Symmetric and Asymmetric Structures of Hurricane Boundary Layer in Coupled Atmosphere–Wave–Ocean Models and Observations

It is widely accepted that air–sea interaction is one of the key factors in controlling tropical cyclone (TC) intensity. However, the physical mechanisms for connecting the upper ocean and air–sea interface with storm structure through the atmospheric boundary layer in TCs are not well understood. This study investigates the air–sea coupling processes using a fully coupled atmosphere–wave–ocean model, especially the couplinginduced asymmetry in surface winds, sea surface temperature, air–sea fluxes, and their impacts on the structure of the hurricane boundary layer (HBL). Numerical experiments of Hurricane Frances (2004) with and without coupling to an ocean model and/or a surface wave model are used to examine the impacts of the ocean and wave coupling, respectively. Model results are compared with the airborne dropsonde and surface wind measurements on board the NOAA WP-3D aircraft. The atmosphere–ocean coupling reduces the mixed-layer depth in the rear-right quadrant due to storm-induced ocean cooling, whereas the wind–wave coupling enhances boundary inflow outside the radius of maximum wind. Storm motion and deep tropospheric inflow create a significant front-to-back asymmetry in the depth of the inflow layer. These results are consistent with the dropsonde observations. The azimuthally averaged inflow layer and the mixed layer, as documented in previous studies, are not representative of the asymmetric HBL. The complex, three-dimensional asymmetric structure in both thermodynamic and dynamic properties of the HBL indicates that it would be difficult to parameterize the effects of air–sea coupling without a fully coupled model.