The Impact of Surface Heat Fluxes Outside the Inner Core on the Rapid Intensification of Typhoon Soudelor (2015)

Typhoon Soudelor was the most destructive tropical cyclone (TC) in the western North Pacific during 2015, undergoing rapid intensification (RI) with the central minimum sea-level pressure drop of 90 hPa in 2 days based on JMA best-track data. In this study, a 1.67-km convection-permitting full-physics model simulation can well reproduce the track and intensification trends of Typhoon Soudelor, as well as the gradually-decreasing vertical wind shear and other environmental conditions.

To investigate how the surface heat fluxes outside the inner core (IC, the region inside 60 km) affect TC structure and intensification rate change, many experiments with the surface wind capped at 1 m/s in the calculation of surface latent and sensible heat flux in different radial extent are performed. It is found that the intensification rate is larger than that of the control experiment (CTRL) during RI when the surface heat fluxes are suppressed in the area 150-km (2.5 times IC size) radius away from the TC center, while the TC is significantly weaker and does not undergo RI when the surface fluxes are also suppressed at 60 to 150-km radius (1-2.5 times IC size). The result remains the same when the experiments restart at 24, 18, 12, 6 hours prior to RI of CTRL. In addition to intensity change, substantial reduction of surface fluxes outside the inner core leads to lower TC strength and smaller radius of maximum wind (RMW), indicating that the most violent winds concentrate in the inner-core region.

In the inner-core feature of each experiments, the RI cases show stronger boundary-layer inflow and upper-level updrafts (downdrafts), higher axisymmetricity and heating efficiency than that of non-RI cases in the burst time before RI of CTRL. Furthermore, the upper-level warm core develops significantly in RI cases during RI, yet there is no evident upper-level warming in non-RI cases. These experiments also suggest that the convective burst in the inner core is not a deterministic factor but plays a supporting role in rapidly-intensifying TC.