Numerical Simulations of Near-Cloud Turbulence in the Upper-Level Outflows of Tropical Cyclones in the Northwestern Pacific Ocean

Various mechanisms of near-cloud turbulence (NCT) events in the outflow of tropical cyclone (TC) in the northwestern Pacific Ocean were investigated using the Weather Research and Forecasting (WRF) model with the finest horizontal grid spacing of 300 m. There were several light-to-moderate turbulence encounters reported by commercial aircraft across North Pacific Ocean from East Asia to North America during two different TC periods (typhoons Hagibis and Faxai in 2019). Locations of most turbulence events were several hundred kilometers away from the center of the TCs and located in the northwestern side of the TCs. In each simulation, Mellor-Yamada-Janjić (MYJ) scheme was used to parameterize local vertical mixings [i.e., subgrid-scale turbulent kinetic energy (SGS TKE)] in free atmosphere, and SGS TKE tendency was analyzed using the TKE budget equation. In the case of typhoon Hagibis, SGS TKE larger than 0.5 m² s^-2 was simulated especially at the bottom of cirrus anvil clouds mainly due to the Kelvin-Helmholtz instability induced by strong vertical wind shear. At the incident location and time (0850 UTC 11 in October), SGS TKE increases drastically due to localized convective instability. This area was also consistent with the region of strengthened inertial instability mainly developed by the strong anticyclonic outflow of the TC, as it moved northward. This confirms that the enhanced inertial instability played an important role in producing convective instability and localized strong SGS TKE directly affecting cruising aircraft. In addition, non-zero TKE at the top of cirrus anvil cloud was found, which was attributed to convective instability induced by differential thermal advection between above and in the outflow of TC. In the case of typhoon Faxai, simulated SGS TKE larger than 0.25 m²s^-2 appeared right above the leading edge of convection within the outflow layer. It showed that localized overturnings of isentropes with convective instability developed above the isolated convective clouds in the northern side of the TC was collocated with observed NCT events. More detailed results will be presented in the conference.

Acknowledgement: This research was performed by the National Research Foundation of Korea Research and Development Program (NRF-2019R1I1A2A1060035), and the Korea Meteorological Administration Research and Development Program under Grant KMI2022-00310.