LOW-LEVEL WIND MAXIMA AND THE TRANSPORT OF PYROGENIC PRODUCTS OVER SOUTHERN AFRICA

Low-level jets (LLJ) over various regions of southern Africa are shown to occur mostly as a result of a thermal gradient established over ge ntly sloping terrain and have been observed to overlie the nocturnal t emperature inversions. Strong low-level winds associated with plain-mo untain and mountain-plain circulations have also been observed over th e central Namib desert and Natal interior. With the exception of the N amib and Natal, where no temperature profiles accompanied wind profile observations, the LLJ is. seen to increase in strength as the nocturn al surface temperature inversion intensifies through the night. The he ight of the LU above the surface also increases as the inversion deepe ns. Wind speeds in all regions exceed 10 m s(-1) in the jet core, whic h typically is located between 200 and 300 m above ground level (agl). Over the central Namib, the summertime plain-mountain wind reaches si milar wind speeds in a jetlike core located about 200 to 600 m agl. Th e Natal mountain-plain winds may exceed 10 m s(-1) in a layer of up to 1000 m deep. It is suggested that all these regional-scale wind syste ms transport pyrogenic products in excess of 400 km during a single ni ght. Subsequent daytime convective lifting of these ozone precursors, coupled with synoptic-scale anticyclonic circulation, provides a trans port mechanism for pyrogenic products over southern Africa toward the Atlantic and Indian Oceans.