梅姬颱風（2010）之海氣交互作用－觀測與海氣耦合模式整合分析

Abstract

When Typhoon Megi (2010) passed through the Philippine Sea and the South China Sea, distinct patterns of oceanic response and significant differences in tropical cyclone (TC) intensity were observed in the two basins. In this study, the dynamics of oceanic responses to Megi is investigated by using a full-physics coupled model based on the WRF (Weather Research and Forecasting) model and 3DPWP (Price-Weller-Pinkel) ocean model. The unprecedented atmospheric and oceanic data used in this study were obtained from aircrafts and research vessels during ITOP (Impact of Typhoons on the Ocean in the Pacific, 2010). The model results are compared with the observation from satellites, the in-situ measurement during ITOP, and the ocean analysis field from the NRL Eastern Asian Seas Ocean Nowcast/Forecast System (EASNFS) with the atmospheric forcing from our analysis and with ITOP data assimilated. The sensitivity experiments with different processes of ocean dynamics, including ocean current shear-induced entrainment, horizontal advection, vertical advection, and pressure gradient, are performed to understand the mechanisms of the TC-induced cold wake formation. The control experiments reasonably well capture Megi’s track. The simulated intensities are close to observations in the Philippine Sea while being diverse in the South China Sea, for which the closest result is taken by a three-dimensional coupled model simulation because of the significant SST cooling. The different oceanic responses in these two basins show that it is valuable to include the three-dimensional dynamic processes in the ocean especially for TCs with slow translation speed and under unfavorable upper ocean thermal structure. In the analysis of a stable boundary layer, the depth and stability of the TC boundary layer are relatively limited and enhanced by the TC-induced SST cooling respectively, and thus the inflow angle would be larger over the cold wake region. From the sensitivity experiments, potential mechanisms of oceanic response to different dynamic processes are evaluated. Strong surface wind stress associated with the TC can generate turbulence in the upper ocean, thus reducing SST through the vertical mixing of cooler water from the deeper ocean. On the other hand, the upper ocean temperature profile would be shifted upward and provides an unfavorable condition for TC intensification due to the effect of the vertical advection. In addition, the horizontal ocean temperature gradient under and around the TC center is reduced due to horizontal advection and pressure gradient terms. Nevertheless, the interpretation of the individual impact of these dynamical processes is not explicit because the nonlinear effects remain extant in the experimental approach of this study. In all, this study provides a comprehensive comparison among atmosphere-ocean coupled model simulations, satellite data, EASNFS ocean analysis, and aircraft and vessel observations during ITOP. The changes of the atmospheric and oceanic structures in correspond to the TC-ocean interactions (e.g., the formation of the cold wake and stable boundary layer, and the enhanced inflow angle) are examined with care. Also, this work shed some lights on the impact of each physical process on the TC-induced variations in the ocean thermal structure.