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

On 11 December 2017, a tornadic supercell initiated and moved 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 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 5 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 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 configuration of the model (SA4.4) performed better than that of 1.5 km grid spacing (SA1.5) version of it. SA1.5 underestimated the low-level 10 warm moisture advection and convergence, and missed the storm. SA4.4 captured the supercell however the mid-level vorticity was found to be one order of magnitude smaller than that of a typical mesocyclone. A grid length of 4.4km is also too coarse to capture the details of a mesocyclone. The SA4.4 also 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 15 storms. Such a study, to the best of our knowledge, has never been conducted.


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 socioeconomic impacts and affect different facets of the society including ecosystems (Meehl et al., 2000). Examples of these 20 extremes include powerful synoptic tropical systems such as Tropical Cyclones Eline (Reason and Keibel, 2004) and Idai 2 (WMO, 2019) that occurred in the years 2000 and 2019, respectively. These severe storms affected millions of people in southern African mainland. Extreme weather events have been observed in other parts of the world, for instance the unusually extended tornado outbreak that occured across the central and eastern United States during May 2003 which resulted in about $829 million of property damage (Hamill et al., 2005). Therefore, there is growing evidence that suggests that more of these 25 extreme events are becoming more common across the globe, and can be expected to increase in intensity and frequency due to climate change (e.g. Handmer et al., 2012;Melillo et al., 2014;EASAC, 2018).
There is no region on Earth immune to extreme or severe weather events, but the most vulnerable communities are especially those from developing countries (Mirza, 2003). For example, many developing countries' economies and livelihoods are highly depended on primary sectors of the economy (UN, 2018a) such as agriculture and fishing of which 30 negative effects have already been experienced due to an increase in high impact weather events (Adger et al., 2003;Awojobi and Tetteh, 2017). Therefore, an understanding of these high impact weather events, and being able to predict them precisely, is of importance for sustainable development as well as mitigation and adaptation strategies.
One of the regions identified as highly vulnerable to climate change and its effects is sub-Saharan Africa (Boko et al., 2007). Most countries in this region of the world are lacking behind in the development scale and have many stressors including 35 high impact weather events (UN, 2018b). Although no single weather event can be confidently attributed to climate change, an increasing number of studies are documenting a positive trend of severe weather events in southern Africa (Davis-Reddy and Vincent, 2017;Kruger and Nxumalo, 2017). These include an increase in the frequency of extreme rainfall events associated with land-falling tropical cyclones and severe thunderstorms, particularly in the eastern parts of southern Africa and south of South Africa (Malherbe et al., 2013;Engelbrecht et al., 2013). 40 A climatological analysis over South Africa indicates an increase in extreme rainfall events, especially during spring and summer months DEA, 2016) mostly in the form of convective precipitation (Tyson and Preston-Whyte, 2000;Hart et al., 2013). The eastern parts of South Africa are dominated by thunderstorms during summer months which often result in fatalities, serious injuries, and extensive damage to infrastructure including crops and livestock (Gijben, 2012;NDMC, 2018). 45 Thunderstorms are primarily classified either as single cell, multicell or supercellall three types could become very severe and destructive, with supercell thunderstorms notoriously known for producing spectacular tornadoes (Houze, 1993).
It should be noted that some severe forms of multicell thunderstorms, and non-severe storms, can also produce tornadoes (and non-mesocyclonic tornadoes), and that not all supercell thunderstorms produce tornadoes (Markowski and Richardson, 2009).
Tornadoes are common around the world, but the most severe are often reported in the United States of America (e.g. Hamill 50 et al., 2005).
South Africa experiences a significant number of severe thunderstorms annually, especially during summer months over the Highveld (Gijben, 2012). A few of these intensify to severe supercell type and often result in strong damaging winds, large hail and tornadoes (Rae, 2014;Glickman, 2000). Tornadoes can occur anywhere in South Africa, but they are frequently 3 reported in the Highveld regions of Gauteng, north-eastern Free State, western Mpumalanga, and south-central KwaZulu Natal 55 (Goliger and Retief, 2007).
Research has shown that the possible two main important factors for the initiation of tornadoes in thunderstorms are: large amounts of low-level water vapour and the presence of boundary layer vertical wind shear (Markowski and Richardson, 2009). A recent study by Dahl (2017) has shown that the tilting of horizontal vorticity alone is not enough to produce a rotating updraft in supercells. For the tilting of horizontal vorticity, a downdraft is needed possibly at the rear-flank of the storm to pair 60 the updraft in the inflow side of the storm (Davies-Jones et al., 2001). However, this downdraft is not necessary if near-surface vertical vorticity pre-existed before the storm genesis, in which case near surface convergence alone is enough for tornadogenesis (Markowski and Richardson, 2009). Davies-Jones (2015) has summarised tornadogenesis in supercells as occurring in three stages: the formation of mesocyclone in the mid-level, rotation at the ground, and tornado formation or failure. 65 On 11 December 2017, a supercell thunderstorm associated with large hail, strong damaging winds and a tornado tracked through Vaal Marina, a town in the extreme south of the Gauteng Province of South Africa. The tornadic supercell led to trees being uprooted, destroyed commercial and residential properties, and left about 50 people injured, including several animals, and over 1100 informal settlement dwellers displaced (SAWS, 2018). The aim of this study is to perform an analysis of this supercell in order to understand the conditions that led to its severity including the formation of a tornado in Vaal Marina. The 70 dynamics and thermodynamics of two configurations of a numerical weather prediction (NWP) model operationally run at the South African Weather Service (SAWS) are also analysed and evaluated to see how they performed in predicting this tornadic supercell.
NWP has advanced over the years (Bauer et al., 2015), with current NWP models operating in the convection-permitting scale where convection schemes are switched off or restricted (Clark et al., 2016). A numerical modelling study by Weisman 75 et al. (1997) found that a 4 km model grid spacing may be sufficient to reproduce mesoconvective circulations and net momentum and heat transport of midlatitude type convective systems. With this understanding the super-parametrization modelling procedure was developed with most of the embedded cloud resolving models running with a grid length of 4 km (Randall et al., 2003;Randall et al., 2016). Other studies, however, argue that models of grid spacing of 1 km or less are the ones adequate to represent dynamics and local processes responsible for triggering convection (see Roberts, 2008, andBryan 80 et al., 2003). In this study, we consider a model run with a grid length of 4.4km and 1.5km.

Area of interest
The focus area of this study is in the northern parts of the Highveld region of South Africa, and covers Vaal Marina located in 85 the extreme south of the Gauteng Province, and the storm track in the Free State and Mpumalanga provinces as shown in Fig.   4 1. The locations of SAWS radars used to track the storm of interest and weather stations where data were analysed for mesoscale circulations, in this study, are also shown. The storm track as shown in Fig. 1 was identified and reproduced using the Thunderstorm Identification, Tracking, Analysis and Nowcasting (TITAN) and QGIS softwares to mark geographic information system (GPS) locations of the cell for each radar scan (for both radars in Irene and Ermelo, individual and merged 90 for validation), from initiation to dissipation of the storm.  The primary model data analysed in this study are from the Unified Model (UM) (version 10.4). The UM is a seamless and deterministic numerical modelling system originally developed by the United Kingdom's Meteorological Office (UKMO) for weather prediction and climate studies (Davies et al., 2005). This model is operationally run four times a day on the SAWS CRAY XC-30 high performance computing system. Initial and boundary conditions from the Global Atmosphere (GA, Walters et al., 2017) are used as the driving data for the purpose of producing short-range operational weather forecasts of up to 3 days 100 ahead.
The UM is run in two configurations of the regional and sub-regional domains with 70 vertical levels. The first configuration runs at a horizontal grid length of 4.4 km (this configuration is hereafter referred to as SA4.4) and covers the domain from 0° S to 36° S and 5° E to 54° E. This setup essentially covers the Southern African Development Community (SADC) except the northern parts of Democratic Republic of Congo. The second configuration is centred over South Africa 105 from 20° S to 36° S and 15° E to 34° E and is run at a horizontal grid length of 1.5 km (this configuration is hereafter referred to as SA1.5). The model configurations analysed in this study were both operationally initialised and run at 00:00 UTC on 11 December 2017.
The dynamical core of the model configurations in this study uses a semi-implicit semi-Lagrangian formulation to solve the non-hydrostatic, fully-compressible deep-atmosphere equations of motion (Wood et al., 2014). On the other hand, physical 110 parameterisations of sub-grid processes are split into slow processes (radiation and microphysics) and fast processes (atmospheric boundary layer turbulence, cloud and surface coupling) which are treated and computed differently.

Radiosonde, satellite, station and radar data
To perform an analysis of the thunderstorm and its proximal environment during its lifecycle, surface weather station data and radar data are obtained from the SAWS weather station and radar networks. Surface observation data from a total of up to 228 115 automatic weather stations across South Africa, were regridded and used for synoptic analysis. The regridding is done using the Cressman objective analysis scheme which is described in detail by Cressman (1959). The regridded surface data points are 25 km apart. Only a selected number of stations proximally closer to the storm track in the Free State, Gauteng and Mpumalanga provinces were used for mesoanalysis (see Fig. 1). The surface stations data used contain temperature, dewpoint temperature, pressure, wind speed, wind direction, and rainfall. A surface chart analysis was also performed using data from 120 these stations.
To capture data from middle to lower levels of the storm, the Irene (25.87° S, 28.22° E) and Ermelo (26.53° S, 30.03° E) weather radars are used to track the storm throughout its cycle, from initiation to dissipation (see Fig. 1). We also include the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (GPM) (IMERG) for precipitation observation (Huffman et al., 2019). 125 6 A 1200 UTC Irene weather office's Radiosonde sounding, on the day considered, is used as a proximity sounding of the atmospheric conditions at Vaal Marina. The Irene weather office is located about 110 km north of Vaal Marina and is therefore within the proximity sounding criteria for the location as defined by Craven and Brooks (2004). Radiosonde sounding from the Bram Fischer International Airport (FABL) in Bloemfontein was also used to capture upstreams winds. 130

Reanalysis data and synoptic analysis
A fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis of the atmosphere (ERA5) on single levels and pressure levels, hourly data are utilised to analyse meteorological conditions of the day. The ERA5 dataset is the state-of-the-art atmospheric reanalysis of the global climate with a horizontal grid length of 0.25°×0.25° and 137 vertical levels up to 1 Pa (Hersbach and Dee, 2016). Parameters analysed from this dataset include 10 m winds, mean sea level pressure, 135 convective and non-convective precipitation, and geopotential heights at 850 -, 500, and 300 hPa levels. A surface chart analysis perfomed by operational forecasters at the SAWS for 12 UTC 11 December 2017 is also used in the synopic analysis and mesoanalysis. \

Dynamic and thermodynamic analysis
In this study, both the 4.4 km and 1.5 km horizontal grid spacing of the model configurations are not high enough to reliably use only wind vectors as a determinant of organised mesoscale rotation. As an alternative, vorticity is used as a measure of 140 rotation.
Vorticity is a curl of velocity and gives the measure of a fluid's infinitesimal rotation (Holton and Hakim, 2013). Ignoring the effects of friction, the Boussinesq approximation of the vorticity equation can be written as, Where ̅ = ( , , ) and ̅ = ( , , ) respectively represents vorticity and velocity vectors in the horizontal (x-,y-axis) and 145 vertical (z-axis), and B is the buoyancy.
It is not uncommon for thunderstorms to rotate, with supercell thunderstorms spectacularly distinguished by an even stronger and persistent vertically rotating updraft (Doswell and Burgess, 1998;Houze, 1993). Consequently, the current study utilises the vertical component of relative vorticity ζ as a diagnostic tool of rotation in the vertical axis. It can be derived from Eq. 1 to get, 150 It should be noted from Eq. 2 that ̂• ∇ × (̂) = 0. This implies that buoyancy is not directly responsible for the generation of vertical vorticity and therefore vertical rotation in thunderstorms. Further simplifying Eq. 2 gives, where the terms on the right-hand side of the equation respectively represents the advection, tilting and stretching terms. 155 In this study our dynamical definition of a mesocyclone follows that of Glickman (2000), which defines it as a cyclonically rotating vortex in a thunderstorm associated with vorticity on the order of 10 −2 −1 or greater. The horizontal scale of this vortex is normally somewhere between 2 to 10 km in a thunderstorm.
To analyse the significance of low-level moisture in the event considered, the convergence of moisture flux is integrated from the surface up to 600 hPa level. Moisture flux convergence (MFC) is a useful diagnostic tool for convection initiation as 160 it combines the effects of moisture advection and convergence and can be computed at any atmospheric pressure level (Banacos and Schultz, 2005). The reason we integrate from the surface to 600 hPa, is because we want to capture the significance of the convergence of low-level moisture fluxes. Ndarana et al. (2020) argues that the flow associated with ridging South Atlantic anticyclones, a low-level synoptic driver in our study, changes completely beyond 600 hPa to become sinusoidal and westerly.
To derive an equation for MFC, the conservation of water vapour is used and is further expanded and written in flux form 165 using the continuity equation to get: where q is the specific humidity and the second and third terms respectively represent horizontal and vertical moisture flux divergence. S represents the sources and sinks of the air parcel's water vapour evaporation rate (E) and precipitation rate (P), assuming that all condensed water precipitates immediately. MFC is computed by taking the negative of the horizontal 170 moisture flux divergence in Eq. 4, which gives, MFC is then integrated from the surface ( ) to 600 hPa and becomes, Media reports indicated that on the late afternoon of 11 December 2017, a severe thunderstorm which resulted in extensive damage, tracked through the extreme south of the Gauteng Province of South Africa (Mitchley, 2017). Some eyewitnesses reported seeing a tornado close to Deneysville near the Vaal Dam, which later made its way to Vaal Marina and Mamello informal settlement (all these locations are hereafter collectively referred to as VAM) (Mashaba and ANA reporter, 2017).

8
Witnesses also reported strong, damaging winds and large hail (SABC, 2017). Visual impacts of the observed severe storm 180 were also captured by some reports, and indicate uprooted trees, damaged houses and shacks, including damaged infrastructure such as overhead power lines (Storm Report SA, 2017). From these vast reports, it is clear that this was a high impact weather system, and therefore important to analyse.

Synoptic setting and meteorological conditions of 11 December 2017
On 11 December 2017, analysis indicates the presence of a ridging anticyclone which extended north of the south-eastern 185 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 (Indicated by letter H over South-West Indian Ocean, SWIO; Fig. 2(d)). The Atlantic anticyclone began ridging behind a cold front which moved over south of the continent between 08 and 09 December and was The analysis also indicates the presence of a mid-tropospheric trough over the south-east of the continent and a ridge extending over Namibia and the western parts of South Africa (Fig. 2(b)). This mid-level circulation pattern resulted in midatmospheric south-westerly winds over the interior of South Africa, which enhanced convective instability over the central-200 east of the country as the dry mid-level air advects over the low-level warm and moist-air in the east. A weak upper-air trough was also present over South Africa (Fig. 2(a)).
These synoptic settings and conditions (as indicated in Fig. 2) resulted in the western-half of South Africa being hot and dry, while the eastern parts were cool and moist with low-level clouds present. The 12:00 UTC surface dew-point temperature analysis for 11 December 2017 conducted by operational forecasters at the SAWS, indicated the presence of a dryline over the 205 10 central Free State Province, which extended through the south-western areas of the North West Province and south-western parts of Botswana (Fig. 3). The dryline separated the hot dry air in the west from the warm moist air in the east, and was moving eastwards after sunrise as differential heating allowed vertical mixing to entrain dry air from the west into the moist boundary layer to the east of the dryline. This dryline was responsible for initiating several severe convective storms in the Free State Province, from which one intensified into a supercell thunderstorm that lasted for 7 hours and resulted in the 210 observed VAM tornado.

Mesoanalysis 215
Surface station data along near thunderstorm track were used for mesoanalysis. Data analysis from radar and four weather stations close to the location of the storm initiation (Potchefstroom, Soweto, Klerksdorp and Kroonstad) indicates that the storm of interest initiated after 13:00 UTC over a dryline. Fig. 4 and 5 indicate that along the dryline, there was a convergence of moist northerly winds and relatively dry westerly winds. Throughout the analysis period (from 10:00 to 22:00 UTC) Potchefstroom and Soweto recorded an advection of moist air from northerly winds, which respectively averaged 25 and 17 220 kts between 13:00 and 14:00 UTC (Fig. 4 and Fig. 5(a) and 5(b)). During the same analysis period, stations at Klerksdorp and Kroonstad reported an advection of warm moist air from north-westerly winds which eventually became hot and dry and changed to westerly after a passage of the dryline (Fig. 4 and Fig. 5(c) and 5(d)). The average hot and relatively dry winds in those stations between 13:00 and 14:00 UTC was 33 kts, with Kroonstad respectively recording 43 and 33 kts. After initiation, the storm initially propagated eastwards, then suddenly changed direction to north-east towards the Vaal Dam as it matured into a supercell thunderstorm through energy supply from a continual merger of several cells (see Fig. 1).
As the supercell approached the Vaal Dam, it intensified further and was now associated with large hail and strong damaging winds, with the first sighting of a tornado near Deneysville (town in the north-western edge of the Vaal Dam). The now tornadic 230 supercell then moved across the Vaal Dam and reached Vaal Marina (town in the north-eastern edge of the Vaal Dam), in the extreme south of the Gauteng Province. In Vaal Marina (and Mamello informal settlement), the supercell was still associated with large hail, strong damaging winds, and a tornado touchdown track of about 1.5 km (1500 m) was confirmed from site visit (and by confirming this in situ data with radar and satellite data).
It might be significant to note that Frankfort station located a little over 50 km south-south-east of VAM (see Fig. 1) 235 reported cool and moist southerly to south-easterly winds almost throughout the day (Fig. 5(e)) while stations north of VAM reported cool to warm and moist northerly to north-westerly winds. There is not enough surface data in the vicinity and south of VAM to know whether Frankfort wind speeds and directions generally represent mesoscale circulation at locations south and proximally closer to VAM since it is the only station available around that area. If it does represent mesoscale circulation south of VAM, then it is more likely that there was convergence of cool and moist air close to VAM before tornadogenesis 240 occurred between Deneysville and Vaal Marina. The 12:00 UTC Irene upper-air sounding (Fig. 6) captured north-north-easterly cool and moist surface winds of 10 kts 245 which back with height (turns counter-clockwise with height, comparative to veering in the Northern Hemisphere) to westsouth-westerly at 500 hPa to become 25 kts. The Bram Fischer International Airport (FABL) sounding indicates much greater mid-level winds of 45 kts upstream. The FAIR and FABL upper-air soundings in Fig. 6 confirm winds captured by the synoptic analysis in Fig. 2 close to the soundings vicinity. This indicates significant directional shearing between the surface and the 14 mid-troposphere, while the speed shearing was respectively weak in FAIR (15 kts at 500 hPa) to moderate (40 kts at 300 hPa). 250 FABL hodograph indicates unidirectional shearing, while that of FAIR is curved and is a common signature for classic supercell development. The FAIR sounding also indicates a 35 kts lower mid-level jet and an upper-level west-north-westerly jet of 65 kts, which also confirm the synoptic analysis in Fig. 2. The lower levels were moist with an average relative humidity of 91% between the surface and 625 hPa, while the mid-levels were dry with relative humidity averaging 26% between 573 hPa and 400 hPa. Therefore, the availability of low-level moisture, dry mid-levels, and vertical wind shearing, provided 255 favourable conditions for the possibility of dynamically and thermodynamically induced organised severe thunderstorms (including supercell type) developing in the vicinity. This analysis indicates that three ingredients important for strengthening and maintaining typical supercell thunderstorms were present, and can be summarised for this case as follows: 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 which enhanced convective instability as the dry mid-level air advects over low-260 level warm and moist-air. A hook echo can clearly be seen from the low-levels of the storm between 15:16 UTC and 15:34 UTC as observed by the first sweep of the Irene radar (at an elevation angle of 0.5°) (Fig. 7). Figure 7(a) indicates a hook echo west of the Vaal 265 Dam at 15:16 UTC which was associated with local strong cyclonically rotating winds as indicated by the radial velocity 15 inbound-outbound maxima in Fig. 7(b) (this could explain the tornado observed near Deneysville). A hook echo was also observed 12 minutes later at 15:28 UTC over Mamello, an informal settlement located in the eastern outskirts of Vaal Marina ( Fig. 7(e)). Figure 7 (f) indicates that this hook echo over Mamello was also associated with local strong circulating winds. This analysis indicates that it is most likely that the tornado that was reported to have devastated VAM touched down multiple 270 times between 15:15 and 15:30 UTC (17:15 and 17:30 local time); but could also have been on the ground during this entire period or for a period longer than the mesocyclone that was captured by the radar. 16 Figure 7: Irene radar equivalent reflectivity factor (dBZ) and radial velocity of scatterers (m/s) of the supercell thunderstorm as plotted using Python ARM Radar Toolkit (Py-ART) (Helmus and Collis, 2016). The left-hand column indicates low-level reflectivity as captured by the first radar sweep at an elevation angle of 0.5° for every 6 minutes, between 15:16 UTC and 15:34 UTC. The right-hand column radar scans have similar properties as the left-hand column radar scans but for the radial velocity of scatterers.
After passing through the VAM area, the supercell continued in the north-easterly direction, weakening and strengthening 275 throughout, before dissipating just before reaching Machadodorp in Mpumalanga Province. Throughout near storm track (except for Frankfort), surface winds were variable between northerly and north-westerly (Fig. 5). The supercell dissipated on approach to Machadodorp where winds were variable between north-easterly to easterly between 19:00 and 20:00 UTC (21:00 and 22:00 local time) and averaged 8 kts.

Unified Model analysis 280
The twenty-four hour rainfall total is shown in Fig. 8 for both SA1.5 and SA4.4, as well as the ERA5 reanalyses and IMERG which represents observations. Both the ERA5 and IMERG captured rainfall in the area where the storm passed, however there are differences in the two datasets. For example, IMERG indicates a large amount of rainfall in the north-east border of the plot. The ERA5 shows large amount of rainfall only in the south-east of the domain shown. The SA4.4 simulated a large amount of rainfall in different parts of the domain, while the SA1.5 only simulated a large amount of rainfall in the south-east 285 border of the domain. We also investigated the timing of rainfall in the SA4.4, and found that the model predicted a precipitating storm propagating north of VAM between 15:00 and 16:00 UTC. The timing of this precipitating storm as predicted by SA4.4 was correct; however, the location was slightly north of the actual observation (cf. Fig. 7).This is a common issue with convection-permitting models which prompted the scientific community to develop new metrics to verify these types of models to avoid penalising them twice for misplacing rainfall (e.g. Davies et al. 2008;Gilleland et al., 2009). On the 290 other hand, SA1.5 failed to predict any precipitation close to VAM between 15:00 and 16:00 UTC or anytime during the analysis period (between 12:00 to 24:00 UTC) (see Fig. 8). An analysis of the 10 m wind directions in SA4.4 and SA1.5 generally agrees with observed surface northerly to north-295 westerly wind directions close to VAM, however the wind speeds are underestimated in both models (Fig. 9). Figures 9(a) and 9(b) shows that SA1.5 and SA4.4 predicted almost similar wind speeds at 15:00 UTC, which are underestimated compared to observations ( Fig. 9(d)). The simulated wind speeds by the UM is more similar to the ERA5 (Fig. 9(c)) , and the similarities are larger with the SA4.4. The ERA5 winds are also lower than those observed using SAWS ground stations. A sample of three stations along the storm track taken proximally closer to the location of initiation, propagation and dissipation stages of 300 the storm (Klerksdorp, Witbank and Machadodorp) indicate that SA4.4 and SA1.5 especially underestimated the wind speeds during the initiation stage of the storm (Fig. 10). The ERA5 winds are stronger than both UM configurations, despite the ERA5's lower resolution compared to these configurations.  representing areas of convergence of moisture and negative values (blue) represents areas of divergence of moisture.

22
The SA4.4 generally indicates westerly and south-westerly 500 hPa winds at 15:00 UTC over VAM (Fig. 10). 310 Considering surface winds, the SA4.4 indicates the presence of vertical shear between the surface and the mid-levels, however the speed shearing is possibly underestimated especially during the initiation phase of the storm. The SA4.4 predicted a southwesterly propagation of 500 hPa vertical vorticity north of VAM between 15:00 UTC and 16:00 UTC (Fig. 10). The maximum negative vorticity at 500 hPa reached by the VAM storm, as predicted by SA4.4, ranged between −0.5 × 10 −2 −1 and −0.75 × 10 −2 −1 from 16:00 UTC and 17:00 UTC. This vertical vorticity was one order of magnitude smaller than a 315 dynamical definition of a mesocyclone, which is an indicator that SA4.4 did not capture the mesocyclone of the VAM storm, and therefore underestimated its severity. However, contrary to supercell storms in the midlatitudes, most storms developing in the subtropics (which is where South Africa is located) do so without strong vertical shear and are rather associated with weak or moderate vertical wind shear (Sansom, 1966;Hand and Cappelluti, 2011). It may therefore be argued that relatively lower amounts of vertical wind shear in the subtropics, and in our cases study, correlates with relatively lower relative vertical 320 vorticity, and therefore relatively weaker mesocyclone according to classical definitions as stated by Glickman (2000).
A relationship between MFC and vertical vorticity was also determined. It was found that SA4.4 predicted vorticity maxima at 500 hPa that is associated with MFC maxima at low-levels. MFC initiates first at the low-level followed by vorticity genesis at 500 hPa. This implies that the development of MFC in the low-levels could be a precursor of vertical vorticity initiation in the mid-levels. Since there is a positive relationship between low-level MFC and mid-level vorticity, it follows 325 that the stronger the low-level MFC, the stronger the mid-level vorticity would be. This finding is in agreement with a study by Banacos and Schultz (2005) Further examination of the VAM storm during its 500 hPa vorticity maxima as predicted by SA4.4 (which occurred between 16:00 UTC and 17:00 UTC), reveals that low-level convergence occurred in the equatorward side of the storm, and divergence in the polarward side of the storm. Figure 12(a) indicates that low-level convergence was associated with relatively 330 higher values of specific humidity. This implies that there is an inflow of low-level moist air which converges in the equatorward side of the storm and then gets uplifted as updrafts. Figure 12(a) also indicates that low-level divergence is associated with relatively lower values of specific humidity which implies that there is low-level diverging dry-air in the polarward side of the storm. Figure 12(b) shows that the low-level diverging dry-air depicted in Figure 12 Indicates 500 hPa convergence/divergence (in 10 −2 −1 ) at contour intervals of 0.1 × 10 −2 −1 and vorticity (in 10 −2 −1 ) shaded fields.
The analysis also indicates a coupling of 800 hPa convergence (divergence) fields with 500 hPa (divergence) convergence fields, which is an indicator that the low-level converging moist-air in the equatorward side of the storm gets uplifted with updrafts to the dry mid-levels then later gets advected to the low-levels with the downdrafts in the polarward side of the storm 340 ( Fig. 12(a) and 12(b)). Figure 12(c) depicts 10 m winds and convergence (divergence) analysis. It should be noted that this analysis indicates that near-surface convergence (divergence) field patterns are similar to those at 800 hPa shown in Fig. 12(a), of which are together coupled with 500 hPa divergence (convergence) fields shown in Fig. 12(b). Figure 12(c) indicates that there are two areas of updrafts origin near the surface (ahead of the storm and at the rear-flank of the storm) which are both located in the 345 equatorward side of the storm, as also confirmed by the mid-level convergence and vorticity analysis (Fig. 12(d)). Figure 12(c) also indicates areas of forward-flank and rear-flank downdrafts which are in the polarward side of the storm as also confirmed by the mid-level convergence and vorticity analysis (Fig. 12(d)). Figure 12 can be summarised as follows. There is a convergence of moist and relatively stronger surface and low-level winds ahead of the storm which are uplifted as updrafts to the mid-levels. Some of these updrafts diverge as they reach the 350 mid-levels and are later advected to the lower-levels and surface as relatively dry downdrafts, which then also diverge ahead of the downdraft area (toward the north-east as part of a front-flank gust front) and toward the rear-flank of the storm (toward the west as part of a rear-flank gust front) on reaching the surface. The north-eastward diverging winds then contribute to uplifting the inflowing moist air ahead of the storm. On the other hand, the westward diverging winds converges with northwesterly inflowing moist-air in the rear-flank of the storm, initiating a second area of updrafts in the rear-flank of the storm. 355 This rear-flank updrafts also diverge in the mid-levels and contribute to the forward-flank downdrafts. The diverging midlevel winds from the rear-flank updrafts also converge with mid-level dry-air which is being entrained from behind the storm and are later advected to the surface as the rear-flank downdrafts.. Figure 12 further indicates that surface and low-level convergence is associated with the storm's cyclonic updrafts, while divergence is associated with anticyclonic downdrafts.
It is interesting to note that this analysis indicates that SA4.4 predicted thunderstorm dynamics consisted with those of a 360 classic supercell. Therefore, from a dynamical perspective, SA4.4 was able to predict the VAM supercell, even though its strength was underestimated, and location incorrect possibly due to a lack of data assimilation.

Summary and conclusion
In this study an analysis of the tornadic supercell that tracked through the northern parts of the Highveld in South Africa on 11 December 2017 was performed. It was found that this supercell initialised in the Free State Province as part of a cluster of 365 multicellular thunderstorms over a dryline, and propagated in a north-easterly direction while strengthening and weakening throughout its lifetime, until it dissipated 7 hours later on approach to Machadodorp in the Mpumalanga Province. It was also found that three ingredients were important in strengthening and maintaining this supercell: 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 which enhanced convective instability. 370 On approach to the extreme south of the Gauteng Province, the already severe supercell encountered even more low-

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NWP models operationally run at the SAWS were also analysed and evaluated to see how they performed in predicting 375 this tornadic supercell. It was found that the 4.4 km grid spacing model (SA4.4) performed better than the 1.5 km grid spacing model (SA1.5) in predicting this supercell. This is in spite of the horizontal resolution of SA1.5 being higher than that of SA4.4, and therefore, expected to be able to represent small-scale atmospheric processes better. The poor performance of SA1.5 seems to have emanated from a significant underestimation in low-level warm moisture advection and convergence (which act as dynamic lifting mechanisms). Keat et al (2019) have found that most rainfall, as simulated by SA1.5, is produced 380 by large storms of at least 50 km in diameter. In our study, radar indicates that the storm being analysed had a diameter of between 20 km and 30 km, which may also provide an explanation of why SA1.5 does not capture any storm, and therefore may be a case of missed convection initiation.
SA4.4 was able to capture the supercell being analysed. However, the severity of this supercell was underestimated, possibly due to an underestimation in the mid-level vorticity which was found to be one order of magnitude smaller than that 385 of a typical mesocyclone. This underestimation in mid-level vorticity was found to be a result of underestimated surface to mid-level vertical wind shear and underestimated low-level horizontal mass and moisture flux convergence. SA4.4 also predicted a correct timing of the supercell, however the location was which is a common issue found in convective permitting models.
From these results, it is recommended that research into the possibility and benefits of implementing dynamic and 390 thermodynamic objective analysis schemes, which are derived from both NWP model data output and near real-time surface and upper-air observations, be conducted for a very short-range objective prediction of severe thunderstorm (including supercells) initiation over the central and eastern parts of South Africa.
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 395 knowledge, has never been conducted. This will help improve representation of regional and local processes in mesoscale NWP models which will in turn improve the skill of severe storm prediction over South Africa.
Data availability. The UM is licensed and legal possibilities of accessing its operational data should be cleared with the SAWS.
Radiosonde data can be obtained from the Integrated Global Radiosonde Archive Version 2 (IGRA 2) via https://doi.org/10.7289/V5X63K0Q. Radar and weather stations data can be obtained from the SAWS. ERA5 data can be 400 obtained from Copernicus Climate Change Service Climate Data Store via https://climate.copernicus.eu/climate-reanalysis.
Author contributions. LEL and MMB conceived and designed the study. LEL, GR, MMB and PM performed the analysis while continuously discussing results with all authors who in turn contributed through their feedback. GR, MG, LEL and NM curated the data. LEL wrote the initial draft of the manuscript with all authors reviewing and contributing in various proportion 26 to sections. All authors edited the final draft and provided significant comments and suggestions for further clarity and 405 improvements.
Competing interests. The authors declare that they have no conflict of interest.