Coupled Atmosphere-Wave-Ocean Parameterization for High-Wind Conditions

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 intensity and to improve coupling parameterization in the extreme high-wind conditions. To resolve the hurricane inner core structure, crucial for intensity forecasting, the model horizontal resolution needs to be at 1-2 km. In this study we have developed a moving-nests modeling system in the PSU/NCAR MM5, which allow us to conduct 5-6 day long simulations at grid spacing of 1.67 km. 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 2002 and 2003. The enthalpy and momentum exchange coefficients under the high-wind conditions are difficult to determine. The stress is supported mainly by waves in the wavelength range of 0.1-10 m, which are unresolved by wave models. We begin by examing key parameterizations including effects of the wave spectral tail on drag coefficients, the source term for sea spray, and subgrid-scale turbulence property at 1-2 km resolution. The components of the coupled model system are the PSU/NCAR MM5, WAVEWATCH III, and the University of Miami HYCOM. Coupled model simulations of the Hurricanes Fabian and Isabel (2003) are compared with observations of surface wave spectra, surface fluxes, and vertical profiles of atmospheric boundary layer and ocean mixed layer from the CBLAST field program in 2003. Remotely sensed SST, surface winds from the tandem scatterometer missions (SeaWinds and QuikSCAT), and rainfall from TRMM TMI and PR are used to evaluate and validate model simulations, develop parameterizations of the air-sea interface, and initialize the models over the open ocean.