The physical and synoptic‐dynamical differences between warm days, hot days, and heatwaves during summer in Victoria, Australia

Abstract

To understand the physical processes leading to extreme and sustained near‐surface temperatures, three classes of summertime days in Victoria, Australia are compared: they are upper‐quartile warm days (above 75th percentile in daily maximum temperature), single hot days (those above the 90th percentile) and three‐day heatwaves (those above the 90th percentile). In each class, the physical processes are similar: in the composite mean there is cold advection in the boundary layer and strong diabatic heating during the day. However, short‐wave radiation, and hence the surface sensible heating, decreases as hot days or heatwaves progress. At night on hot days and heatwaves, a nocturnal jet advects warm air into the region above the boundary layer, and this warmer air is mixed to the surface after sunrise. During the day, the temperature advection anomaly is warm (relative to warm days), with stronger anomalous warm advection on heatwaves than hot days. The stronger and slower‐moving anticyclones during heatwaves account for their longevity. The arrival of a coastal front terminates the sequence of high surface temperatures. Ten‐day backward trajectories computed from ERA5 show that the air mostly originates from the South Indian Ocean 10 days before warm, hot and heatwave days. During heatwaves, the air descends from greater heights, resulting in stronger adiabatic compression and hence greater warming.