OBSERVATIONS OF A CUTOFF LOW OVER SOUTHERN AUSTRALIA

The structure and evolution of an intense cut-off low is documented in detail unprecedented for the Australian region, by combining high tim e-resolution radiosonde profiles with co-located wind-profiler observa tions and routine Australian Bureau of Meteorology analyses. The cut-o ff low develops over the ocean south of Australia and subsequently int eracts with a subtropical frontal system over the central part of the continent. Throughout the period of observation, the strongest tempera ture gradients lay through the subtropics. Moreover, the subtropical t emperature gradients strengthened while the midlatitude temperature gr adients associated with the cut-off low weakened. The Australian Burea u of Meteorology's 150 km Regional Assimilation and Prognosis System a nalyses capture the broad-scale structure and evolution of the system reasonably well. The system documented here is similar in structure to the instant occlusion investigated by Browning and Hill (1985), the m ajor difference being that our cut-off low develops poleward of a subt ropical front. The cut-off low was cold-cored and produced a very deep tropopause fold. Over Adelaide, the tropopause descended to an altitu de of about 5 km, and the temperature at 500 hPa fell to -30 degrees C . The western edge of the low-level cold dome was marked by a strong s econdary warm front, which passed over Adelaide about 8 hours after th e tropopause height minimum. The warm front was characterized at the s urface by an absolute minimum in the pressure, and a temperature rise of about 7 degrees C. Significantly, this increase in temperature took place in the early hours of the morning, against the diurnal trend. T he cut-off low produced very high rainfalls, resulting in flash floodi ng and much damage in the Adelaide Hills area. The most severe convect ion took place in the region behind the lowered tropopause, around the passage of the warm front.