A study of two satellite image gust front events over South Africa

Two examples of gust fronts radiating out from convective activity were seen on satellite imagery during March over South Africa. The gust front was clearly evident on the Meteosat infrared satellite imagery over the north-eastern Free State on the evening of the 10th March 1999, while double gust fronts occurred over the western part of the Northern Cape on the evening of the 31st March 1999.

To the best of the authors knowledge these have not previously been noted on satellite imagery, nor documented, over South Africa. However, similar gust fronts or waves have been witnessed and documented elsewhere. In a satellite image study of four simultaneous large-amplitude wave families, Reeder and Christie (1998) state that large amplitude internal waves (which are waves found inside a fluid) are generated in the northern Queensland region of Australia with predictable regularity. In the Australian context the waves are generated in the evening, or early hours of the morning, and propagate on the strong surface based nocturnal inversion that commonly forms in the region.

The Reeder and Christie (1998) study documents four wave formations, all visible on a single satellite image. The study is of particular interest, because three of the wave formations are believed to have been of convective origin. One wave formation is very similar to the wave or gust front over the north-eastern Free State. While, in a separate instance, a pair of waves over Queensland, spawned by two different regions of thunder activity, produced new convective cells at their point of intersection. This also happened over the Northern Cape. The intersection of waves are expected to be "preferred sites for convection because air parcels experience larger vertical displacements there" (Reeder and Christie, 1998). Theory dictates that the parcel displacement, at the intersection, would be greater than that provided by a simple linear superposition of the two individual wave families (Grimshaw and Zhu 1994).

This object of this paper is simply to document a phenomenon which has not before been noted in our part of the world. That it was seen twice in one month makes it doubly worthwhile. Apart from the satellite imagery, radar images give an idea of the evolution of the thunderstorm which spawned a gust front over the north-eastern Free State, while Automatic Weather Station (AWS) recordings at Kroonstad and Welkom in the Free State provide evidence of the passage of the gust front. AWS information at Brandvlei in the Northern Cape, while less distinct, also bears testimony to one of the gust fronts in that area.

(NB It is understood that this is an abstract for approval or rejection and that images will possibly be added to the photo ready copy for preprints).