Effects of Surface Waves and Upper Ocean on Hurricane Structure and Intensity in a Fully Coupled Model

One of the uncertainties in tropical cyclone prediction is that the air-sea interaction is the inner core region is largely unknown with very little observations. The extreme high winds, large ocean waves, and copious sea spray push the surface-exchange parameters for temperature, water vapor, and momentum into untested new regimes. The objective of this study is, through numerical simulations using a nested-grid high-resolution, coupled atmosphere-wave-ocean model, to understand the physical processes governing the storm structure and intensity. We developed a model evaluation and validation procedure using observations including both satellite remote sensing and in situ measurements from the Coupled Boundary Layer Air-Sea Transfer (CBLAST) Hurricane field programs in 2003 and 2004. The components of the coupled model system are the PSU/NCAR MM5, WAVEWATCH III, the WHOI 3DUOM and the UM HYCOM. The coupled modeling system has have tested in a number of Atlantic hurricanes including Bonnie (1998), Floyd (1999), Fabian (2003), and Frances (2004). To better understand the effects of air-sea coupling on hurricanes, we first conducted an uncoupled control simulation for each storm using MM5 only. The fully coupled simulations are compared with the observations and the uncoupled simulations. In general, the coupled model improves the simulated storm intensity over the uncoupled simulations that tend to overestimate storm intensity in all cases. It is because of the storm-induced cooling due to vertical mixing in the upper ocean in the coupled model, whereas the uncoupled MM5 uses a constant SST through out of the simulation. The storm-induced cold wake in the SST is well simulated in the coupled model compared with the coupled model compared with the satellite observations. The maximum cool is to the rear right of the storm as observed in Frances and many other storms previously. The storm-induced surface wave field is high complex and asymmetric around a hurricane. It is clearly evident in the coupled simulation of Hurricanes Floyd and Frances, in terms of both significant wave height (SWH) and wave length. The highest SWH and largest wave length are found in the front right quadrant as observed in CBLAST-Hurricane field program. These characteristics in the wave fields will produce a relatively weaker surface stress in the front right quadrant and a stronger stress in the rear left quadrant of the storm. The combined effects of the atmosphere-wave-ocean coupling give a rise in the asymmetry in the air-sea fluxes. The uncoupled MM5 simulation produced a more symmetric net heat flux compared to that of coupled model simulation. The asymmetry in surface heat flux has also been observed in CBLAST-Hurricane 2003-2004.