Summertime easterly surges in southeastern Australia: a case study of thermally forced flow

During the summer of 1999/2000, wind profiling stations were operated at Wagga Wagga and Canberra Airports. Time series from the Canberra station clearly showed the passage of evening easterly surges. These surges were often observed to reach Wagga WaggaAirport about six hours later, aroundmidnight. Data from the Bureau ofMeteorology's observing network allow us to investigate the conditions under which these wind surges occur while the wind profiler data allow us to contrast their characteristics and vertical structure at the two stations. A three-day set of observations provides examples of surges that are driven by pressure gradients produced by differential heating; and for this period, our observations support earlier arguments that mesoscale forcing due to both the plateau effect and land-sea temperature contrasts are important in the development of these surges. At the same time, we argue that continental-scale thermal forcing associated with the development of the easterly inland trough, is important in driving the surges several hundred kilometres inland to Wagga Wagga. The contrasting case is when the inland trough dominates over the mesoscale forcing, and there are periods where the winds are easterly throughout the day. Under these conditions there is no night-time surge, and the nocturnal boundary layer is typical of summer in inland Australia, with the rapid acceleration of boundary-layer winds after dark and the development of a strong nocturnal jet.