GCM, particularly those related to improved resolution of the topography, such as its influence on surface air temperature and large-scale precipitation. The regional model, unlike the GCM, is also able to resolve tropical cyclones, which affect eastern tropical regions of southern Africa in summer. The hydrological cycle is stronger in the RCM, with consequent increases in the intensity of rainfall, in the magnitude of the moisture fluxes and in soil moisture compared to the driving GCM. The largest errors in temperature and precipitation in the RCM (and GCM) control climate occur in summer. There are positive biases in precipitation, and negative biases in surface air temperature over much of southern Africa in this season. These errors are due to errors in both the internal model physics and the lateral boundary conditions inherited from the GCM. An additional RCM experiment, where the model is forced by reanalysis data (i.e. quasi-observed) aids in the identification of the sources of these errors. A 30-year (2071-2100) RCM climate change experiment (using the A2 emissions scenario) demonstrates a mean surface air temperature increase of 3.7°C in summer and 4.0°C in winter over southern Africa by the 2080s. The regional model predicts a drying over much of the western subtropical subcontinent in summer, associated with a decrease in both the number of rain-days and the intensity of rainfall. In contrast, equatorial regions tend to become wetter, due here to an increase in rainfall intensity rather than a change in the number of rain-days.