Physical Basis And Numerical Study Of The Coupled Atmosphere-Wave Model

Understanding the processes and mechanisms of air-sea momentum, heat and mass transfers is essential to the study of air-sea interaction, and has taken on community interests in the past. As ubiquitous phenomena on the sea surface, surface waves can influence air-sea momentum, heat and mass exchanges through sea-state-dependent surface roughness. Meanwhile, sea spray produced by bursting bubbles in whitecaps and wind tearing of breaking wave crests can modify transfers of momentum, heat and mass across the air-sea interface directly or indirectly. Reliable understanding and parameterizing the impacts of wave state and sea spray on air-sea momentum and heat fluxes is crucial to air-sea interaction research, and is the physical basis of the coupled atmosphere-wave model.Based on previous studies and measurements, a parameterization of sea surface areodynamic roughness, which adapts to from low-to-moderate wind conditions to extremely-high wind conditions, is given by taking wave state and sea spray effects into account. The relationship between drag coefficient and wind speed corresponding to the new proposed parameterization agrees well with the existing field and laboratory observational data. According to this relation, under low-to-moderate wind conditions when the sea spray effects can be neglected, the nondimensional areodynamic roughness first increases and then decreases as the wave age increases. While under high wind conditions the drag coefficient decreases with increasing wind speed because of the modification of logarithmic wind profile by spray droplets. In addition, drag coefficients corresponding to different wave states reach their maximum value when the wind speed is between 25 to 33 m s-1.Parameterization of sea surface scalar roughness under low-to-moderate wind is reviewed, and dissipative heating and sea spray effects on air-sea heat flux are discussed in this paper. Taking into account the effect of wave state on whitecap coverage, a sea spray generation function (SSGF) for bubble-derived droplets is presented. Combined it with the wave-state-dependent SSGF for spume droplets (Zhao et al., 2006), a SSGF applicable to both bubble-derived and spume droplets with wave state effects included is obtained. Applying this SSGF to Andreas (1992)'s method for estimating sea spray heat flux and considering the thermodynamical feedback of sea spray, an algorithm to estimate wave state affected sea spray heat flux is accomplished. Given the atmospheric and oceanic environment, sea spray heat flux estimated by this algorithm increases with wind speed, wave age and windsea Reynolds number as well. Based on the discussion of air-sea momentum and heat fluxes, the next generation Weather Research and Forecasting (WRF) model is coupled with the third generation wave model WAVEWATCH III to establish a coupled atmosphere-wave system. In order to investigate the impacts of different sea-state-dependent roughness, dissipative heating and sea spray heat flux on typhoon system, the coupled model is applied to an idealized typhoon. It is found that sea-state-dependent roughness increases sea surface friction, reduces typhoon intensity and surface wave height. Whereas the impacts of different sea-state-dependent areodynamic roughness parameterizations on air-sea heat flux and typhoon track are insignificant. When using the new presented wave state and sea spray affected areodynamic roughness parameterization, the minimum central pressure for the coupled simulation is reduced by about 5% relative to the uncoupled simulation, and the simulated maximum significant wave height is also reduced by about 6%. The inclusion of dissipative heating increases the air-sea heat flux, thus intensifies the typhoon system, leading to an 8-10% increase of minimum central pressure, a 6-10% increase of maximum wind speed at 10-m height and a 10-15% increase of maximum significant wave height. Incorporating sea spray heat flux substantially strengthens the typhoon system, and increases surface wind and wave height. It also influences air-sea heat fluxes directly, and significantly increases the air-sea latent heat and water vapor fluxes. In addition, it should be pointed out that the impacts of sea-state-dependent sea surface roughness, dissipative heating and sea spray heat flux on the typhoon system are not isolated, but dependent upon each other. With all the three effects included, the minimum central pressure in the coupled simulation is 10.7 hPa deeper than in the uncoupled simulation, corresponding to a 14% increase of the intensity of typhoon system. The simulated maximum wind speed and significant wave height increase by 18% and 4% respectively. The simulated total air-sea heat flux for the coupled simulation also increases about 10%, while the air-sea latent heat flux increases more significantly by about 18%.The coupled atmosphere-wave system is further used in the simulation of Typhoon Matsa (2005). The effects on the typhoon system of the coupling between atmosphere and wave models are investigated through the analyses and comparisons of the results of the coupled and control simulations. It is shown that, considering the impacts of sea state, dissipative heating and sea spray heat flux, the coupled experiment simulates a stronger typhoon whose central pressure and maximum sustained wind agree better with the data in the best track from RSMC. In addition, comparison with the observed significant wave heights of Jason-1 altimeter indicates that the significant wave heights simulated by the coupled experiment are better consistent with the observations along the satellite tracks.