Abstract:

In order to strengthen the understanding of the low visibility impact events on the navigation meteorological conditions of the Yangtze River trunk line and improve the level of weather forecast for channel impact, using observational data from the National Meteorological Station and ERA5 reanalysis data (the fifth ECMWF reanalysis), we conducted an analysis of the spatiotemporal distribution characteristics of fog days at 51 stations along the Yangtze River, as well as the weather conditions and meteorological element changes during fog occurrences. The main findings are as follows: (1) Most stations along the river experienced a high incidence of fog from November to next January. The Sichuan and Chongqing area exhibited a consistently high fog occurrence throughout the year, while in the Hubei-Anhui Plain region fog incidents are frequent in spring. Thick fog and heavy fog predominantly occurred during the late-night and early morning hours, with strong fog typically occurring about 2 hours later. (2) In winter, fog along the Yangtze River primarily occurred in the Sichuan section (Yibin-Chongqing), the southwest and central sections of Chongqing (Chongqing-Wanzhou), followed by the Anhui section (Anqing-Hexian) and the Jiangsu section (Dantu-Taicang). (3)When fog was present along the river, the average 10-minute wind speed ranged from 0 to 3 m·s^-1, occasionally exceeding 4 m·s^-1. Northerly wind is the main wind, followed by easterly wind and westerly wind.(4) Mountainous areas along the Yangtze River exhibited a high proportion of rain and fog, with a notable frequency of thick fog, which was strongly correlated with precipitation. In contrast, in plain areas, radiation fog in early morning was more prevalent, and the occurrence of thick fog was often not directly linked to precipitation. The proportion of rain and fog in the eastern plain area was similar to that in mountainous areas, with relatively minor station-to-station fluctuations. (5) Strong fog weather events were associated with four primary near-surface weather situations: the low-pressure rear type, low-pressure trough type, weak high-pressure type, and high-pressure bottom type. Among them, the weak high-pressure type had the highest incidence, followed by the low-pressure trough type, while low-pressure rear type and high-pressure bottom type were less common.

Key words: the Yangtze River channel, fog, meteorological factors, circulation situation

摘要：

为了加强对长江干线通航气象条件低能见度影响事件的了解，提高航道影响天气预报水平，利用国家级气象站观测资料及欧洲中期天气预报中心（ECMWF）全球再分析资料（ERA5），对长江沿线51站2016—2020年的雾日时空分布特征、雾发生时的天气形势和气象要素变化特征进行分析。结果表明：（1）大多数长江沿线站点雾高发于11月至次年1月，其中川渝一带雾常年高发，鄂皖平原地区雾春季多发。浓雾和大雾多出现在半夜和清晨，且强浓雾通常在雾发生2 h后出现。（2）冬季，长江沿线雾集中出现在四川航段（宜宾—重庆）、重庆西南部及中部航段（重庆—万州），其次是安徽航段（安庆—和县）和江苏航段（丹徒—太仓）。（3）雾出现时，10 min平均风速通常在0~3 m·s^-1，少数地方超过4 m·s^-1；以偏北风为主，其次为偏东风和偏西风。（4）沿江山区雨雾占比大，浓雾频次较高，与降水呈现出较好相关性；平原地区清晨辐射雾占比较大，浓雾发生通常与降水无直接关系；东部平原地区雨雾占比基本与山区相同，但各站点波动较小。（5）出现强浓雾天气时，近地面天气形势主要有4类：低压后部型、低压槽型、弱高压型和高压底部型。其中，弱高压型是高发天气型，其次是低压槽型，低压后部型和高压底部型较为罕见。

关键词: 长江沿线, 雾, 气象要素, 环流形势