Coastal wind and dispersion conditions at Koeberg: analysis of data with application to mesoscale forecasting

Coastal wind data collected at the Koeberg nuclear power station north of Cape Town were analysed to determine the interaction of synoptic and thermal cycles. A spectral analysis over periods ranging from 2 h to 32 days highlighted the dominant 24-h land-seabreeze cycle, most evident in the across-shore U wind component, during summer. An analysis of wind direction persistence showed that winds parallel to the coast are less variant than the short-lived across-shore flows. The persistence analysis indicated that wind directions change out of a 22,50 sector more than half the time after one hour, making forecasting more difficult. Spatial variations in the dispersion climatology were revealed by the comparison of wind, temperature, lapse rate and turbulence data from a network of weather stations within a 20-km radius of Koeberg. Calm summer daytime conditions were isolated as having the most spatial structure. The marine areas to the northwest of Koeberg were found to have cool temperatures and low turbulence. To the south the steep topographic relief increased turbulence, while inland a thermal internal boundary layer formed, and turbulence values were double those at the coast. Using a scattergram technique, horizontal turbulence was found to be inversely proportional to wind speed, while vertical turbulence rose linearly with speed in a 1:10 ratio. Real-time displays of 24-h and 7-day trends are suggested as a means to assess the potential mesoscale variability in dispersion conditions near the Koeberg power station