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.