EVIDENCE FOR A MESOSCALE THERMAL CIRCULATION OVER DRY SALT LAKES

Because of albedo and thermal conductivity effects, dry salt lakes in central Australia exhibit a characteristic thermal "signature" within the overall landscape, having much cooler daytime surface temperatures and warmer nocturnal temperatures than the surrounding environment. Various aspects of a possible mesoscale thermal circulation arising from such a thermal differentiation have been investigated for The Salt Lake, a moderately sized (70 km2) dry salt lake in northwestern New South Wales (30-degrees-05'S, 142-degrees-08'E). Surface temperatures across The Salt Lake and surrounding landscape determined by field and polar orbiting (NOAA-9) satellite observations show marked variation, with the salt crust remaining up to 16 K cooler than the surrounding sand country during the day and up to 8 K warmer at night. That this thermal forcing can provide a horizontal pressure gradient sufficiently strong to induce air motion is confirmed by analysis of hourly output from a network of anemographs which were established around the perimeter of The Salt Lake during a five-month autumn and winter period. An extremely regular pattern of nocturnal near-surface convergence (lake warm) and daytime divergence (lake cool) is shown, a pattern which is enhanced when data are stratified for anticyclonic conditions with a weak pressure gradient (approximately 24% of total observations). Wind roses produced for a number of sites around the lake show a distinct diurnal reversal in near-surface airflow, with generally onshore flow by day and offshore flow at night. Confirmation that a diurnally reversing thermal circulation does operate under certain conditions at The Salt Lake is provided by pilot balloon analysis of winds aloft and by preliminary mesoscale modelling of airflow. This circulation, which has not previously been described in the literature, may have important environmental and climatic implications in the Australian context.