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 gently sloping terrain and have been observed to overlie the nocturnal temperature inversions. Strong low-level winds associated with plain-mountain and mountain-plain circulations have also been observed over the central Namib desert and Natal interior. With the exception of the Namib and Natal, where no temperature profiles accompanied wind profile observations, the LLJ is seen to increase in strength as the nocturnal surface temperature inversion intensifies through the night. The height of the LLJ above the surface also increases as the inversion deepens. Wind speeds in all regions exceed 10 m s−1 in the jet core, which typically is located between 200 and 300 m above ground level (agl). Over the central Namib, the summertime plain-mountain wind reaches similar wind speeds in a jetlike core located about 200 to 600 m agl. The 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 systems transport pyrogenic products in excess of 400 km during a single night. Subsequent daytime convective lifting of these ozone precursors, coupled with synoptic-scale anticyclonic circulation, provides a transport mechanism for pyrogenic products over southern Africa toward the Atlantic and Indian Oceans.