Impacts of SST Patterns on Rapid Intensification of Typhoon Megi (2010)

Typhoon Megi (2010) was one of the most intense typhoons that attained the minimum central pressure below 890 hPa. To understand how the changes in sea surface temperature (SST) distribution associated with the typhoon passage influenced the intensification process of Typhoon Megi, numerical simulations were conducted by a high-resolution three-dimensional atmosphere–ocean coupled regional model, CReSS–NHOES (Aiki et al., 2015). The coupled model is composed of the Cloud Resolving Storm Simulator (CReSS; Tsuboki and Sakakibara, 2002) for the atmospheric part and the Non-Hydrostatic Ocean model for the Earth Simulator (NHOES; Aiki et al., 2006, 2011) for the oceanic part. Three sensitivity experiments were conducted with different SST representations, one using CReSS-NHOES (hereafter 3dO) and the other two using CReSS (hereafter 1dO and FO). The three-dimensional structure of ocean was only considered in the 3dO experiment. In the 1dO experiment, a simple thermal diffusion model is used to express temperature changes due to ocean vertical mixing. The effect of ocean upwelling, however, is not included. The FO experiment with a time-fixed SST does not consider the development of SST. All experiments used the same initial and boundary conditions and model specifications for the atmosphere, started at 0000 UTC 14 October 2010, and had an integration time of 9 days. The computational domain is 5°N–25.5°N and 109°E–150°E. The domain consists of 2048×1024, the horizontal grid size of all models is 0.02 longitude by 0.02 latitude.

Typhoon Megi was formed on 14 October and traveled westward in relatively large translation speed faster than 5 m s^–1 as intensifying gradually. Around 0000 UTC 17 October 2010, the storm started to intensify rapidly (hereafter, RI) and attained the minimum central pressure of 885 hPa at 1800 UTC 17 October 2010. Although all experiments represent the relatively accurate tracks over the Philippine Sea, the intensity of simulated storm differs largely among the experiments; the minimum central pressure of the storms are 892, 901, and 839 hPa in the 3dO, 1dO and FO experiments, respectively. The 3dO experiment represents the evolution and maximum intensity of Typhoon Megi, because only the coupled model successfully simulates changes in the local SST pattern associated with the typhoon passage. The simulation results showed a close relationship between the radial SST profiles and the RI process; the high SST in the eye region facilitated tall and intense updrafts inside the radius of maximum wind speed and led to RI, while high SST outside this radius induced local secondary updrafts that inhibited RI. Thus, the changes in the local SST pattern around the storm center affected the RI by modulating the radial structure of core convection.