South African coastal lows, busters & gravity waves

By Ian Hunter (South African Weather Service)

One of the most impressive meteorological phenomena on the east coast of South Africa is the locally-named 'Buster', which is essentially a gust front that occurs following the passage of a coastal low. Figure 1 (source: SAWS) provides a typical example from SAWS's automatic weather station at Virginia Airport, just north of Durban. Note the sudden changes in wind direction and speed — and the jump in air pressure — all coinciding at 16h00. Some busters are associated with wind gusts of over 90km/h. However, by no means all coastal lows are linked to such rapid changes, or strong opposing winds.

Referring again to the multiple time series: prior to the coastal low, the overnight winds veered W–NW as a shallow land breeze undercut the large-scale circulation. Then, at approximately 9h00 am the NE wind broke through to the surface. As is often the case the wind eased off a few hours after the peak SW wind had passed in the evening.

The coastal low that moved up the east coast on 9 September 2008 was not a particularly impressive example. The satellite image in Figure 2 (source: SAWS) shows why this particular coastal low was chosen to illustrate the basic features of the weather system. Note that this high resolution visible image was taken half-an-hour after the coastal low passed through Durban, i.e. it would appear that these gravity waves (as revealed by the cloud street formation) could have been triggered by the pressure jump. On the other hand the cloud lines extended well to the southeast and coastal lows are literally that, their circulation does not extend far offshore (less than 100 km). The inset is an infrared version of the visible image, which shows the cloud to be at a low level (dark = relatively high temperatures = low cloud tops).

Figure 3 (source: SAWS) depicts the temperature and moisture sounding as recorded by the 12:00 UTC radiosonde ascent from Durban International Airport. This is a particularly intense subsidence inversion for the South African east coast. On the other hand the temperature drops by several degrees not far above the surface, i.e there is a shallow, moist layer of very unstable air. Low level moisture content would have been higher over the warm ocean. The cloud lines form where upward motion causes condensation, as the waves propagate parallel to the coast.

Comparing the times that the coastal low passed through other automatic weather stations on the KwaZulu-Natal coast the propagation speed of the Buster was roughly 40km/h. It might seem a simple matter to predict exactly when it will reach, say Durban, knowing at what time it passed through e.g Port Edward. Unfortunately this is a system which is continually being generated (by upper, offshore flow interacting with the escarpment). It can slow down, speed up, or even fill up, before it ever reaches Durban.

Note: gravity waves are oscillations which have gravity as their restoring force. Wind waves and swell in the ocean are also gravity waves.

Additional content

Met-9 IR10.8 image (13:00 UTC)
Met-9 IR10.8 animation (04:45–14:45 UTC)