Atmospheric and Land Surface Contributions to Heatwaves: An Australian Perspective

Heatwaves are associated with prolonged hot conditions that accompany a quasi-stationary anticyclone. This is a consequence of different heat mechanisms including subsidence, horizontal heat advection, and sensible heating from the land surface. Understanding the relative contribution of each of these heat mechanisms is important for improving the predictability of heatwaves. However, the evolution of heatwaves can vary depending upon antecedent conditions, and therefore, it is necessary to examine the role of both atmospheric processes and land surface conditions together. Using a regional climate model, we simulate five heatwaves that occurred over different parts of Australia. Using Lagrangian back-trajectories to inform the atmospheric flow preceding each of the heatwave events, we perturb the evaporative fraction where and when the flow enters the boundary layer. We aim to understand the role of land surface energy partitioning on two of the heat mechanisms and identify under what circumstances it has limited influence on a heatwave. We find that the sensitivity to changes in land surface energy partitioning varied considerably between the events. Advection was critical by either enhancing the accumulation of heat into the affected location or for dissipating the event by ventilating the region with cooler air. Convective precipitation was triggered when increasing the evaporative fraction due to a warm atmosphere and more moisture supplied by the land surface. Finally, the contribution of each of the heat mechanisms varied between the events due to atmospheric and land surface processes and therefore generalizing the contribution of these mechanisms to heatwaves should be avoided.