基于ERA-5高分辨率资料的高原低涡客观识别方法研究

Study on an Objective Identification Method for Qinghai-Xizang Plateau Vortices Based on ERA-5 High-Resolution Data

利用ERA5的500 hPa逐小时再分析资料(0.25°×0.25°),以高原低涡的天气学特征为依据,设计研究基于高时空分辨率网格资料的高原低涡客观识别算法,并对1990~2019年暖季的青藏高原低涡进行识别,建立高原低涡数据库。该算法通过客观标准提取低涡特征点、并采取DBSCAN算法(一种基于密度连通性质进行聚类的方法)进行特征点聚类分析,以相邻时次之间低涡的重合状况、临近程度和既往生命时长为依据对低涡进行追踪匹配,得到合理的低涡路径,将孤立的低涡连接成动态发展的低涡过程。利用《青藏高原低涡切变线年鉴》(简称《年鉴》),针对2017年暖季(5~9月)低涡对比分析与本文的客观识别结果的异同,结果表明:客观识别低涡的月分布特征以及移出高原低涡的占比与《年鉴》相似,典型的长生命低涡能被正确识别,可见客观识别算法及其数据库具备一定的可靠性和实用性。但客观识别的低涡总数量多于《年鉴》,并且源地为“西部型”的低涡也较多。究其原因,更精细的网格尺度能将低涡在更早的生命时期提取出来,可能会将《年鉴》中的“东部型”低涡进一步追溯到更靠西的位置。此外由于青藏高原西北部观测站点较为匮乏,导致生成于高原西北部的低涡在《年鉴》中未能体现出来,也是低涡数量存在差异的重要原因。

Using ERA-5500 hPa hourly reanalysis data(0.25°×0.25°)and based on the synoptic characteristics of the Qinghai-Xizang Plateau vortex(QXPV),an objective identification algorithm based on grid data with high spatial and temporal resolution was developed to determine the QXPV during the warm season from 1990 to 2019 and establish the QXPV database.The algorithm extracted the QXPV feature points by objective criteria,and afterward,the DBSCAN(Density-Based Spatial Clustering of Applications with Noise)algorithm was adopted for feature point clustering.According to the degree of overlap and distance between QXPV occurring in adjacent time and the lifespan,a reasonable path of the QXPV was acquired to connect the isolated QXPV into a dynamic vortex process.Subsequently,this study compared the QXPV that occurs in the warm season of 2017(May–September)identified by the QXPV and Shear Line Yearbooks(the Yearbook)and the objective identification.The findings reveal that the monthly distribution characteristics of the objective identificated QXPVs(OI-QXPV)and the QXPV proportion moving out of the plateau are similar to those in the Yearbook.Moreover,the usual long-life QXPV can be correctly identified,demonstrating that the objective identification algorithm and its database have certain reliability and practicability.However,the total number of OI-QXPV is more than that in the Yearbook,and more OI-QXPVs are arising from the western part of the plateau than in the Yearbook QXPVs(YB-QXPV).This is because the vortex can be extracted from earlier lifetimes due to a finer grid scale,making it possible to trace the“eastern type”vortex in the Yearbook further to the west.Moreover,due to the lack of observation stations in the northwest part of the Qinghai-Xizang Plateau,the QXPVs produced in this area are not reflected in the Yearbook,which is another crucial reason for the difference in the number of QXPVs.