Study of Processes Leading to Tropical Cyclone Structure and Intensity Changes

University Of Hawaii, Honolulu

Investigators

Abstract

The major goal of this research is to improve understanding of the processes that lead to structure and intensity changes of tropical cyclones (TC). The working hypotheses are (1) any environmental flow introduces quasi-stationary asymmetric forcing which reduce the capability of axisymmetrization potential of the symmetric circulation, and thus limiting the maximum potential intensity that the TC otherwise can achieve given favorable tropical oceanic conditions, and (2) external forcing affects tropical cyclone structure and intensity changes either directly or indirectly through the mesoscale processes occurring in the inner core region (within about 200 km from the cyclone center), including interactions among inner, outer spiral rainbands and eyewall convection; concentric eyewall cycle; and eyewall breakdown and the associated mixing process. The following scientific questions will be addressed: a. How do the inner and outer spiral rainbands form, propagate, and interact with each other and with eyewall convection, affecting tropical cyclone structure and intensity changes? b. How important is the environmental asymmetric forcing to the formation of tropical cyclone concentric eyewall and the subsequent eyewall replacement? What determines the timing and lifetime of the concentric eyewall cycle and the associated tropical cyclone intensity change? c. How does the inner core of a tropical cyclone respond to vertical shear of the environmental flow and what are the subsequent changes in storm structure and intensity? How sensitive are the structure and intensity changes to both the magnitude and vertical profile of the shear? To answer these questions, the Principal Investigator (PI) will use the quadruply nested, movable mesh, fully compressible, nonhydrostatic tropical cyclone model (TCM4) developed by the PI to conduct a series of well-designed idealized numerical experiments. The PI will analyze satellite and radar data for tropical cyclones over both the western North Pacific and Atlantic and the global reanalysis products to help design the idealized model initial conditions. Comprehensive diagnostics and budget analyses of the numerical results will be performed to elucidate the physical mechanisms. The intellectual merit of the project: This research will further understanding of the roles of inner core dynamics and their interaction with the large-scale environment in leading to the structure and intensity changes of tropical cyclones, in particular, their contributions to the formation of concentric eyewall structure and the size changes of the eye, eyewall, and the storm scale circulation. The broader impacts of the project: Research on tropical cyclone structure and intensity changes is of high priority within the U.S. Weather Research Program (USWRP), an interagency activity designed to perform research necessary to improve understanding and prediction of tropical cyclone motion, structure, and intensity. This research potentially will contribute to improved understanding of the physical processes that cause the tropical cyclone structure and intensity changes, leading to improved prediction of tropical cyclone structure and intensity by numerical models. In the process of carrying out this research, the PI will train a graduate student in the science of tropical cyclones, especially in the area of tropical cyclone modeling and numerical prediction.