The accuracy of typhoon track prediction has improved in recent years, whereas that of typhoon intensities prediction has not been improved yet. One of the main reasons is the difficulty of representing momentum and heat transfer mechanisms across the sea surface in typhoons.
On the sea surface under a typhoon, friction occurs between the atmosphere and the sea surface, resulting in the transfer of the typhoon’s kinetic and thermodynamic energy at the sea surface (Fig. 1). The energy transfer due to friction has a significant impact on the track and intensity of a typhoon.
In this R&D item, we aim to investigate the air-water momentum and heat transfer (flux) at high wind speeds, using the world’s largest typhoon simulation tank located at Research Institute for Applied Mechanics, Kyushu University. We also aim to formulate the momentum and heat fluxes across the sea surface at high wind speeds and elucidate the momentum and heat transfer mechanism.

Although the air-sea momentum and heat fluxes have been investigated in our previous studies, the simulation tank used there had a short fetch. Therefore, the results may not accurately represent the air-sea momentum and heat fluxes in the wide ocean field. Our goal is to clarify the momentum and heat transfer mechanism and develop the reliable model formulae applicable to wide ocean.

①More accurate measurement of momentum flux

Using the momentum budget method and profile method, we estimated the momentum flux and drag coefficient at a fetch distance of 20 m under conditions with wind speeds up to 40 m/s. Momentum budget method successfully achieved precise observations at medium to high wind speeds. Next, we examined the peak enhancement factor (γ_D, Fig 2) that accounts for wave spectra. Our results showed good agreement with previous experiments. The wind wave development was suppressed at high wind speeds. These results confirmed that changing in the wind wave development under high wind speeds are related to regime shifts in momentum transport, regardless of fetch length.

②Estimation of heat flux

Heat transfer experiments were conducted. At low wind speeds, the heat transfer coefficient C_K​ obtained was consistent with previous experiments. In contrast, at high wind speeds, the C_K were smaller compared to those from experiments conducted in smaller tanks, indicating that under high wind speed conditions, C_K depends on the fetch length of the tank.

③Measurement of scattered droplets

On water surfaces with breaking waves under high wind speeds, various droplets are scattered into the air, as illustrated in Fig 3. Such droplets are expected to influence the heat budget at the air-water interface. We precisely measured the droplet dispersion and proposed a new model that incorporates the effects of the number density of droplets.

The new model indicates that droplet dispersion from breaking surfaces could significantly affect heat transport under high wind speed conditions.