Roll cloud on the South African east coast

Authors


A recent article about a roll cloud off eastern Florida (Hartung and Sitkowski, 2010) brought to mind a type of roll cloud in association with a Buster that is experienced along the eastern coast of South Africa in KwaZulu-Natal Province. ‘Buster’ is the name given by local ski-boat fishermen to a fresh to strong (8–14ms–1) southwesterly wind that follows the passing of a coastal low, which is preceded by a northeasterly or sometimes a northwesterly wind (Tyson, 1965; de Villiers, 1985a). The South African coastal low is a lee low that occurs as a consequence of the interaction of a marked coastal escarpment and a northwesterly offshore flow off the South African plateau ahead of an approaching cold front (Hunter, 1987). As the low moves northeastwards along the coast, it eventually dissipates on the wider coastal plain of northern KwaZulu-Natal and southern Mozambique (Torrance, 1995). Note that, due to the northeast to southwest orientation of the KwaZulu-Natal coast and rising ground inland to the coastal escarpment, winds along the coast are predominantly either from the northeast or the southwest. A related coastal-low wind phenomenon occurs along the southeast coast of Australia where it is called a ‘southerly Burster’, or ‘Buster’ (Colquhoun et al., 1985). In similar manner to the coastal bore event discussed by Hartung and Sitkowski (2010), the southwesterly buster, under ideal circumstances, is also characterised by abrupt pressure, temperature, humidity and wind fluctuations (Preston-Whyte, 1975; Hunter, 1987) and may briefly produce a rise in surface temperature (de Villiers, 1985b).

The leading edge of the Buster, marked by a roll cloud (de Villiers, 1985a), is more common in summer when it is superseded by a stratiform cloud layer (Jackson, 1966). On one occasion, the author observed a roll cloud of sufficient depth to produce a light rain shower. However, this is a rare occurrence given that the cloud feature is often limited to a shallow depth of maritime moisture below a temperature inversion. The cloud, photographed on 7 December 1975 at 1210 and 1220 local time (LT = utc + 2 hours) in Figures 1 and 2, was first observed approaching Cape St Lucia Lighthouse (107m above sea level, 23°30's 32°24'E) on the northern KwaZulu-Natal coast at 1200 LT. The cloud base was estimated to be about 300m above sea level. Although the cloud was approaching from the southwest, the wind at the lighthouse was northeasterly at 9ms–1. Upon the cloud's arrival at the lighthouse at 1225 LT, the surface wind, which had temporarily fallen calm, abruptly began blowing from the southwest at 10ms–1, gusting to 15ms–1. The disturbed sea surface caused by the southwesterly wind can be seen in the photographs by the light area within the cloud shadow. The change from a fresh northeasterly wind in advance of the coastal low to a strong Buster often takes just a matter of minutes, as can be seen in the example at Durban (south of Cape St Lucia), over a year before in Figure 3. On this occasion the wind changed from northeasterly at about 10ms–1 to southwesterly 15ms–1 gusting to 20ms–1 and more.

Figure 1.

Photograph looking to the southwest at the Buster cloud approaching Cape St Lucia Lighthouse 7 December 1975 at 1210 local time. (Photograph: M. P. de Villiers.)

Figure 2.

Photograph looking to the southwest at the Buster cloud approaching Cape St Lucia Lighthouse 7 December 1975 at 1220 local time. (Photograph: M. P. de Villiers.)

Figure 3.

Autographic wind speed (top) and direction (bottom) recording the passage of a Buster at Durban 28 October 1974 1000 to 2000 local time (utc+2). The time is shown between the speed and direction graphs.

The wind is a particular hazard to aircraft landing and taking off and has been known to flip parked untethered light aircraft. It has also been known to sink or swamp yachts in Durban harbour and on more than one occasion caused chaos during yacht regattas in the harbour and off the coast, as well as being a danger to ski-boat fishermen off the coast (de Villiers 1985a; 1996). The damaging effect of the wind quickly dissipates inland due to the marked rise of the escarpment to nearly 400m about 18km from the coast.

Apart from the cumulus cloud formed at the meeting of the offshore, warmer and drier northeasterly air and the onshore, cooler and moister southwesterly air, the most pronounced feature in the photographs is the wedge-shaped formation on the leading edge of the cloud (Figures 1 and 2). This feature persisted from first sighting until it passed the lighthouse when it was obscured by the following cloud. This was the only time, during many years of living on the KwaZulu-Natal coast, that the author saw such a wedge-shaped cloud formation. A possible explanation is that the advancing cooler and moister air from the southwest forced the warmer and drier air from the northeast to rise and penetrate into the cloud, thereby causing a dry slot with the wedge of moist air below it (de Villiers, 1985a). The convective nature of the cloud in the photographs, due to interaction between cooler and warmer air, is also compatible with the evolution described by Hartung and Sitkowski (2010), whereby roll clouds decayed when there was an increase in cellular convection due to interaction with a lake sea-breeze front.

The depth of the coastal low is dictated by a subsidence inversion at escarpment level that separates warm offshore air from cooler surface maritime air. The inversion lowers with the approach of the coastal low and Berg wind conditions develop when the subsidence inversion extends to the surface (Preston-Whyte, 1975; Hunter, 1987).

A Berg wind is the local name for the offshore Föhn-type wind. The latter wind is a warm and dry wind which occurs in stable air to the leeward of a range of mountains and is characterised by a potential temperature that increases markedly with height (UKMO, 1991). Berg winds occur with low frequency along the South African coast and blow for a few hours, mainly in the latter part of the winter and the early summer (Tyson, 1965). When the wind backs from northeasterly to northwesterly and remains moderate, or stronger, for a while, hot and dry Berg wind conditions are experienced. The surface temperature on the coast can exceed 40 °C and fall 15–20 degC with the arrival of the Buster. During the more humid summer months this results in distinct thermal discomfort (Preston-Whyte, 1975). An example (Figure 4) is shown at Durban when the surface temperature reached about 38 °C. Further north along the coast it rose to 44 °C (de Villiers, 1996).

Figure 4.

Automatic weather station recording of surface air temperature at Durban during a Berg wind from about 0315 to 1000 local time (utc +2) and the following Buster on 12 October 1995.

In view of the characteristics described by Hartung and Sitkowski (2010), the southwesterly Buster and its associated roll cloud may be regarded as the South African equivalent of a coastal bore. Finally, the sudden arrival of the Buster and its associated weather is an awesome weather phenomena that can only be fully appreciated by one who has been in its path when it passes (de Villiers, 1996).

Acknowledgements

The author is grateful to the South African Weather Service for permission to use the two photographs and the two diagrams from their Newsletters and Internal Forecasting Report.

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