Abstract:

Based on the daily mean flow of Yichang station, precipitation of 287 weather stations in the upper reaches of the Yangtze River and NCEP reanalysis data with 2.5°×2.5° spatial resolution from 1981 to 2012, 128 medium and small flood processes in the upper reaches of the Yangtze River were selected according to the defined criteria, then the monthly distribution characteristics of floods and flood-causing precipitation processes and their weather causes were analyzed. The results indicated that the medium and small floods in the upper reaches of the Yangtze River mostly occurred in July to September, the duration got gradually longer from June to September, and the maximum duration of discharge occurred in July, while the maximum peak of discharge and flood volume appeared in August. In June, the location of subtropical high and South Asia high were to the east and south compared with the normal, the cold air moving southward under Baikal lake trough guidance converged with the southwest jet over Chongqing-Yichang and Wujiang basins, which caused the uneven distribution of rainfall in sub-basins, the center position of rainfall was to the east, heavy rainfall days were fewer but intensity was larger. In July, the subtropical high and South Asia high were to the west and north, many short wave troughs appeared in the mid-high latitude, southwest warm airflow was strong, convergence zone was wide, which resulted in wide precipitation range and uniform distribution, and more strong precipitation days. In August, the subtropical high was moving further to the west and north, circulation in mid-high latitude was straight, convergence center was located in the northwest of the upper reaches of the Yangtze River, coordinating with high heat and high humidity environment field, which was easy to cause local heavy precipitation, the center of rainfall was located in Min-Tuojiang basin and the area precipitation was larger. After September, with the subtropical high moving to westward and South Asia high moving to southeastward, the surface cold air was very active and stronger owing to influence of deep northeast trough. Combined with weak southwest warm and wet airflow, the precipitation intensity weakened, area precipitation was small, and the center of rainfall was located in Min-Tuojiang and Jialingjiang basins.

Key words: medium and small floods, flood volume, area precipitation, weather causes

摘要：

基于1981～2012年宜昌站日均流量、长江上游287个气象站逐日雨量及NCEP再分析资料，选取长江上游中小洪水过程128例，分析其洪水、致洪降雨月分布特征，并研究其天气成因。结果表明：长江上游流域中小洪水多发于7～9月，且过程持续时间6～9月呈逐渐变长趋势，其中过程洪量7月最大，洪峰流量及次洪量最大值出现在8月。6月副热带高压（以下简称副高）及南亚高压偏东偏南，贝加尔湖槽引导冷空气南下，与西南急流在重庆—宜昌、乌江汇合，造成降水各子流域分布不均，中心位置偏东，强降水日数少但强度较大；7月副高及南亚高压西伸北抬，中高纬地区多短波槽活动，西南气流强盛，辐合区范围大，造成降水范围广且分布均匀，强降水日数多；8月副高进一步西伸北抬，中高纬环流平直，辐合中心位于长江上游流域西北部，配合高热高湿的环境场，造成降水分布不均匀且局地性强，中心位于岷沱江，过程面雨量大；9月以后，副高继续西伸，南亚高压南压东移，受深厚东北槽影响，地面冷空气活跃，西南暖湿气流减弱，降水中心位于岷沱江、嘉陵江，强度减弱，过程面雨量小。

关键词: 中小洪水, 次洪量, 面雨量, 天气成因