Due to the unique topography and geographical location, severe snowfall is the main disastrous weather in winter in the Hunan Province of China. Based on the daily precipitation data in Hunan Province from 1961 to 2021, the regional heavy snowfall processes are classified by using the synoptic diagnostic method. In addition, the water vapor transport characteristics of typical heavy snowfall processes are analyzed by the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) air mass backward trajectory model. Then, the responses of the differences in water vapor transport to heavy snowfall under different weather situations are discussed. The results show that the spatial distribution of climatic mean heavy snowfall days in Hunan Province is extremely uneven, and the heavy snowfall days decrease from north to south, with the most in the Dongting Lake area and the least in the Nanling Mountains. In the past decades, snowstorms mainly occur in local areas, and there are fewer widespread snowstorms. The frequency of heavy snowfall days generally shows a decreasing trend, with three peaks all appearing before 1990. After the 2010s, the number of days and stations of heavy snowfall decreased noticeably, and so did the number of regional heavy snowfall processes. This result indicates that global warming has remarkable effects on the snowstorm events in Hunan Province. Heavy snowfall mainly occurs from December to February, and peaks from mid-January to early February. Over the past 61 years, more than 50% of heavy snowstorm events occurred after 2000. According to the main weather systems affecting regional heavy snowfall processes, these weather processes in Hunan Province can be classified into three categories: southern branch trough (SBT) type, blocking high collapse (BHC) type, and stepped trough type. Among them, the SBT type accounts for more than 60% of the heavy snowfall events in Hunan. In terms of the SBT type and the stepped trough type, the water vapor from the high-latitude inland and low-latitude sea surface accounts for a comparable proportion, each accounting for nearly 50%. For the SBT type, the proportion of the water vapor from warm-humid airflows is slightly higher than that from cold-humid airflows. However, in terms of the stepped trough type, the water vapor transported by cold-humid airflows from the north contributes more than that by warm-humid airflows. For the BHC type, the specific humidity and the water vapor from the high-latitude inland contribute 70% of heavy snowfall processes. In addition, the contribution of the two southwesterly water vapor channels to heavy snowfall processes is small. The water vapor sources differ remarkably for different heavy snowfall types, but all of them are dominated by the water vapor transport in the middle and lower troposphere, which is the main reason why the formation of snowfall areas under different weather types is obviously different.

中文翻译：

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湖南省大雪气候特征及典型降雪事件水汽输送特征

由于独特的地形和地理位置，大雪是湖南省冬季的主要灾害性天气。根据湖南省1961—2021年逐日降水资料，采用天气诊断方法对区域性强降雪过程进行分类。此外，利用混合单粒子拉格朗日积分轨迹（HYSPLIT）气团后向轨迹模型分析了典型大雪过程的水汽输送特征。然后讨论了不同天气条件下水汽输送差异对强降雪的响应。结果表明:湖南省气候平均暴雪日数空间分布极不均匀,暴雪日数自北向南递减,洞庭湖地区暴雪日数最多,南岭山区最少。近几十年来，雪灾主要发生在局部地区，大范围雪灾较少。大雪日数总体呈减少趋势，其中3个高峰均出现在1990年之前。2010年代以后，大雪天数和场次明显减少，区域性大雪过程次数也明显减少。这一结果表明，全球变暖对湖南省雪灾天气影响显着。大雪主要发生在12月至2月，1月中旬至2月上旬为高峰。61年来，50%以上的强降雪事件发生在2000年以后。根据影响区域强降雪过程的主要天气系统，湖南省的这些天气过程可分为三类： 南方支槽（SBT）型、堵高塌陷（BHC）型、阶梯槽型。其中，SBT型占湖南强降雪事件的60%以上。就SBT型和阶梯槽型而言，来自高纬度内陆和低纬度海面的水汽所占比例相当，均接近50%。对于SBT型，暖湿气流中的水蒸气比例略高于冷湿气流中的水蒸气比例。但就阶梯槽型而言，北方冷湿气流输送的水汽贡献大于暖湿气流。对于BHC型，比湿和来自高纬度内陆的水汽贡献了70%的强降雪过程。此外，西南两条水汽通道对强降雪过程的贡献较小。不同强降雪类型的水汽来源存在显着差异，但均以对流层中下层水汽输送为主，这也是不同天气类型下降雪区形成明显不同的主要原因。