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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-34
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-34
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  03 Aug 2020

03 Aug 2020

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A revised version of this preprint is currently under review for the journal WCD.

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

Lesetja E. Lekoloane1,2, Mary-Jane M. Bopape1, Gift Rambuwani1, Stephanie Landman1, Puseletso Mofokeng1,3, Morne Gijben1, and Ngwako Mohale1 Lesetja E. Lekoloane et al.
  • 1South African Weather Service, Pretoria, 0001, South Africa
  • 2Global Change Institute, University of the Witwatersrand, Johannesburg, 2050, South Africa
  • 3School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Johannesburg, 2050, South Africa

Abstract. On 11 December 2017, a tornadic supercell initiated and tracked through the northern Highveld region of South Africa for 7 hours. A tornado from this supercell led to extensive damage to infrastructure and caused injury 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 see how they performed in predicting this tornadic supercell. It was found that this supercell initiated as part of a cluster of multicellular thunderstorms over a dryline, with three ingredients being important in strengthening and maintaining it for 7 hours: significant surface to mid-level vertical shear, an abundance of low-level warm moisture influx from the tropics and Mozambique Channel, and the relatively dry mid-levels. It was also found that the 4.4 km grid spacing model (SA4.4) performed better than that of 1.5 km grid spacing (SA1.5). SA1.5 underestimated mid-level vorticity due to a significant underestimation in low-level warm moisture advection and convergence. SA4.4 captured the supercell but underestimated its severity due to an underestimation in mid-level vorticity found to be one order of magnitude smaller than that of a typical mesocyclone. This was a result of underestimation in 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.

Lesetja E. Lekoloane et al.

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Lesetja E. Lekoloane et al.

Lesetja E. Lekoloane et al.

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Latest update: 03 Dec 2020
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Short summary
We analysed a tornadic supercell that tracked through the northern Highveld region of South Africa for 7 hours. We found that atmospheric conditions were conducive for tornado-associated severe storms over the region. A 4.4 km resolution model run by the South African Weather Service was able to predict this supercell, including its timing. However, it underestimated its severity due to underestimations of other important factors necessary for real-world development of these kinds of storms.
We analysed a tornadic supercell that tracked through the northern Highveld region of South...
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