Landfall-Induced Changes in Tropical Cyclone Veronica’s Surface Winds

Abstract Severe Tropical Cyclone Veronica impacted the Australian northwest coast in March 2019. Synthetic aperture radar observations showed that the eyewall wind maximum shifted from the left-forward quadrant, characteristic of the motion-induced asymmetry for Southern Hemisphere storms, to the right-forward quadrant as Veronica approached land. Analysis of a large body of similar observations showed that the majority of cyclones within 200 km of land have an eyewall surface wind asymmetry with the strongest winds in the offshore-flow semicircle, similar to that in Veronica, rather than an asymmetry consistent with motion. In contrast, cyclones further from land mostly have the expected motion-induced asymmetry. Simulations with a dynamical tropical cyclone boundary layer demonstrate that the rightward shift of the wind maximum in Veronica was consistent with asymmetric surface friction due to the proximity to land. Analysis of the simulated vertical structure revealed that the asymmetry comprised two counterrotating spirals, similar to those described in a companion paper. The effect of this change in surface wind asymmetry on storm impact is argued to be likely modest, because it occurs largely in the offshore flow, usually away from people and infrastructure. Significance Statement Landfall is the time at which tropical cyclones are generally most dangerous, so understanding the changes that occur during landfall helps to mitigate impact. Significant changes were observed in the near-surface winds of Tropical Cyclone Veronica as it approached the coast, and the wind maximum in this Southern Hemisphere cyclone shifted from its normal position in the left-forward quadrant to the right front. We use a climatological analysis to show that similar changes are common, and a dynamical model to show that they are caused by asymmetric friction due to the land–sea boundary.