A modified Supercell Composite Parameter for supercell thunderstorms over the Gauteng Province, South Africa

Abstract

South Africa is a country prone to frequent outbreaks of thunderstorms which are often of a severe nature. Supercell thunderstorms are a particular class of thunderstorm which are typically long-lived and are associated with severe thunderstorm phenomena in as many as 9 out of 10 instances. In particular, the southern African moist Highveld climatological region is known to experience one of the highest rates of occurrences of thunderstorms and lightning within southern Africa and indeed the world. The domain for this Highveld-based study chiefly encompassed the industrialised metropolis of the Gauteng province. The population and infrastructure of this province are vulnerable to a variety of adverse effects relating to severe thunderstorms, comprising one or more of strong damaging winds, large hail, urban flooding or even tornadoes of varying intensity. In this study a sample of 15 supercell (SUP) proximity soundings of upper air data for Irene, Gauteng was compared against a large, independant sample of 510 Irene proximity soundings for austral summers during the period 2007 to 2011, representing non-supercell (NON-SUP) thunderstorm days. Hypothesis testing as well as box and whisker representations of the SUP versus NON-SUP samples were applied to various thunderstorm parameters to determine which SUP parameters exhibit the greatest statistical departure from their NON-SUP counterparts. Selected quartiles of SUP parameters so identified were further utilised to formulate a Modified Supercell Composite Parameter (MSCP) tuned to Gauteng supercells. MSCP as well as formulations of the Supercell Composite Parameter (SCP) were subsequently applied to five case-study events where some of the events were associated with observed supercells and some not.

The results strongly indicated that, in a short-term forecast context, MSCP has useful discriminatory ability to provide quantitative predictive guidance as to the relative likelihood of the development of supercell thunderstorms in the Gauteng area. Prior to this research, neither the SCP nor the MSCP were in operational use in South Africa. To the best of the author’s knowledge this work is the first of its kind in southern Africa, especially with consideration to the unique high-altitude Highveld domain of the Gauteng province. In the light of the research results presented herein, it is proposed that MSCP be jointly implemented on point-based (upper air sounding analysis) platforms as well as that of gridpoint-based (deterministic NWP) platforms for short-term predictions of supercell-type thunderstorm conditions in Gauteng province.