Intellectual content of the proposal: The accurate prediction of the size and intensity of tropical cyclones remains one of the prominent challenges in modern meteorology. In this project, two distinct but complementary tools will be used together to understand what processes control and modulate both the horizontal and vertical structure of the tropical cyclone wind field. These tools will be: 1) high-resolution simulations using a version of the Weather Research and Forecasting Model (WRF) which has been modified to support tropical cyclone simulations in highly idealized and controlled environments, and 2) a linear but sophisticated model of asymmetric and symmetric vortex dynamics that has been developed and used by the Principal Investigator (PI) over the last six years, known as Three-Dimensional Vortex Perturbation Analysis and Simulation (3DVPAS). Since pioneering work of Kerry Emanuel in the 1980s on the theory of the vertical structure of a mature hurricane, little attention has been paid to the role that vertical structure plays in controlling current intensity and future intensity changes. The PI will use the WRF model to simulate cyclones with a variety of vertical and horizontal structures, embedded in environments with controlled mean flow and mean shear speeds. 3DVPAS will be used to assess the implications of these structure changes for future intensification, and to analyze the roles of convective asymmetries (such as spiral bands) in changing the vertical and horizontal structure of the symmetric wind field. Results from both methods of study will be compared and validated against radar observations of the hurricane wind field, obtained from NOAA/HRD and during the Rainband Intensity Experiment (RAINEX).
Broader impacts of the proposal: The broader impacts of this work will be to increase understanding of the evolution of the size and structure of the hurricane wind field. This potentially will lead to better forecasting of not only track and intensity, but also of the appropriate sizes for the watch and warning zones for landfalling storms, which have enormous cost impacts for the affected areas. Further development and eventual dissemination of the idealized modeling system will benefit the research community.
The goals of this project were to comprehensively explore the structure of the wind fields around hurricanes, with particular emphasis on the vertical structure (i.e., how the wind changes with height above the surface), which had previously received very little attention. To achieve these goals, the PI and two graduate students used observational data sets taken by aircrat in hurricanes, meteorological theory, and advanced computer simulations of hurricanes. A number of important results were found. First, the previous view that the outward slope of the hurricane eyewall was related to the strength of the hurricane was shown to be incorrect; instead, it is only related to the size of the hurricane itself, with lager storms sloping outwards more than smaller hurricanes. This information can be used to evaluate computer forecast models, and to improve them to make them more realistic. The dynamics of spiral rainbands, and their interactions with the hurricane itself, were also studied extensively. Many features of the anomalous wind fields around the rainbands were shown to be closely related to the structures of precipitation embedded within them. A number of prior theories regarding the cause of these rainbands have also been challenged, paving the way for further investigations. As part of this project, the PI also developed a numerical modeling framework that allows investigators to more closely control the environments around hurricanes that they simulate. This allows better determinations of cause and effect for changes in hurricane structure and intensity that these simulations can illustrate. The modeling framework has been made available to other scientists for their own use.