Investigating the Atypical Wind Structure of Severe Tropical Cyclone Veronica (2019) During Landfall

Understanding the development of the tropical cyclone wind field during landfall is paramount in quantifying the risk to communities. While the wind itself can cause extreme damage, the azimuthal and radial wind circulations are also key components in modulating the storm's intensity and lifetime via the transportation of heat and moisture.

Here, we investigate a case study of Severe Tropical Cyclone Veronica (2019) with high-resolution, convection-permitting ensemble simulations using the Met Office Unified Model. Veronica exhibited a highly asymmetrical structure, with a polygonal eyewall which rotated cyclonically over time and anticyclonically with altitude. Satellite observations revealed that the surface tangential wind maximum rotated from the left-of-motion quadrants to the front-right. A breakdown in the off-shore wind speeds of Veronica occurred during a partial landfall, where the storm remained almost stationary for hours. The ensemble aligned with observations and demonstrated a perseverance of the inland wind circulation despite the partial eyewall breakdown off-shore, particularly in the mid-boundary layer. As a result, the boundary layer winds became asymmetrical with the tangential wind peaking over land, co-located with a strong low-level outflow in the left-of-shear quadrants.

During the evolution of the eyewall, complex patterns of azimuthal wavenumber-1 and wavenumber-2 asymmetries were present in the vorticity field. The azimuthal propagation speed of the wavenumber-2 asymmetry is consistent with vortex Rossby wave theory. We investigate the relationship between the landfall, shear and vorticity asymmetries to understand the atypical evolution of the tropical cyclone wind structure. It is hoped that this work will provide more clarity on how a storm such as Tropical Cyclone Veronica could persist over land while seemingly weakening over the sea, which could enrich operational forecasting and lead to better-prepared coastal communities.