NUMERICAL SIMULATION ON RE-INTENSIFICATION OF TROPICAL REMNANT RE-ENTERING THE SEA: A CASE STUDY

When Typhoon Toraji (2001) reached the Bohai Gulf during 1-2 August 2001, a heavy rainfall event occurred over Shandong province in China along the gulf. The Advanced Research version of the Weather Research and Forecast (WRF-ARW) model was used to explore possible effects of environmental factors, including effects of moisture transportation, upper-level trough interaction with potential vorticity anomalies, tropical cyclone (TC) remnant circulation, and TC boundary-layer process on the re-intensification of Typhoon Toraji, which re-entered the Bohai Gulf after having made a landfall. The National Centers for Environmental Prediction (NCEP) global final (FNL) analysis provided both the initial and lateral boundary conditions for the WRF-ARW model. The model was initialized at 1200 UTC on 31 July and integrated until 1200 UTC on 3 August 2001, during which Toraji remnant experienced the extratropical transition and re-intensification. Five numerical experiments were performed in this study, including one control and four sensitivity experiments. In the control experiment, the simulated typhoon had a track and intensity change similar to those observed. The results from three sensitivity experiments showed that the moisture transfer by a southwesterly lower-level jet, a low vortex to the northeast of China, and the presence of Typhoon Toraji all played important roles in simulated heavy rainfall over Shandong and remnant re-intensification over the sea, which are consistent with the observation. One of the tests illustrated that the local boundary layer forcing played a secondary role in the TC intensity change over the sea.