Numerical simulation of a mesoscale convective system over the east coast of South Africa

Weather stations over northern KwaZulu-Natal, South Africa, recorded over 100 mm of rainfall during the night of 11-12 February 2005. This heavy rainfall was associated with a mesoscale convective system (MCS) that was initiated from small convective storms beginning early in the afternoon on 11 February 2005. An analysis of Medium Range Forecast (MRF) model output suggests that a combination of synoptic features contributed to the evolution of the system. It is likely that the high-lying topography of the eastern escarpment and high diurnal surface heating provided the trigger for the event. Although MCSs are not uncommon in this region, very little work has been done on South African cases. The MCS is investigated with a non-hydrostatic numerical model (mesoscale model version 3; MM5) to help determine which processes were important in its initiation and development, as well as what factors contributed to the associated heavy rainfall. The model results suggest that the eastern escarpment played a key role in triggering the convective event, as well as influencing the low-level winds that advected moisture into the region. It was also apparent that the Agulhas Current played an important role in supplying moisture to fuel the extreme rainfall. The development of the MCS and the heavy nocturnal rainfall was due to a combination of the continuous moisture supply into the region, a conditionally unstable atmosphere, and uplift due to low-level convergence and the local topography.