1998年长江流域特大洪涝期水汽输送过程的诊断分析

利用基于拉格朗日方法的气流轨迹模式（HYSPLIT_V4.9），结合轨迹聚类法和气块追踪法，探讨1998年6月12日—8月27日期间长江流域强降雨的水汽输送轨迹、主要水汽源地及其水汽贡献，发现此次强降水过程的水汽源地主要为印度洋、孟加拉湾—南海和太平洋；不同降水阶段水汽输送轨迹、水汽源地存在差异。降水第一阶段水汽主要来自孟加拉湾—南海，水汽输送贡献为35%。降水第二阶段水汽主要由印度洋、孟加拉湾—南海和太平洋三个区域共同提供，水汽输送贡献分别为32%、28%和31%。降水第三阶段则是来自印度洋和孟加拉湾—南海的水汽输送占主导地位，它们对降水的水汽输送贡献分别为33%和41%。降水第四阶段水汽主要来源于孟加拉湾—南海，贡献为40%。强降水过程中大气环流的调整，导致了不同阶段水汽源地的变化及各源地水汽贡献的差异。

Diagnostic analysis of water vapor transport process during the catastrophic flood period over Yangtze River Basin in 1998

Large-scale water vapor transportation is an important condition for persistent heavy rainfall.The water vapor transportation and its sources are important factors influencing summer precipitation in China, and they have been highly concerned by the domestic and international meteorological circles.In the summer of 1998, there was a persistent heavy rainfall in the Yangtze River Basin, resulting in a basin-wide large flood, which caused heavy losses to the people''s lives and property.This paper has divided the heavy rainfall process over the Yangtze River Basin from 12 June to 27 August in 1998 into four precipitation stages.The variation features of water vapor transportation, main water vapor sources and their contributions in the different stages have been discussed by using the NCEP reanalysis data and the daily precipitation data of 754 weather stations in China, as well as the airflow trajectory model based on Lagrangian method (HYSPLIT_V4.9), which combined with the trajectory clustering method and the air parcel tracing method.The results suggest that the water vapor of the heavy rainfall mainly comes from three regions, namely the Indian Ocean, the Bay of Bengal-South China Sea and the Pacific Ocean.The water vapor of the Indian Ocean flows into the Yangtze River Basin through Southern India Peninsula, the Bay of Bengal and northern Indo-China Peninsula, while the water vapor of the Bay of Bengal-South China Sea region passes Indo-China Peninsula and the north part of South China Sea into the Yangtze River Basin.The water vapor of the Pacific region flows into the Yangtze River Basin along with the southeast airflow on the edge of subtropical anticyclone.Every precipitation stage has its own water vapor trajectory and sources.The transportation of water vapor originating from the Bay of Bengal-South China Sea is most important and accounts for 35% of the total vapor transportation during the first stage, while the water vapor from the India Ocean and the Pacific Ocean accounting for only 21% and 19%, respectively.During the second stage, the Indian Ocean, the Bay of Bengal-South China Sea and the Pacific Ocean are three major contributors, accounting for 32%, 28% and 31% respectively.During the third stage, the water vapor is mainly provided by the Indian Ocean and the Bay of Bengal-South China Sea, accounting for 33% and 41%, respectively, and the water vapor contribution rate from the Pacific Ocean is only 5%.During the fourth stage, the Bay of Bengal-South China Sea is dominant, the same as during the first stage, and accounts for 40% of total moisture transportation, while the water vapor from the India Ocean and the Pacific Ocean only accounting for 23% and 22%, respectively.The adjustment of atmospheric circulation during the heavy rainfall process leads to the differences of water vapor source and its contribution rate.The enhancement of cross-equatorial jet stream in Somalia during the second and third stages causes its water vapor contribution rate significantly higher than that during the first and fourth stages.The intensity change of the South China Sea monsoon results in the differences of water vapor contribution from the Bay of Bengal-South China Sea during the different stages.