The Impacts of Ocean Cooling and Ocean Surface Waves on Typhoon Modeling for 2018 Typhoon JEBI

Tropical cyclones have a very close relationship with the oceans, and their passage cools the oceans and generates ocean surface waves that change the ocean conditions. This is one of the major factors determining the rapid intensification of tropical cyclones. However, it is still poorly understood due to the immaturity of models and the difficulty of observations. Typhoon modeling is often based only on meteorological models. It does not consider typhoon-induced ocean response and wave effects that control the boundary conditions between a typhoon and an ocean. In recent years, tropical cyclones in the northwestern Pacific have been predicted to intensify due to the warming ocean caused by global warming. Especially in Japan, typhoons have caused severe wind and flood damage. Understanding atmosphere-ocean interaction in extreme conditions contributes to improving intensity prediction and directly affects the assessment of coastal disasters such as storm surges and high storm waves. Therefore, highly accurate typhoon modeling is required considering typhoon-induced ocean response and wave effects that control the boundary conditions between a typhoon and an ocean.

This study aims to clarify the impacts of ocean cooling and the bulk equation of the atmosphere-ocean momentum exchange that depends on sea surface waves caused by the typhoon passage on typhoon intensity (sea level pressure and maximum wind speed) and structure. We targeted Typhoon Jebi, which caused significant coastal disasters in Japan. We use the coupled atmosphere-ocean-wave transport: COAWST Modeling System (Warnar et al., 2010) considering the impact of ocean responses and wave effects on typhoon modeling. This model consists of WRF: atmosphere, ROMS: ocean, and WW3: waves. The bulk exchange formulas in wave coupled were TAYLOR & YELLAND: TY (Taylor and Yelland, 2001), DRENNAN: DRN (Drennan et al., 2003), and OOST (Oost et al., 2005). TY is wave height and wave gradient dependent. DRN is wave height and wave age dependent. OOST is wavelength and wave age dependent.

Jebi-induced SST cooling shows that the SST decreases up to 3 degrees or more on the right side of the typhoon track, and the spatial distribution of SST in model results generally agrees with the observed results in Himawari-8. The ocean coupling weakened the intensity of Jebi in the region where the SST cooling occurred due to the decrease in thermal energy from the ocean to the typhoon. The result of Jebi intensity shows that the sea level pressures were 954.6 (WRF), 962.2 (WRF+ROMS), 962.5 (WRF+ROMS+WW3-TY), 962.1 (WRF+ROMS+WW3-DRN), 962.0 (WRF+ROMS+WW3-OOST) hPa at 00 UTC on 4 Sep (48 hours after the start of the calculation). However, in the wave-coupled case, the wave effect may suppress the intensity decrease due to differences in the bulk equation for momentum roughness. Furthermore, this affected the wind speed distribution due to the breaking down structure of the axial objectivity in the region affected by the ocean response. The wind speed distribution and intensity changes differed depending on whether the bulk equation was dependent on wave height, wavelength, wave gradient, or wave age. Therefore, it is important for intensity prediction and coastal hazard assessment to clarify the impact of ocean cooling and wave effects on typhoons.

In the future, we will conduct detailed validation compared to observations and examine the bulk formulas (including enthalpy roughness length), considering other wave effects to propose an optimal bulk formula for typhoon modeling.