A statistical model of the seasonal-diurnal wind climate at Adelaide

The seasonal-diurnal variation of surface winds observed at Adelaide Airport (South Australia) over 40 years is modelled in terms of the zonal-meridional wind vectors. Calms are assessed separately from the wind vectors. The joint probability density function of the wind vectors is modelled as the sum of a number of disjoint bivariate normal distributions in the zonal-meridional plane. An overview of the data suggests three diurnal mechanisms: sea-to-land breezes, land-to-sea breezes and downslope drainage flows off the Adelaide Hills, together with two non-diurnal large-scale weather mechanisms that are modulated by the annual migration of the sub-tropical ridge. It is shown that, typically, two of the three diurnal components contribute to the joint probability density function for any given hour of day and month, together with the two large-scale weather components. Optimisation is used to quantify the model parameters directly from the joint probability density function. The robustness of the methodology is verified using standard "Bootstrapping" methods. The model elucidates the principal climate mechanisms at Adelaide in terms of the seasonal-diurnal variation of the joint statistics of wind speed and direction using just the mean, annual, semi-annual, diurnal and semi-diurnal harmonics.