A mesoscale atmosphere ocean wave coupling model and numerical simulations on a strong typhoon process in South China Sea.

The exchange of heat, momentum, and moisture between air and sea has long been recognized as a fundamental process in the development of mesoscale atmospheric phenomena such as tropical cyclones, boundary layer jets, and heavy precipitating systems. The air-sea exchange takes place at the air sea interface in the wave boundary layer, so ocean waves play potential impacts on the air-sea interaction process. The mesoscale atmospheric model MM5(V3) and the thirdgeneration of ocean wave model Wave Watch Ⅲ are coupled each other in a two way coupling manner using a Linux IPC (interprocess communication) technique. The coupled model is used to simulate a strong typhoon process in the South China Sea and the interaction between atmosphere and ocean waves is studied. The two models are coupled using the parameterization scheme of sea surface roughness proposed by Smith in 1992. In the parameterization scheme, the sea surface roughness length depends on the wave age, which is calculated in the ocean wave model. In the coupling model run, the atmospheric model provides the sea surface wind to drive the ocean wave model, and the calculated wave age is inputted to the atmospheric model to calculate the sea surface roughness using the special PBL module. In this way, the two models are coupled each other and output both atmospheric and ocean wave parameters simultaneously. Three different experimental schemes are designed in this paper to valuate the function of the coupled model, which are named control, one way and two way coupling runs, respectively. The control run is an ordinary atmospheric model without any oceanic feedback, and the oneway coupling considers the driving wind field from the atmospheric model and its impact on the ocean waves only. The two-way coupling runs the ocean wave model and atmospheric model simultaneously using the coupling scheme mentioned above. In the coupling run, the process of typhoon is simulated by the coupled model and the exchange time step is 15 minutes. The experiment results show that the coupled model shows very good capability in simulating the typhoon process in the South China Sea. The typhoon simulated by the coupled model is stronger than that by the uncoupled atmospheric model and much more agrees with the observations. The tracks of typhoon simulated by the coupled and uncoupled models show little difference. The simulated sea surface heat flux of coupled model increases significantly in the welldeveloped stage of the typhoon, it is higher than that of uncoupled atmospheric model. In the spiral rainfall belt, particularly to the right section of the typhoon track, the simulated precipitation in the coupled model is stronger than that in the uncoupled model. The coupled model behaves very well in simulating the ocean wave distribution of the typhoon system, particularly for significant wave heights. With the feedback of ocean waves, the simulated significant wave heights are higher and in closer agreement with observations. In view of the dependence of sea surface roughness on ocean wave age, the ocean waves affect both dynamical and thermal processes. In this test, the feedback of ocean waves is more effective in the dynamic interaction processes by momentum fluxes in the initial stage of typhoon development, which reduces the intensity of the typhoon system. When the system is well developed, the feedback mechanism is more effective in the thermal interaction processes by heat fluxes, which intensifies the system.