A dynamic and thermodynamic analysis of the 11 December 2017 tornadic supercell in the Highveld of South Africa

Lesetja E Lekoloane,Morne Gijben,Puseletso Mofokeng,Tshifhiwa Gift Rambuwani,Ngwako Mohale,Mary-Jane M Bopape,Thando Ndarana,Stephanie Landman

doi:10.5194/wcd-2-373-2021

Abstract

Abstract. On 11 December 2017, a tornadic supercell initiated and moved through the northern Highveld region of South Africa for 7 h. A tornado from this supercell led to extensive damage to infrastructure and caused injury to and displacement of over 1000 people in Vaal Marina, a town located in the extreme south of the Gauteng Province. In this study we conducted an analysis in order to understand the conditions that led to the severity of this supercell, including the formation of a tornado. The dynamics and thermodynamics of two configurations of the Unified Model (UM) were also analysed to assess their performance in predicting this tornadic supercell. It was found that this supercell initiated as part of a cluster of multicellular thunderstorms over a dry line, with three ingredients being important in strengthening and maintaining it for 7 h: significant surface to mid-level vertical shear, an abundance of low-level warm moisture influx from the tropics and Mozambique Channel, and steep mid-level lapse rates. It was also found that the 4.4 km grid spacing configuration of the model (SA4.4) performed better than the 1.5 km grid spacing version. SA1.5 underestimated the low-level warm moisture advection and convergence, and missed the storm initiation. SA4.4 captured the supercell; however, the mid-level vorticity was found to be 1 order of magnitude smaller than that of a typical mesocyclone. A grid length of 4.4 km is too coarse to fully capture the details of a mesocyclone, which may also explain why the model underestimated the surface to mid-level wind shear and low-level horizontal mass and moisture flux convergence. Future investigations will involve experimental research over the Highveld region of South Africa to understand mesoscale and local dynamics processes responsible for tornadogenesis in some severe storms. Such a study, to the best of our knowledge, has never been conducted.

Highlights

Human survival and development are directly and indirectly impacted by weather and climate extremes (Nuttall, 1998; IPCC, 2014)
The dynamics and thermodynamics of two configurations of a numerical weather prediction (NWP) model operationally run at the South African Weather Service (SAWS) are analysed and evaluated to see how they performed in predicting this tornadic supercell
On 11 December 2017, the analysis indicates the presence of a ridging anticyclone which extended north of the south-eastern subcontinent and in the process of breaking off from the parent Atlantic anticyclone to merge with the south Indian Ocean anticyclone located east of South Africa

Summary

Introduction

Human survival and development are directly and indirectly impacted by weather and climate extremes (Nuttall, 1998; IPCC, 2014) In their different spatial and temporal scales and reach, extreme weather events can result in prolonged negative socio-economic impacts and affect different facets of the society, including ecosystems (Meehl et al, 2000). Examples of these extremes include powerful synoptic tropical systems such as Tropical Cyclone Eline (Reason and Keibel, 2004) and Tropical Cyclone Idai (WMO, 2019) that occurred in the years 2000 and 2019, respectively. There is growing evidence suggesting that these extreme events are becoming more common across the globe and can be expected to in-

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