Storm surge along the east coast of South Africa

by Ian Hunter, Jannie Stander, Estelle de Coning (South African Weather Service) and Jochen Kerkmann (EUMETSAT)

The resultant southerly to south-easterly swell peaked at roughly the same time as the equinoctical spring high tide early the following morning, Monday 19 March.

There was extensive damage to coastal infrastructure, with initial repair costs estimated at roughly half-a-billion rand. The total cost of the storm event — including loss of tourist revenue — is likely to be close to one billion South African Rand. Beach houses were undermined and partially destroyed. Damage to an outfall pipe resulted in raw sewerage flowing into the surf zone. Thousands of tons of sand have been removed from the beaches. The whole economy of the area is based heavily on tourism.

In the Airmass RGB image on 17 March 2007 12:00 UTC (Figure 1), high clouds appear as white, mid-level clouds as light ochre colour. The cold front associated with an intensifying upper air trough can be seen south of the country. A cold air mass can be seen by the bluish color bounded by the warmer greener air mass. In addition, typical signs of future intensification of the upper air trough and (low-level) cyclogenesis can be seen in the comma-shape cloud pattern to the rear of the cold front and in the PV (Potential Vorticity) anomaly (or dynamic tropopause anomaly, see Interpretation) indicated by the red color which marks an area of dry, descending stratospheric air. It is well known that a PV anomaly overrunning a (low-level) baroclinic zone (e.g a cold front) induces a cyclonic circulation (cyclogenesis).

In the Manual of Synoptic Satellite Meteorology the situation/process described above is called 'Instant Occlusion'. The name results from the resemblance of the mature stage of development of the cloud system with that of an occlusion. But the meteorological background of the development is completely different from an occlusion process where warm air is lifted from the surface. The process of the Instant Occlusion (which is one form of rapid cyclogenesis) is characterised by the mergence of a distinct cold air, comma-like cloud feature with a cloud band associated with the polar front (cold front). The adjective instant represents the fact that the occlusion appearance is assumed suddenly without the usual development at a front.

Twelve hours later, on 18 March 2007 at 00:00 UTC, the Airmass RGB shows further thickening (white elements) in the upper level clouds of this newly developed cyclone (see 00:00 UTC image, and Interpretation). The PV anomaly is clearly visible (red colour). Physically, the red stripe that connects the PV anomaly to the polar area further south represents a deformation zone, but because of its appearance it could also called the 'umbilical cord' of the PV anomaly. One can also observe the blue colour intruding into the vortex, indicating the cold-cored nature of the upper air vortex; which is yet another confirmation of the baroclinicity of the system.

Considering 18 March 2007 at 12:00 UTC (Figure 2), the Airmass RGB clearly shows the fully developed cyclone (COL) and the advection of cold air towards the southern coast of South Africa on the rear side of the cyclone. As the cyclone moves slowly towards the south it causes a prolonged situation of strong southerly winds on its western side. This fetch zone of winds up to 60 knots can be best observed from remote sensing instruments that measure winds over the oceans, like scatterometers and passive microwave instruments. As an example, the bottom image shows the wind vectors as derived from QuikScat data. The area of strongest winds is marked by purple colour.

Additional content

RGB composite VIS0.8, IR3.9r, IR10.8 (18 March 2007, 12:00 UTC)
RGB composite NIR1.6, VIS0.8, VIS0.6 (18 March 2007, 12:00 UTC)
RGB composite WV6.2-WV7.3, IR9.7-IR10.8, WV6.2 (20 March 2007, 12:00 UTC)
Sea level pressure analysis (18 March 2007, 18:00 UTC)
Report on the storm surge of 19 March 2007 (Hunter, Stander & De Coning)