对强台风“纳沙”（1117）登陆海南岛前后降水非对称性的分析

利用NCEP 1 °×1 °再分析资料和卫星资料，以2011年强台风“纳沙”为例，分析了“纳沙”登陆海南岛前后的降水特征，并分析了“纳沙”周围TBB、湿度、水平风速和垂直速度在其路径两侧分布的不对称性，并从空间结构的分布上讨论了降水分布的可能成因。结果表明:登陆海南岛前后，“纳沙”的降水在其路径两侧的分布呈显著的不对称性，强降水主要集中在其路径左侧。“纳沙”除温度距平的分布较对称外，其它物理量在台风周围的空间结构均表现为显著的不对称性:（1）TBB，在路径左侧的强对流云系的强度和范围均比右侧大；（2）湿度，路径左侧的湿区范围比路径右侧大；（3）水平风速，台风位于海上和登陆时，路径右侧的最大风速比左侧强，台风登陆时其左右两侧最大风速相差20 m/s；在登陆前和登陆后路径右侧的相等大风速区范围比左侧大；（4）垂直速度，路径左侧的上升运动比右侧强，尤其在台风登陆时左侧的垂直上升速度比右侧大-2.4 Pa/s。通过对比上述物理量的非对称分布与降水分布可知，湿度可能是台风降水非对称分布的原因之一，而垂直速度可能是造成“纳沙”非对称降水的主要原因。另外，从垂直风切变作用进一步探究台风降水非对称性的形成机制，结果发现“纳沙”登陆前后的强降水均集中在顺切变方向及其左侧。垂直风切变可较好地解释路径左侧的强垂直上升运动和强降水区。此外海南岛的地形条件也导致“纳沙”在登陆期间海南岛西部的降水显著增加。 

Analysis on Asymmeric Precipitation ofthe Strong Typhoon Nesat (1117) Beforeand After Landing on Hainan Island

Using the NCEP 1 °×1 ° reanalysis data, satellite data, this paper takes the strong typhoon Nesat in 2011 for example to analyze the precipitation characteristics of the typhoon before and after landing on Hainan Island, and the asymmetry characteristics of TBB, humidity, horizontal wind speed and vertical velocity surrounding the typhoon on the left and right sides of the path, and discuss the possible causes of precipitation distribution from the aspect of the distribution of spatial structure. The results are showed as follows. Before and after landing on Hainan Island, the precipitation distribution of typhoon Nesat showed significant asymmetry, and heavy rainfall mainly concentrated on the left side of the path. In addition to relatively symmetricaltemperature anomalies, other physical quantities surrounding the typhoon showed significant asymmetry in the following points: (1) TBB. The intensity of strong convective clouds was greater, its range on the left side was larger than on the right side; (2) Moisture. The range of wet area on the left side was larger than on the right side; (3) Horizontal wind field. The maximum wind speed on the right side were stronger than on the left side when the typhoon was at sea and landed, especially when the typhoon landed maximum wind speed difference between the sides was up to 20m/s; the range of equal maximum wind speed on the right side was wider than on the left side before and after the landing of the typhoon; (4) Vertical velocity. Upward motion on the left side was stronger than on the right side, especially vertical rising velocity on the left side was -2.4 Pa/s stronger than on the right side when the typhoon landed. By comparing the distribution of the above physicswith the distribution of precipitation, we found that humidity may be one reason for the asymmetric distribution of precipitation, and the vertical speed may be the main reason for asymmetric precipitation of typhoon Nesat. From the vertical wind shear, we further explored the role of the formation mechanism of asymmetric typhoon precipitation and found that the heavy rainfall was concentrated in the shear direction and along the left side before and after the typhoon landed on Hainan Island. Vertical wind shear can better explain the strong vertical upward movement and heavy precipitation area on the left side of the path. In addition, Hainan Island terrain conditions also led to a significant increase in typhoon precipitation in the western part of theisland during the landing of typhoon Nesat.