Abstract
It is well-recognized that tropical cyclone (TC) intensity is strongly modulated by air-sea interactions. However, how and to what extent air-sea interactions affect TC structure remains an open question. The overall goal of this study is to better understand the physical processes of the atmosphere-wave-ocean couplings and their impact on TC structure. Because the boundary layer connects the air-sea interface to TC convection, it is also important to understand how the couplings modulate boundary layer structure. In this study, coupled atmosphere-(wave)-ocean models and observations from two field programs are used in this study: Coupled Boundary Air-Sea Transfer (CBLAST, 2003-04), and Impact of Typhoons on the Ocean in the Pacific (ITOP, 2010). High-resolution numerical experiments with and without ocean and/or wave couplings are conducted for Hurricane Frances (2004), Typhoon Choiwan (2009), and Typhoon Fanapi (2010). Results show that both ocean- and wave-couplings cause significant changes in TC and TC boundary layer structures. In particular, a stable boundary layer forms over the storm-induced cold wake. Tracer and trajectory analyses in a coupled-model simulation suggest that the stable boundary layer thermodynamically suppresses convection in and downstream of the cold wake, and dynamically causes the surface wind to turn further inward. The stabilized air tends to stay in the boundary layer longer and penetrate further into the eyewall. This stabilized air then brings extra energy into the eyewall due to enhanced fluxes downstream of the cold wake. The boundary layer in a TC has been seen as a passive layer that is driven by both the TC vortex above and by the ocean underneath. This study shows that the boundary layer, when in the presence of the storm-induced cold wake, can actively influence TC structure through the formation of an internal stable boundary layer. Although the stable boundary layer is a consequence of the TC-induced cold wake that has a negative impact on TC intensity, it appears counter-intuitive that the stable boundary layer has a positive impact on TC intensity via this separate mechanism. In summary, we find that atmosphere-wave-ocean coupling affects boundary layer structure and the physical properties of the near-surface air flow in TCs, which in turn changes the convective organization and eventually affects TC structure, energetics and intensity. This indicates that atmosphere-wave-ocean coupling affects TC structure via complex physical processes. Hence it is difficult to parameterize the atmosphere-wave-ocean coupling processes in TCs without a fully coupled model.