Controls governing airflow over the South African lowveld

A wind climatology has been derived for the lowveld region of South Africa, using averaged monthly wind roses for 16 sites, and wind velocities record ed by three automatic weather stations over a five-year period. The seasona l pattern of near-surface airflow is related to the frequency of occurrence of the major synoptic circulation types. Southeasterly flow is most preval ent over the region, particularly in summer, and is associated with easterl y waves, continental anticyclones and ridging anticyclones. Flow from the n orthwest is linked with westerly waves and coastal lows, and is more freque nt in the winter months. Short-term variations in the wind regime were inve stigated using a sodar sited at Skukuza in the Kruger National Park. Circul ation is characterized by a distinct diurnal oscillation between westerly a nd easterly winds, in response to thermo-topographic forcing. The regional mountain-plain and plain-mountain winds frequently extend beyond 800 m abov e ground level, and may have speeds exceeding 10 m s(-1). The evolution of a nocturnal low-level jet, with speeds approaching 20 m s(-1) in the core, is documented. Winds through the depth of the mixing layer were investigate d by tracking radiosonde balloon ascents. Horizontal wind velocities up to the 500 hPa level are largely determined by synoptic forcing. Above 5 km, w ind direction is constant with height and frequently is from the southwest or west, in response to the position of the subtropical anticyclone. Elevat ed stable layers in the atmosphere exert little control over horizontal air movements in the region. Southeasterly winds at lower levels do not facili tate the export of aerosols and trace gases off the subcontinent. Above 5 k m, westerly flow allows pollution to be transported over the Indian Ocean.