Establishment of an Objective Standard for the Definition of Binary Tropical Cyclones in the Western North Pacific

Abstract

To develop an objective standard for defining binary tropical cyclones (BTCs) in the western North Pacific (WNP), two best-track datasets, from the China Meteorological Administration and the Joint Typhoon Warning Center, were adopted for statistical analyses on two important characteristics of BTCs—two TCs approaching each other, and counterclockwise spinning. Based on the high consistency between the two datasets, we established an objective standard, which includes a main standard for defining BTCs and a secondary standard for identifying typical/atypical BTCs. The main standard includes two requirements: two coexisting TCs are a pair of BTCs if (i) the separation distance is ⩽ 1800 km, and (ii) this separation maintains for at least 12 h. Meanwhile, the secondary standard defines a typical BTC as one for which there is at least one observation when the two TCs approach each other and spin counterclockwise simultaneously. Under the standard, the ratio of typical BTCs increases as the BTC duration increases or the minimum distance between the two TCs decreases. Then, using the JTWC dataset, it was found that there are 505 pairs of BTCs during the period 1951–2014, including 328 typical BTCs and 177 atypical BTCs, accounting for 65.0% and 35.0% of the total, respectively. In addition, a study of two extreme phenomena—the maximum approaching speed and the maximum counterclockwise angular velocity in typical BTCs—shows that the configuration of the circulation conditions and the distribution of the BTCs favor the formation of these extreme phenomena.

摘要

为了发展客观标准来定义西北太平洋双台风活动(BTC),采用中国气象局(CMA)和联合台风预警中心(JTWC)两个热带气旋最佳路径数据集对BTC的两个重要参数——两者相互靠近和逆时针互旋——进行统计分析。基于两个数据集统计结果的高度一致性,我们建立了BTC客观定义标准——包括定义BTC的主要标准和识别典型/非典型BTC的二级标准。主要标准要求BTC应具备两个条件:(i)两TC中心距离≤1800km且(ii)至少维持12 h;二级标准要求典型BTC应满足至少有一次观测同时出现两TC彼此靠近且逆时针旋转。根据该标准,典型BTC的比率随BTC持续时间的增加或两个TC之间的最小距离的减小而增加。在此基础上,使用JTWC数据集,发现在1951年至2014年期间共有505对BTC,包括328个典型BTC和177个非典型BTC,分别占总数的65.0%和35.0%。此外,对两个极端现象(典型BTC中的最大接近速度和最大逆时针角速度)的研究显示,环流配置和BTC的地理分布有利于这些极端现象的形成。