Ridging South Atlantic high pressure systems (ridging highs) are often accompanied by cut-off low (COL) pressure systems aloft, but may also occur without them, in which case a linear baroclinic wave would be observed propagating across the South African domain in the upper troposphere. Using 41 years of ERA-5 reanalysis data, this study documents differences between the characteristics of the prevailing dynamical processes and associated local eddy kinetic energy generation, its downstream transfer and dissipation during these two scenarios. The study shows that when COLs are present then baroclinic conversion is strong and it is confined east of the Greenwich Meridian, whereas it is located downstream of South Africa and it is much weaker, when ridging occurs without COLs. The differences in strength and locations of the baroclinic conversion are associated with the differing jet streak configurations between the two scenarios; which lead to Rossby wave breaking and the absence thereof when there are COLs and when ridging occurs without COLs, respectively. The presence of breaking during COLs leads to trans-ridge downstream development that facilitates energy transfer from the midlatitudes into the South African domain. When there are no COLs present, the trans-trough downstream development is stronger than it is across the upstream ridge. Barotropic conversion from eddy kinetic energy to mean kinetic energy occurs in the South African domain during COLs, but occurs much further downstream when there are no COLs during ridging highs. The difference in the characteristics identified in this study can be traced back to the differences in the potential vorticity anomaly structures, which are largely due to whether the waves break or do not during the evolution of ridging events in the South African domain.

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.

Thank you so much for taking the time to review and give feedback to our manuscript, which has led to an improved one. In the revised manuscript, we address all the major and minor comments the reviewer made, including those raised regarding tornadogenesis and model analysis. In the attached supplement we make a point-by-point response to the reviewer's comments.

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.