Meiyu identification of Yangtze-Huaihe region in Anhui province based on Meiyu monitoring indices

According to the definition of the onset and the ending of the Meiyu period in China’s national standard “Meiyu Monitoring Indices” (GB/T 33671—2017), Meiyu processes and their signatures in the Yangtze-Huaihe region of Anhui province during 1957—2020 were identified. The identified signatures of the Meiyu process were compared with those identified by methods proposed before the release of the national standard to investigate the impact of the Meiyu identification method on its signatures. The results show an overall consistency between the Meiyu processes identified based on the national standard and other methods. The identified Meiyu processes based on rain day (which is defined as the days with the precipitation greater than half of the average daily precipitation during June-July) have generally more years with no Meiyu and later onset of Meiyu than those identified based on the national standard. A total of 13 empty Meiyu seasons are identified during 1957—2020, which results in an empty Meiyu frequency of about 20%. For the years with Meiyu, their average onset and ending dates are 19 June and 13 July respectively, which results in an average length of Meiyu season of 23.7 days. The average amount of precipitation-concentration period and the whole period of Meiyu season are 270.7 and 263.0 mm respectively, accounting for about 1/4 of annual precipitation and 3/4 of precipitation during June-July. When the study area is small, instead of the day after the last rainy day, the ending date of a Meiyu season should be determined based on the conditions of atmospheric circulation and regional rain days. It is also shown that there are severe droughts in the Yangtze-Huaihe region of Anhui province, which are more likely to appear during empty or dry Meiyu seasons, especially when there are consecutive empty or dry Meiyu seasons. Conversely, in the same region, severe floods are more likely to appear in years with heavy and concentrated precipitation during the Meiyu season, especially for those with precipitation larger than 400 mm in Meiyu season, e.g. the year 2020.