Relation of Convective Bursts to Changes in the Intensity of Typhoon Lionrock (2016) during the Decay Phase Simulated by an Atmosphere-Wave-Ocean Coupled Model

抄録

 Typhoon Lionrock (2016) made landfall in the Pacific side of northern Japan. One of the intriguing events was the occurrence of consecutive deep convections (convective bursts: CBs) before making landfall on 31 August. Lionrock paused the decay of the intensity although sea surface cooling (SSC) was induced distinctly by Lionrock along the track. To examine the influence of CBs on changes in the storm intensity during the decay phase, numerical simulations were conducted with a 3-km-mesh coupled atmosphere-wave-ocean model. The coupled model successfully simulated the occurrence of CBs north of the near-surface convergence area, which was formed by the confluent of the storm’s tangential winds with near-surface frictional spiral inflow from the surrounding region where the significant wave height was high, while maintaining the relatively fast translation and the asymmetric TC structure. Lower tropospheric horizontal moisture fluxes became enhanced around the convergence area although SSC resulted in reduction in air-sea latent heat fluxes within the storm’s inner core. Local occurrences of upward moisture fluxes associated with CBs led to increases in mid-to-upper tropospheric condensational heating on the upstream side. This resulted locally in increases of lower-tropospheric pressure gradient forces on the upstream side. This helped pause the decay of the simulated storm intensity even during the decay phase. Sensitivity experiments regarding the effect of ocean coupling demonstrated that the vertical moisture fluxes and the number of CBs could increase around the surface frictional convergence area ahead of the storm when no coupled model was used. This suggests that the storm in mid-latitude could rapidly increase maximum surface wind speeds locally under a favorable oceanic condition. The number and distribution of CBs are indeed sensitive to oceanic conditions and are considered to affect the storm-track simulation and maximum surface winds.