CHANGES IN ATMOSPHERIC DISPERSION POTENTIAL ACROSS THE SW CAPE COAST

Abstract

Changes in the potential for dispersion of atmospheric pollutants across the SW Cape coast, to the northeast of Cape Town, are analysed using case studies of 10 m meteorological network and doppler acoustic sounder data. The analysis focuses on summer fair weather inversion conditions associated with coastal lows, and cases distinguish between SSE trade winds and weaker WSW seabreezes. The more common SSE wind regime was characteristed as a topographically channelled low level (300m) jet. The spatial distributions of air temperatures and horizontal wind deviations were closely related and across coast gradients in the first 10 km exceeded 5⁰C and 15⁰, respectively. The acceleration and deflection of SSE wind flow by nearby Table Mountain and a daytime land-sea temperature difference of over 2NC accounted for much of the spatial structure. Sounder transects made along a route 12 km inland over a two week period when seabreezes were prevalent indicated sharp variations in wind direction, speed, turbulence and air temperature in the 0—500 m layer. During lighter seabreeze wind conditions, a thermal internal (convective) boundary layer was well described by sounder transects. The TIBL depth was observed to grow at a rate of 1:20 in the first 5 km and 1:50 further inland. The meteorological gradients are discussed with regard to the potential for dispersion of atmospheric pollutants in the coastal zone and the ability of tower-mounted sensors to adequately parameterise such processes