An Observational And Simulation Study On Precipitation Associated With The Landfalling Typhoon KROSA(0716)

Based on the in situ observations, radar and Japanese reanalysis data, the influence of orography and cold air on the severe precipitation during the landfall of Typhoon KROSA (0716) was analyzed. The results show that the coastal mountains obviously increased and blocked the typhoon precipitation. There is a significant positive correlation between the precipitation and the height of the mountains. Due to KROSA being intense, the heaviest precipitation is concentrated mainly in the crests and surrounding regions of the mountains. The stronger the winds were, the stronger the vertical velocity forced by the orography became. Then falling-down velocity of raindrops was reduced and the precipitating areas were drifted downstream. In addition, the stronger winds would cause the longer drifting distance. Cold air was advected toward Zhejiang and Fujian and swirled into the inner circulation of the typhoon from the southwest before KROSA made landfall. The existence of the cold air reduced the precipitation in Jiangshu and Fujian. Some weak cold air at low levels met the warm air in the vicinity of Zhejiang, Jiangshu and Anhui, which generated several intense convective rainbands and caused heavy precipitation in Hangzhou.A numerical simulation (control experiment) was performed to further investigate the influence of the interaction of the typhoon circulation with the environment on the precipitation. The results showed that the simulated storm track and precipitation distributions successfully. Furthermore, the activity of cold air is found to be consistent with the observation. The cold air involved into the outer circulation of the typhoon from the low levels met the warm air from the south-east and formed a front associated with the rainbands over Hangzhou. The pattern of mean precipitation in the orographic area along the coast of Zhejiang consisted with that upstream, which suggested that the variety of the orographic precipitation in the coast of Zhejiang was significantly affected by the precipitation derived from the rainbands. The vertical velocity forced by the orography corresponded to the simulated vertical velocity at 1-km level, indicating that the lower-level upward motions were dominated by the orographic force. The lifting condensation was located on the windward slop, the generated radar echoes were drifted to the crest and even leeside. Descent on the leeside caused adiabatic heating and reduced relative humidity, which made weak echoes on the leeside.Two sensitive experiments were conducted, where the orography of Zhejiang was removed and reduced half, respectively. The results showed that the modeled storm tracks and strengths were not sensitive to the orography, while the precipitation varied significantly. As the orography was removed, the precipitation over Zhejiang became more uniform and less heavy, and the orographic precipitation almost disappeared. When the orography over Zhejiang was reduced half, the orographic precipitation still occurred, but with less magnitude compared with the control experiment. The two experiments showed the importance of the orography for the typhoon precipitation when a typhoon makes landfall. A comparison of the precipitation over sea areas indicates that the activity of rainbands over sea areas is less sensitive to the oragraphy.