北雁荡山地形对2020年“黑格比”台风暴雨影响的数值模拟

2020年第4号台风“黑格比”在浙南登陆后过境北雁荡山期间在山区引发了特大暴雨。基于中尺度数值模式WRFV4.0.2对台风进行高分辨率数值模拟,分析北雁荡山地形对此次台风暴雨的作用,并设置了升降地形敏感性试验。结果表明:数值试验较好地模拟了台风移动及特大暴雨的落区和强度,台风大风区明显不对称分布,台风登陆后第一、四象限过境山区,其东侧强偏南气流向山区输送了充足水汽。台风登陆前山区低空存在一条由台风内核拖曳出的狭长螺旋辐合带,水汽通量辐合与风场辐合相一致。台风眼墙过境时沿着降水中心的迎风坡有强烈上升运动,动力条件极好,水汽输送带由近地面向对流层低层延展,山区有零星对流单体触发加强。台风后部环流影响时在高海拔山区风速减弱、绕流激发了中尺度低涡,强降水中心迎风坡上出现持续性、停滞不动的强正涡度中心,是特大暴雨发生的主要原因。地形敏感性试验中无地形时降水减幅40%~50%,地形高度翻倍降水增幅超过60%。

Numerical Simulation of North Yandang Mountain Topography Influence on Typhoon Hagupit Rainstorm in 2020

Typhoon Hagupit (No.04,2020) caused torrential rainfall in the mountain region as it passed over North Yandang Mountain after making landfall in the south coastal areas of Zhejiang Province. Based on the Weather Research and Forecast (WRF) version 4.0.2 mesoscale numerical model, a high-resolution numerical simulation is conducted on typhoon Hagupit to analyse the effect of North Yandang Mountain on the occurrence of this heavy rainstorm caused by the typhoon. Sensitivity experiments, which involve adjusting the terrain height by lifting or reducing it, are carried out to investigate the role of terrain. The results are as follows: The track and intensity of the typhoon were well simulated by the numerical experiment over time. The simulated torrential rain triggered by the typhoon was consistent with the observation, including the rainfall area and intensity. The distribution of the strong wind centre caused by typhoon Hagupit was noticeably asymmetric. The first and fourth quadrants of typhoon Hagupit successively passed over the mountain region after the typhoon made landfall, which transported sufficient water vapour to the mountain region through a strong southerly jet on the east side of the typhoon. A long and narrow spiralling convergence band was observed at low altitudes above the mountain region, which was brought from the inner core of typhoon Hagupit before the landfall. The location and distribution of the water vapour flux convergence band were consistent with the wind convergence band. Strong upward motion occurred on the windward slope along the heavy rain centres when the eyewall of the typhoon passed over, indicating favourable dynamic conditions for the formation of torrential rain. The water vapour transport band extended upward from the near surface into the lower atmosphere, and scattered convective cells started to be triggered and enhance in the mountain region simultaneously. As the back of typhoon Hagupit passed over, the winds weakened in the high altitude area of the mountain region, resulting in flow around and the formation of mesoscale vortices, which contributed to the heavy rainstorm. The primary reason for the appearance of torrential rainfall was the presence of a strong, persistent, and fixed positive vorticity centre on the windward slope of the heavy rain centre. In sensitive experiments involving the terrain height, it was observed that the accumulated precipitation decreased by 40% to 50% when the terrain height in North Yandang Mountain was set to zero, while it increased by over 60% when the terrain height was doubled in the mountain region.