【作者】 周玲丽；

【导师】 徐世浙； 翟国庆；

【作者基本信息】 浙江大学 ， 气象学， 2009， 博士

【摘要】 我国是一个多台风的国家,而浙江省又是我国每年遭受台风灾害最严重的沿海省份之一。为了能深入了解严重影响浙江地区的强台风短时暴雨天气的发生发展机制以及在发生演变过程中的影响因子,揭示暴雨的地面启动机制,并为精细化预报提供科学依据,本文利用中尺度数值模式WRF（Weather Rearch Forecast）对近年来严重影响浙江省的强台风短时暴雨个例进行模拟,结合WRF-Var三维变分系统每6小时一次将常规观测资料、雷达反射率和径向速度资料同化到模式中,用以改进模拟结果。对比多种方案的模拟结果和实况验证表明,WRF能够较好地模拟出台风中尺度短时暴雨过程,对台风路径、台风强度变化和降水的模拟基本符合实况,能够反映台风实际的演变过程;同化实况资料能在一定程度上明显改进模拟效果,是提高模拟精度的一种有效途径。利用模式输出的高时空分辨率结果进行诊断分析发现,暴雨发生前低层有气流的汇合,高层有气流的辐散,有利于上升运动的维持和加强,为暴雨的发生提供了有利的环境条件;暴雨区的水汽来源为台风东北象限的低空东南风急流,当急流靠近暴雨区时,能直接将水汽源源不断地输送而来,但当急流距离较远时,水汽会经由其他气流输送到暴雨区。在暴雨发生期间近地面边界层中会出现中-β尺度强辐合带,同时在实况雷达回波上显示为强回波带,沿着辐合带在下一小时会出现强降水,雨带的分布、移动和发展随辐合带变化而出现相应变化。不同台风中的中尺度强辐合带的发展高度各不相同,但辐合带上的强辐合中心区域都分布在近地面和边界层顶之间,辐合在垂直方向上有时会随高度向台风外围方向倾斜。强辐合中心顶部有低空急流,辐合区内有气流汇合上升。研究还发现,辐合在垂直方向上的发展和分布是由辐合在垂直方向上的轴线决定的,辐合带在垂直方向上沿着这条轴线分布,辐合带的发展高度受垂直轴线高度的影响和制约。辐合的垂直轴线出现在水平风梯度小于-1.2×10^-4s^-1的区域内。辐合垂直轴线一般在近地面开始出现,随后便向上发展。在向上发展的过程中,可能会出现向台风环流外围方向倾斜或水平转向的特点,有时则会基本保持直立状态;有时辐合垂直轴线到达边界层顶高度后,便会停止发展,而有时则会继续向上发展到对流层中低层。产生辐合垂直轴线的原因主要有两种:一种是由不同方向气流分量之间直接相向汇合形成的,另一种是由于同方向气流的风速递减导致空气质点堆积,由此产生辐合。汇合气流基本都来自台风环流中的低空东南风急流。研究结果还显示,台风短时暴雨的触发机制主要三种情况:一种是“高能舌”型。这种类型的热力能量场由于有弱湿冷空气的侵入,会在暴雨区边界层内出现向上凸起的相当位温高值区,暖湿空气在辐合的作用下产生上升运动,快速释放出水汽和能量,触发对流的发生;同时冷空气与下层暖湿空气在边界层内形成“上冷下暖”的不稳定层结,辐合的形成能触层结中不稳定能量的释放,进一步促进对流天气的发展。另一种是“能量锋”型。这是由于不同属性的较强的干冷和暖湿气流在暴雨区汇合,产生很高的相当位温梯度,从而在暴雨区出现能量锋区。在能量锋区的驱动作用下,暖湿空气沿着锋面爬升并释放能量和水汽,触发对流的发生。产生降水后气流冷却下沉达到近地面,一方面加强了地面冷空气,使能量锋区得到维持;另一方面气流下层后流回到上升区,与上升的暖湿气流构成中尺度垂直环流圈,并在边界层内形成强辐合,进一步促进气流的上升运动,对对流运动起到正反馈作用。第三种类型是“强迫抬升”型。在这种情况下,边界层内都是具有高相当位温的暖湿气流,属性单一,层结稳定,虽然蕴含有丰富的水汽和能量,但是没有外力的作用下,不容易触发对流天气。因此,只有当暴雨区内出现较强的辐合时,才能提供一个强冲击力,使暖湿气流被迫抬升,触发对流的发生,开始产生降水。当气流上升到边界层以上时,由于产生降水释放出凝结潜热使该高度上的空气加热,进一步促进了上升运动的发展,同时还与对流层中层冷空气构成不稳定层结,对对流起到了一个正反馈作用,使降水出现增幅。除此之外,暴雨区强辐合带上还都对应有强的正相对涡度区,涡度正中心总是分布在近地面强辐合中心顶部,涡度正值区域的范围不仅覆盖了近地面的辐合区,还一直向上延伸到了辐散中心所在的高度,为暴雨的持续提供了有利的动力条件。最后,本文总结了台风短时暴雨天气过程的气流结构模型,提取了若干个预报信息因子,建立了台风短时暴雨预报的初步模型,为这类天气的预报提供了一些科学的参考依据。更多还原

【Abstract】 Zhejiang province is one of the most-hit areas by typhoon in China every year. In order to further understand the origin and development of the short-time rainstorm process caused by landfalling typhoon impacting Zhejiang province seriously, discover the ground trigger mechanism of the rainstorm and provide some scientific references for the refined short-range forecast, the advanced research WRF modeling system is used to simulate this kind of rainstorm processes in recent years. With the WRF-Var 3-D assimilation system, conventional surface, upper air data and radar reflectivity as well as radial wind are assimilated into WRF every 6 hours. Results show that the typhoon track and intensity development and precipitation are well simulated by WRF, and the assimilation of observed data into model can to some extent improve the simulated results.Results reveals that there’s confluent stream in the low level and effluent stream in the upper level before the rainstorm happens, which provides favorable environment for the rainstorm. The water vapor comes from the low-level southeast jet stream to the northeast of typhoon center. When the jet stream is close to the rainstorm zone, the water vapor will be transferred there directly by the jet stream, while the jet stream is far away from the rainstorm zone, the water vapor will be transferred by other streams. During the rainstorm process, there’s a meso-β-scale convergence belt appearing within the planetary boundary layer which can be observed by radar. There will be strong precipitation occurring along the convergence belt in the next hour. The distribution of precipitation is similar to the convergence belt, traveling and developing with the convergence belt. The convergence belt develops upward and inclines to the outside of typhoon, but the height is not fixed. However, the strongest convergence center always locates within the planetary boundary layer. On the top of the convergence center, there’s a low-level jet stream. Below the jet stream, there’s stream conflux and ascent in the convergence zone. The vertical distribution and development of convergence belt is up to the vertical convergence line, which appears in the area with horizontal wind gradient less than -1.2×10^-4 s^-1. The vertical convergence line first appears on the surface and then develops upward, either remaining upright or inclining to the outside of typhoon. Sometimes the vertical convergence line stops developing when it reaches the top of boundary layer, sometimes it keeps developing upward to the middle level of troposphere. The vertical convergence line is mainly caused by the direct confluent of streams from different directions and the velocity degression in the same stream. The streams come mainly from the low-level southeast jet stream to the northeast of typhoon center.The analysis also indicates that there’re generally three types of trigger mechanisms of the rainstorm. In the first type, there’s weak cold wet air intruding into the warm wet air zone and causing convergence, which forces the warm wet air to ascend and release vapor and energy rapidly, and finally trigger convection. Besides, the cold wet air and the warm wet air below make the air unstable in the zone. Once the convergence formed, it will also lead unstable energy to release and intestify the rainstorm. In the second type, there’s a strong cold dry stream butting up against the warm wet stream in the rainstorm zone which causes an energy front. When the convergence forms. The warm wet air will ascend along the front and release vapor and energy quickly, which can bring about convection. After the precipitation produced, the air cools down and descends to the surface, which strengthens the strong cold stream and maintains the energy front. Then desending air flows back to the ascending area and makes confluent with the warm wet air, causing convergence and vertical circulation, which could intestify the ascending air and strengthen convection. In the third type, the structure in the rainstorm zone is stable and all the air in the planetary boundary layer are warm and wet. When convergence forms, it forces the warm wet air to ascend, which starts the convection and precipitation. After the air ascends beyond the planetary boundary layer, the latent heat released by solidification warms the air, which intestifies the ascending movement and forms unstable structure, then makes the precipitation stronger. Besides, there’s strong positive relative vorticity appearing in the rainstorm zone, which will make the precipitation persistent and developing. Finally, the structure of short-time rainstorm process caused by landfalling typhoon is summarized, and some forecast factors are extracted to build a forecast model for this kind of rainstorm and provide some scientific references for future forecast.更多还原