Addison Alford - April 8

School of Meteorology Colloquium

Using ground-based fixed and mobile weather radars and numerical modeling to evaluate hurricane structure and dynamics at landfall

Addison Alford

Thursday, April 8^th

11:00 am

Join Google Meet:

https://meet.google.com/tbg-ozvi-wsv

Hurricane observations during landfall in the United States have received relatively little attention in the research community compared to hurricanes observed over the open ocean. Aircraft-based observations using in situ and remotely sensed data have elucidated much about the structure and evolution of hurricanes. However, gathering high temporal (3-5 minutes) and high spatial (500-1000 m) resolution observations over contiguous spatial domains (e.g., 10,000 km2) is not possible with aircraft due to instrumentation limitations, operational missions, and required flight paths in hurricanes. Using ground-based fixed and mobile Doppler weather radars afford continuous observations of processes that not only affect landfall, but also offer comparison to aircraft observations of processes that occur over the open ocean.

            The Shared Mobile Atmospheric Research and Teaching (SMART) Radars (SRs) are a pair of mobile Doppler, dual-polarization radars operated by the University of Oklahoma. Having sampled 15 landfalling tropical storms and hurricanes, datasets collected by the SRs likely provide key insight into hurricane dynamics and landfall processes previously difficult to observe. In Major Hurricane Harvey (2017), SR and coastal WSR-88D data were combined to retrieve three dimensional wind fields as Harvey intensified offshore before making landfall near Port Aransas, TX on 26 August 2017 at 0300 UTC. Dual-Doppler analyses were retrieved approximately every five minutes at 1 km spatial resolution over a spatially contiguous region on the order of 10,000 km2, making Harvey one of the best sampled, dual-Doppler observed landfalling hurricanes to date.

            A detailed analysis of asymmetric dynamic processes in the form of the excitation of vortex Rossby waves (VRWs) will be presented. Analogous to planetary Rossby waves, VRWs propagate on the radial gradient of storm relative vorticity and are a mechanism hypothesized to symmetrize the hurricane vortex, resulting in a stronger symmetric (average) circulation. In Hurricane Harvey, VRWs will be shown to form from asymmetric convection in the eye, leading to outward propagating spiraled rainbands radially outward of the eyewall. Impacts of VRWs on the symmetrization and intensity change in Harvey will be documented in observations for the first time. As vortex Rossby wave-driven processes are currently not well understood, this presentation will also detail a numerical simulation of Hurricane Harvey using a very high spatial resolution (<500 m) run of the Hurricane Weather Research and Forecasting model (HWRF) of Hurricane Harvey. Using the dual-Doppler derived wind fields, the observed  VRW-driven rainbands will be compared to those in the simulation. Additionally, the intensity change in HWRF associated with asymmetric dynamics will also be compared to the observed VRW-driven intensity change.