Disentangling the near‐surface physical processes from the synoptic‐dynamics in heatwaves using a single column model and trajectory analysis

Abstract This study identifies the physical and dynamical processes required for high near‐surface temperatures and the conditions needed for them to remain high for at least 3 days (the definition of a heatwave). The focus region for the study is Victoria in southeastern Australia. Summer days are grouped into warm, hot, and heatwave days depending on whether and how long they lie in the upper quartiles or deciles of daily maximum surface temperatures. Warm, hot, and heatwave days with similar 350 K potential vorticity anomalies are paired using k ‐means clustering, the idea being to pair days with similar upper‐level synoptic patterns but very different near‐surface temperatures. Using the single‐column configuration of the Unified Model, the physical and dynamical processes are systematically swapped between the members of each pair. In this way, the relative importance of each process is quantified. The warm horizontal advection associated with the nocturnal jet and the higher initial temperatures at 0000 h local standard time are most important for the high temperatures on hot days. To understand why hot days have higher initial temperatures, 10‐day backward trajectories from the European Centre for Medium‐Range Weather Forecasts Reanalysis v5 reanalyses are analysed. Ten days before hot days, the air parcels start at higher heights and higher potential temperatures compared with warm days. They descend and are adiabatically warmed before they are diabatically heated after entering the boundary layer. Near‐surface physical processes, including surface sensible heating, do not account for the prolonged high near‐surface temperatures during heatwaves. Instead, the slow‐moving upper‐level anticyclone over the Tasman Sea, which delays the passage of coastal fronts, gives heatwaves their longevity.