Characteristics of tropical cyclone outflow over the western North Pacific

Abstract

This study employs the self-organizing map method to investigate the upper-tropospheric outflow patterns of tropical cyclones (TCs) over the western North Pacific from 1979 to 2019, using the 200 hPa horizontal wind fields from the ERA5 reanalysis datasets. According to the number and orientation of TC outflow channels, as well as the wind speed, the outflow patterns are classified into five categories: southwestward single-channel pattern S1 (26.1%); northwestward single-channel pattern S2 (23.6%); northeastward single-channel pattern S3 (23.6%); double-channel outflow pattern D (20.8%); and high latitude outflow pattern H (6.0%). Composite analysis shows that the orientations of the TC outflow channels are aligned with the direction of the environmental vertical wind shear and closely related to the distribution of the environmental inertial instability, upper-level divergence, and inner-core convective activities. TC intensity and intensity changes for different outflow patterns are also significantly different. Patterns S1 and S2 usually appear in the development phase and are thus prone to TC intensification, while patterns S3 and H usually occur in the weakening phase and are thus prone to TC weakening. The double-channel pattern (D) has the largest mean intensity and accounts for more than 60% of super-typhoon samples.

摘要

本文基于欧洲中期天气预报中心第五代大气再分析数据集 (ERA5) 的200 hPa水平风场数据, 借助自组织映射网络方法对1979–2019年间西北太平洋热带气旋的流出层特征进行了客观分型研究. 根据流出通道的数量, 方向以及风速大小等特征, 西北太平洋热带气旋流出层主要可分为五种流出类型: 西南向单流出通道型 (26.1%); 西北向单出流通道型 (23.6%); 东北向单流出通道型 (23.6%), 双流出通道型 (20.8%) 以及高纬度流出通道型 (6.0%). 不同流出通道类型对应的热带气旋高层热力, 动力结构具有显著差异, 流出通道的方向与高层惯性稳定度, 散度场, 以及热带气旋内核区对流活动的分布等密切相关. 同时, 流出通道类型与热带气旋的强度及强度变化也具有密切联系. 西南和西北向单通道流出型主要发生在增强阶段, 具有东北向单流出通道和高纬流出通道的热带气旋则更倾向于衰减. 双流出通道型的热带气旋样本具有最大的平均强度, 且热带气旋强度越强, 双流出通道出现的频率越大, 占强热带气旋样本的60%以上.