The life cycle of the heatwave boundary layer identified from commercial aircraft observations at Melbourne Airport (Australia)

Although undoubtedly important to the dynamics of heatwaves, the role of the boundary layer has received relatively little attention. Here, a 16‐year (2003–2018) record of commercial aircraft observations centred on Melbourne Airport (37.7°S, 144.8°E) is used to investigate the structure and evolution of the boundary layer during summertime heatwaves in Victoria. Composite means show that the daytime boundary layer is deeper during heatwaves than at other times, whereas the heatwave boundary layer during the night and early morning is shallower and more stable. A strong northerly nocturnal jet forms slightly below the top of the inversion, presumably transporting hotter air above the boundary layer overnight. A deep mixed layer develops rapidly after sunrise, mixing downward high‐momentum air from the nocturnal jet. For heatwaves lasting for 3 days, the nocturnal jet progressively strengthens each night. Heatwaves end in the afternoon or evening following the passage of a strong coastal front and the beginning of postfrontal cold air advection. During heatwaves, the daytime boundary layer is deep and warm; the night‐time boundary layer is shallow and capped by a stable layer. A strong nocturnal jet forms and strengthens from heatwave day 1 to day 3. A strong coastal front terminates heatwaves at Melbourne.