Controls governing airflow over the South African lowveld : research article

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