Indicative significance of five patterns of wind at the Nanyue Mountain Meteorological Stationon various severe precipitation events in Hunan

Using routine upper-air and surface observations, hourly wind variations at the Nanyue Mountain Meteorological Station (called Nanyue Station for short) and hourly rainfall data of 96 meteorological stations over Hunan between April and September from 2006 to 2014,we make a synoptic system classification for 117 severe precipitation cases during the corresponding period, and then discuss the indicative significance of five wind patterns at Nanyue Station on different types of severe precipitation forecast in Hunan. The results indicate that there are 5 different types of wind variations at Nanyue Mountain, which correspond to 6 different patterns of severe precipitation events in Hunan.Of the five hourly wind variation patterns, four have indicative significance to the change of the movement and intensity of the severe precipitation belt. The time when south wind at Nanyue Station exceeds 10 m·s-1 is about 2-6 hours ahead of severe precipitation beginning in northern and northwestern Hunan. When the south wind reaches the low-level-jet criterion of 12 m·s-1, the strongest precipitation can be expected to occur in central and northern Hunan about 2-5 h later. It usually has forecast lead time of 2-4 h for enhanced rain in southern Hunan when the south wind at Nanyue falls below 8 m·s-1 or shifts to north wind. When the dominant condition at Nanyue is south wind of 12-14 m·s-1, the most severe precipitation always occurs in northern and northwestern Hunan. If the wind speed at Nanyue exceeds 14 m·s-1, rain area will move northwards to the north parts of the Yangtse River. When the dominant at Nanyue is north wind of 1-3 m·s-1, moderate to severe precipitation occurs in central and southwestern Hunan. When the northerly wind speed is between 3-5 m·s-1, precipitation will occurs in central,southwestern and southeastern Hunan. If northerly wind exceeds 5 m·s-1, precipitation in central a d southwestern Hunan will weaken or stop completely, but in southeastern Hunan it will increase. During the heavy rain period caused by typhoon, if wind at Nanyue appears north andsouth erratically alternating and the wind speed is less than 5 m·s-1, severe precipitation usually occurs in southeastern Hunan.