Impact of Largescale Vorticity on Tropical Cyclone Wind Asymmetry

Tropical cyclones pose the most risk to communities and infrastructure when making landfall, making it important to accurately forecast storm intensity and size during these periods. Unfortunately, there is still a large range of uncertainties surrounding the dynamics of tropical cyclones during landfall, which can affect the forecast quality.

In this case study, we analyse Severe Tropical Cyclone Veronica (2019) which made landfall in north-western Australia and remained almost stationary on the coast for approximately 24 hours. Veronica experienced a breakdown from the offshore side while maintaining intensity inland, which is atypical since landfall is usually linked to weakening from increased friction and shear, as well as decreased surface enthalpy fluxes. These changes in the wind field were mainly found within the boundary layer, suggesting the importance of (and leading to a focus on) the boundary layer dynamics. While this case is Australian, we note that there was no evidence that the storm was fuelled by hot, moist sand as has been suggested in previous studies.

Here, we utilise observations, full-physics ensemble simulations, and boundary layer simulations to veer from the purely thermodynamic theories and explore the possibility of kinematic explanations. In particular, we hypothesise that the eyewall breakdown offshore and expansion of the wind field inland could be related to a larger-scale vorticity gradient. There is also evidence to suggest that substantial alterations to the vorticity field were made by a spiral rainband feature being maintained by surface frictional convergence, and that the eyewall itself was destabilised by vortex Rossby waves.

Understanding the interaction with the large-scale vorticity environment with the internal tropical cyclone dynamics will provide valuable insight into the asymmetric storm breakdown at landfall. Ultimately, this enhanced understanding should contribute to informing forecasts and improving preparedness, by allowing forecasters to evaluate whether the larger-scale environment could enhance the tropical cyclone risk to coastal communities.