The internal variability of the RegCM3 over South Africa

The aim of this study is to investigate the internal rainfall variability of a nested model system and of a regional climate model (RCM). Four solutions obtained through perturbing the wind fields at initialization for the ECHAM4.5 atmospheric general circulation model (AGCM) are used to force the RegCM3 RCM over South Africa. To determine the amount of variability introduced by non-linearities in an RCM, four additional RegCM3 simulations are made through initializing the RegCM3 on different days but using a single realization from the ECHAM4.5. The simulations are made for one dry and one wet, El Nino-Southern Oscillation (ENSO) associated summer season defined as December to February. The rainfall variability associated with non-linearities in the nested system is high to an extent that ensemble members produce anomalies that have opposite signs in the same season. However, the sign of the ensemble average anomaly generally corresponds with the observed anomaly. The internal rainfall variability of the RCM is small when seasonal totals are analysed while with the daily rainfall totals the variability is larger. The number of events that fall into the three rainfall categories (i.e. below-normal, normal and above-normal) for the RegCM3 ensemble members are close to one another however the timing of the events is different. The results suggest that in seasonal operational forecasting making ensemble members associated with the internal variability of an RCM is not necessary because the information obtained from the ensemble members is almost similar. Copyright (C) 2007 Royal Meteorological Society.