Downdrafts and Convective Gusts in High-Resolution Simulations: An Australian Case Study

Abstract

Convective downdrafts are a crucial part of convective systems and can cause local damage by generating severe surface gusts. However, fully representing the mechanisms behind strong downdrafts and gusts in models remains difficult. We conducted short-duration regional simulations using a nested downscaling approach with the Weather Research and Forecasting (WRF) model including a cloud-resolving innermost subregion at various grid spacings down to 200-m. The model generated a strong gust event characterized by a pronounced, coherent downdraft that we used to study the forcing mechanisms. The 1-km simulation underestimated the downdraft and 5-km didn’t represent it at all. In addition, the 1-km simulated weaker downdrafts exhibited a vertical momentum budget different to that of the 200-m simulation. The perturbation pressure vertical gradient and thermal buoyancy are the main contributors to the downdraft acceleration in the 200-m simulation, with condensate loading playing a less significant role than other processes at mid and low levels. Coarser resolutions underestimate the role of perturbation pressure, resulting in a smaller contribution to downward acceleration compared to thermal buoyancy and condensate loading. This highlights the potential importance of the spatial distribution of downdraft forcing mechanisms as a key consideration for why coarser grid spacings may fail to adequately capture more realistic downdrafts. The findings presented here are relevant for risk assessment applications and parameterizations of downdrafts, demonstrating benefits of large-eddy simulations (LES) for wind hazard analysis and highlighting the need for care when interpreting results from coarser models.

Affiliated Institutions

• NSW Department of Planning and Environment AU
• The University of Melbourne AU
• UNSW Sydney AU

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