4:15 PM - 4:30 PM
[AAS06-08] Vortex Structure in Rapid Intensification of Numerically Simulated Typhoon Nanmadol (2022)
Keywords:Typhoon, Mesoscale meteorology, Numerical modeling, Atmospheric boundary layer
Recent advances in satellite observation networks, numerical models, and assimilation techniques have improved the track forecast of tropical cyclones (TCs). On the other hand, the intensity forecast of TCs still needs to be improved. In particular, the rapid intensification (RI) mechanism of TCs, which causes a decrease in the central pressure and an increase in the maximum wind speed at a short time scale of one day, needs to be understood. One of the characteristics observed in the RI period of TCs is the contraction of the eyewall with an increase in the maximum wind speed. In this study, a cloud-resolving model is used to clarify the wind speed increase and the eyewall contraction processes in Typhoon Nanmadol (2022) which experienced the RI, and the vortex structure in the simulated typhoon is validated with aircraft observations.
The numerical model used in this study is the Cloud-Resolving Storm Simulator model which includes the cloud microphysics, turbulence, atmospheric-cloud radiation, and surface exchange processes. The cumulus parameterization is not used. The experimental design has a horizontal resolution of 2 km and 96 vertical layers. The vertical grids are aligned with stretching so that the vertical resolution becomes finer in the lower layer. The altitude of the lowest layer is 100 m and the model-top height is 38 km. The integration period is from 0600 UTC on 14 September to 0000 UTC on 18 September 2022 before the RI in Nanmadol. An aircraft of the Tropical cyclones-Pacific Asian Research Campaign for Improvement of Intensity estimations/forecasts penetrated the typhoon center in the daytimes of the RI stage on 16 September and the mature stage on 17 September. In this study, the dropsonde data in the aircraft observations are used.
The track and central pressure of the typhoon in the numerical experiment were in good agreement with the best track of the Japan Meteorological Agency. In the simulated typhoon, the radius of the maximum wind speed (RMW) decreased from 70 km to 30 km and the maximum wind speed increased from 40 m s-1 to 60 m s-1 during the RI. The evolution of the simulated vortex structure was similar to the dropsonde observations during the RI. In the numerical experiment and observations, the height of the maximum tangential wind (HMW) of the typhoon in the mature stage was located at 1 km or less.
To understand the contraction process of the RMW, the tangential wind budget analysis was carried out. The results showed that the net positive tendency of the tangential wind was exhibited in the inside of the RMW during the RI stage, and it was contributed by the inward transport of angular momentum by the strong axisymmetric inflow below the HMW and the upward transportation of the angular momentum by the asymmetric updrafts above the HMW. Corresponding to the tangential wind tendency, the supergradient wind of 5 m s-1 or more was always observed inside the RMW in the eyewall contraction. The strong supergradient wind corresponds to the horizontal convergence and upward motions through the pumping in the boundary layers. The persistent supergradient wind inside the RMW is linked to the long-lasting contraction of the eyewall and the RMW. The radius of the supergradient wind maximum and the RMW almost overlapped and the contraction ceased as the typhoon intensity approached the mature stage. This result suggests that the eyewall contraction requires (1) the arrival of the boundary layer inflow to the inside of the RMW and (2) the asymmetric upward motions that transport angular momentum to the free atmosphere. In future works, we will investigate how these factors are determined.
The numerical model used in this study is the Cloud-Resolving Storm Simulator model which includes the cloud microphysics, turbulence, atmospheric-cloud radiation, and surface exchange processes. The cumulus parameterization is not used. The experimental design has a horizontal resolution of 2 km and 96 vertical layers. The vertical grids are aligned with stretching so that the vertical resolution becomes finer in the lower layer. The altitude of the lowest layer is 100 m and the model-top height is 38 km. The integration period is from 0600 UTC on 14 September to 0000 UTC on 18 September 2022 before the RI in Nanmadol. An aircraft of the Tropical cyclones-Pacific Asian Research Campaign for Improvement of Intensity estimations/forecasts penetrated the typhoon center in the daytimes of the RI stage on 16 September and the mature stage on 17 September. In this study, the dropsonde data in the aircraft observations are used.
The track and central pressure of the typhoon in the numerical experiment were in good agreement with the best track of the Japan Meteorological Agency. In the simulated typhoon, the radius of the maximum wind speed (RMW) decreased from 70 km to 30 km and the maximum wind speed increased from 40 m s-1 to 60 m s-1 during the RI. The evolution of the simulated vortex structure was similar to the dropsonde observations during the RI. In the numerical experiment and observations, the height of the maximum tangential wind (HMW) of the typhoon in the mature stage was located at 1 km or less.
To understand the contraction process of the RMW, the tangential wind budget analysis was carried out. The results showed that the net positive tendency of the tangential wind was exhibited in the inside of the RMW during the RI stage, and it was contributed by the inward transport of angular momentum by the strong axisymmetric inflow below the HMW and the upward transportation of the angular momentum by the asymmetric updrafts above the HMW. Corresponding to the tangential wind tendency, the supergradient wind of 5 m s-1 or more was always observed inside the RMW in the eyewall contraction. The strong supergradient wind corresponds to the horizontal convergence and upward motions through the pumping in the boundary layers. The persistent supergradient wind inside the RMW is linked to the long-lasting contraction of the eyewall and the RMW. The radius of the supergradient wind maximum and the RMW almost overlapped and the contraction ceased as the typhoon intensity approached the mature stage. This result suggests that the eyewall contraction requires (1) the arrival of the boundary layer inflow to the inside of the RMW and (2) the asymmetric upward motions that transport angular momentum to the free atmosphere. In future works, we will investigate how these factors are determined.