Observations of a cut‐off 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 time‐resolution radiosonde profiles with co‐located wind‐profiler observations and routine Australian Bureau of Meteorology analyses. The cut‐off low develops over the ocean south of Australia and subsequently interacts with a subtropical frontal system over the central part of the continent. Throughout the period of observation, the strongest temperature gradients lay through the subtropics. Moreover, the subtropical temperature gradients strengthened while the midlatitude temperature gradients associated with the cut‐off low weakened. The Australian Bureau of Meteorology's 150 km Regional Assimilation and Prognosis System analyses 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 major difference being that our cut‐off low develops poleward of a subtropical front. The cut‐off low was cold‐cored and produced a very deep tropopause fold. Over Adelaide, the tropopause descended to an altitude of about 5 km, and the temperature at 500 hPa fell to −30°C. The western edge of the low‐level cold dome was marked by a strong secondary warm front, which passed over Adelaide about 8 hours after the tropopause height minimum. The warm front was characterized at the surface by an absolute minimum in the pressure, and a temperature rise of about 7°C. Significantly, this increase in temperature took place in the early hours of the morning, against the diurnal trend. The cut‐off low produced very high rainfalls, resulting in flash flooding and much damage in the Adelaide Hills area. The most severe convection took place in the region behind the lowered tropopause, around the passage of the warm front.