Effects of Atmosphere-Wave-Ocean Coupling on Tropical Cyclone Structure

It has been known that air-sea interaction plays an important role in determining tropical cyclone (TC) intensity. Warm sea surface temperature (SST) and deep ocean mixed layer are favorable conditions for TC development and intensification. On the other hand, TC-induced SST cooling represents a negative feedback that is a self-limiting factor on TC intensity. Recent studies have shown that TC-induced surface waves have a major impact on air-sea momentum, heat, and moisture fluxes, which are highly complex and asymmetric around the storm. The question of how these complex air-sea interactions affect TC structure and intensity remains to be unresolved. This study aims to better understand the asymmetry in air-sea fluxes and its impact on storm structure using the University of Miami fully coupled atmosphere-wave-ocean model (UMCM, Chen et al. 2007, 2009). The components of the coupled model consist of MM5, WRF, WAVEWATCH III, and 3DPWP.

Coupled model simulations of Hurricane Frances (2004) and Typhoon Jangmi (2008) are used in this study. Observations from the Coupled Boundary Layer Air-Sea Transfer (CBLAST) and Tropical Cyclone Structure (TCS-08) are used for evaluating model results in both cases, respectively. To isolate the effect of the air-sea coupling, uncoupled atmosphere, coupled atmosphere-ocean, and fully coupled atmosphere-wave-ocean simulations are conducted. One of the most distinct features of the coupled model simulations is that the air-sea fluxes are highly asymmetric in TCs, similar to that observed. Although the TC-induced air-sea fluxes as well as the hurricane boundary layer (HBL) are asymmetric, the TC vortex and convective inner core are quite symmetric during the intensification stage of both storms. It indicates that the effects of the air-sea fluxes on convection in TCs are non-local. To understand the physical processes of air-sea fluxes on the HBL, deep convection, and the TC vortex structure, a combined trajectory and tracer analysis is used to quantify the linkage from the air-sea interface to the HBL and troposphere in TCs. These results will provide some insights not only on the energetics in TCs, but also the physical processes of air-sea coupling and their impact on TC structure and intensity.