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.

① More accurate measurement of momentum flux

Using the typhoon simulation tank and the momentum budget 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. This momentum budget method successfully achieved precise observations at medium to high wind speeds. By combining this with the profile method using vertical wind speed distribution measurements obtained by Pitot tubes (Fig. 2), we proposed a more reliable model formula.

② Estimation of heat flux

Previously, the estimation of heat flux in the typhoon simulation tank was rough due to the non-uniform water temperature. To address this, we established new environment by introducing new water pump. With stabilized water temperatures, we successfully estimated the heat flux with the targeted precision.

③ Measurement of scattered droplets

Under high wind speeds with breaking waves, various sizes of droplets are scattered into the air. These droplets may be involved in the heat transfer, but this has not been sufficiently verified. Therefore, we introduced a system to measure droplets in the typhoon simulation tank and began observing the droplets (Fig. 3).