Irene weather office sounding data are considered a proximity sounding for the Gauteng province of South Africa. Sounding-derived parameters are analysed for 35 austral summers from 1977 to 2012. The goal of this study is to provide a climatology of sounding-derived parameters commonly used as ingredients to forecast heavy rainfall. The emphasis is placed on identifying those variables that distinguish between climatology and heavy rainfall events. Special attention is given to how the critical values associated with heavy rainfall change from early to late summer.
During early summer (October to December), the atmospheric circulation over Gauteng is markedly extra-tropical in nature. Heavy rainfall occurs in a conditionally unstable atmosphere and is associated with conditions conducive to the development of severe storms, such as large wind shear and convective available potential energy (CAPE) values, strong upper tropospheric winds and large temperature lapse rates. In late summer (January to March), the atmosphere takes on distinct tropical characteristics and becomes increasingly convectively unstable. During this time of year there is abundant moisture in circulation and the storms that develop are highly efficient in producing precipitation. Forecasting heavy rainfall in early summer requires different techniques than in late summer.
Sounding parameters, which provide information about the moisture content of the atmosphere, are capable of distinguishing between climatology and heavy rainfall during all summer months. The only other variables capable of doing this are the average meridionial wind direction in the 800 to 600 hPa layer, the mean layer equivalent potential temperature, the Showalter Index (SI), the K-index (KI) and the elevated K-index (EKI). However, critical values associated with heavy rainfall for all these parameters change month by month.