山东一次极端雨雪过程积雪特征分析及模式产品检验

Study on snow cover characteristics and model verification of an extreme rain-snow event over Shandong

利用地面自动气象站资料、人工加密积雪深度逐时观测资料和ERA5再分析资料,对山东2021年11月6—8日极端雨雪过程积雪特征进行分析。结果表明:(1)降水量突破同期历史极值导致此次雨雪过程成为极端天气事件,积雪深度是预报难点。(2)暴雪和积雪集中分布在山东的中北部地区,有量积雪的范围与降雪量R≥5 mm的分布范围基本一致。积雪深度具有明显的时间变化特征。(3)在山东典型回流暴雪天气形势下,有利的水汽、动力条件和冷空气降温作用,造成山东出现极端雨雪。低层的强冷平流降温导致降水发生相态转换,山东中北部出现暴雪及严重积雪。(4)积雪区降雪含水比差异大,平均降雪含水比为0.53 cm·mm^(-1),比历史平均值偏低。积雪深度与高空温度、相对湿度和垂直速度的配置有关,低的温度有利于降雪和积雪。地理位置、鲁中山地地形和地面风速对积雪深度有影响,海陆差异较纬度差异影响大,海拔高度影响较小。(5)欧洲中期天气预报中心业务预报模式积雪产品对山东积雪有较好的预报能力,时效近、误差小,但存在预报总体偏弱、北部偏小和中南部偏大的特点。

Diagnostic analysis is performed to study the snow cover characteristics of an extreme rain-snow event over Shandong from 6 to 8 November 2021 by using data of automatic weather stations,artificial densified hourly snow depth observations,and ERA5 reanalysis data.Main results are as follows.(1)The rain-snow process is an extreme weather event for its precipitation higher than the extremum of the same period in history,and snow depth poses a challenge to forecasters.(2)Snowstorm and snow cover area are concentrated in the central and northern parts of Shandong,the range of quantitative snow cover is basically consistent with that of snowfall R≥5 mm,and the snow depth has obvious temporal variations.(3)The extreme rain-snow event is caused by cold air under favorable water vapor and dynamic conditions and typical backflow snowstorm weather situation.The low-level strong cold advection leads to the precipitation phase transformation,and snowstorm and severe snow cover occur in the central and northern parts of Shandong.(4)The snow-to-liquid ratio differs greatly in snow-covered area and the mean value is 0.53 cm·mm^(-1),which is lower than the historical average.Snow depth is related to the configuration of temperature,relative humidity,and vertical velocity in the upper level;low temperature is conducive to snowfall and snow cover.The geographical location of Shandong,mountainous topography of central Shandong,and surface wind speed have influence on snow depth.The sea-land difference has greater influence on snow depth than latitude difference does,while altitude has less influence.(5)The snow cover products of ECMWF weather model perform better in forecasting the snow cover in Shandong with small errors,though the forecast is overall weaker in that the snow cover is less in the northern part and more in the central-southern part than the observational data.