AGS-PRF: Extreme Near-Surface Wind Speeds Associated with Coherent Small-Scale Vortices within the Boundary Layer of Tropical Cyclones

Stern Daniel P, State College PA

Investigators

Abstract

Intellectual merit Tropical cyclones possess some of the strongest near-surface winds found anywhere on earth, yet the mechanisms by which the extreme (>80-100 m/s) local wind speeds characteristic of intense hurricanes are produced remain poorly understood. Preliminary research has shown that the strongest winds are often found in close proximity to intense (10-25 m/s) updrafts and are associated with coherent small scale (1-3 km) vortices. In this project, the structure, dynamics, and impact of these intense updrafts and their associated vortices will be investigated. This will be accomplished through the analysis of high-resolution simulations of both real-data and idealized tropical cyclones, using the Weather Research and Forecasting Model (WRF) and the Bryan Cloud Model (CM1). Comparisons will be made to a dataset of dropsonde observations previously collected by the principal investigator. Finally, the linear vortex model of Nolan (3DVPAS) will be utilized to determine whether these vortices are a consequence of a dynamical instability of the boundary layer in hurricane eyewalls. These extreme updrafts (which are ubiquitous in category 4 and 5 hurricanes) might influence the overall intensity of hurricanes, through their effects on turbulent mixing in the boundary layer, axisymmetrization, and surface fluxes. Therefore, in addition to investigating the vortices themselves, the study will also use Large Eddy Simulations (LES) to determine the impact that these features may (or may not) have on the larger scale structure of the storm. This project will be undertaken at NCAR, with Dr. Richard Rotunno as the sponsoring scientist, and Dr. George Bryan as a collaborating scientist. Broader impacts As the most severe wind-damage in landfalling tropical cyclones has been observed to be highly localized, it is possible that these small-scale vortices are responsible for such occurrences. It is therefore imperative to increase our understanding in this respect, and this project could lead to improved modeling and better forecasts of the surface wind field at landfall. These features also pose a danger to research aircraft, and so a proper understanding of the risk of encountering these updrafts would be beneficial. Therefore, this work has the potential to be highly relevant to the tropical cyclone research community, to the wind engineering community, and to society at large.