P26 Using WRF to study the development of tropical temperate troughs over southern Africa

Viguad, Nicolas, International Research Institute for Climate and Society, Benjamin Pohl, and Julien Cretat, CRC University of Bergundy, France

The Weather Research and Forecasting model (WRF) forced by ERA40 re-analyses, is used to examine, at regional scale, the role of key features of the local atmospheric circulation on the origin and development of Tropical Temperate Troughs (TTTs) representing a major contribution to South African rainfall during austral summer. A cluster analysis applied on 1971-2000 ERA40 and WRF simulated daily Outgoing Longwave Radiation (OLR) reveals for the November to February season three coherent regimes characteristic of TTTs over the region. Ensemble experiments generated using perturbed initial conditions were designed for the case of austral summer 1996/1997 to examine the reproducibility of TTT events. Three types of simulations are then examined: (i) reference experiments with no nudging and no oceanic mixed layer suggest, by comparison to simulations with spectral nudging, the importance of westerly waves and their phasing regarding the development of rain-producing continental TTT events, (ii) experiments with an oceanic mixed layer (OML) on the other hand helped identifying the influence of oceanic surface conditions within the Agulhas current region of the South West Indian Ocean (SWIO) regarding TTTs persistence, warmer sea surface temperatures being associated with increased moisture advection from the SWIO where latent heat release is enhanced, favoring baroclinic instability and thus sustaining convection activity locally.