Analysis of a backflow heavy snowfall event in central North China using multi-source data
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摘要: 2020年1月5日07时至6日04时(北京时,下同)华北中部出现一次回流暴雪天气,过程最大降雪量15.5 mm。文中应用ERA5再分析和多种高分辨率观测资料分析了此次暴雪的大尺度天气背景和本地动、热力状况,探讨了暴雪落区、强度演变和降雪微物理特征及成因。结果表明,受河套地区地面倒槽和东北平原高压影响,900 hPa以下东北气流(被称为“回流”)自东北平原经渤海抵达华北平原,早于降雪7 h开始影响华北中部,受太行山阻挡在华北平原形成浅薄的近地面中尺度辐合线,对应暴雪落区;暴雪落区位于500 hPa高空槽前、700 hPa南北走向切变线东侧,850 hPa受西南低涡外围东南气流影响。降雪前1 h石家庄市观测到800 m以下转为东北风,1 km以下气温迅速下降至−5—−1℃,形成“冷垫”;暴雪区上空700 hPa附近低空急流较降雪早2 h出现,随后急流变厚、向下伸展至2 km高度,其下部暖湿空气沿“冷垫”爬升触发降雪,急流风速增至极值(19 m/s)和急流指数达峰值(约8)与大于1 mm/h强降雪时段重合,此时700 hPa上下为上升运动和水汽输送的大值中心。本次降雪粒子直径多为0.35—0.55 mm,降雪强度与粒子数浓度呈线性正相关;降雪云层位于1.3—5.5 km高度,大致以3 km (约−10℃)为分界线,下层为冰雪混合层,上层为冰雪层,冰雪层相对湿度与地面雪花粒子浓度及降雪强度呈正相关。基于雨滴谱仪探测资料反演的地面反射率因子与降雪强度拟合关系为Z=149.85R1.14。Abstract: A backflow heavy snowfall event occurred in central North China from 07:00 BT 5 January to 04:00 BT 6 January 2020, producing a maximum snowfall of 15.5 mm. The ERA5 reanalysis and high-resolution observation data from multiple sources are utilized to analyze synoptic background and local dynamic and thermal conditions of this event as well as the spatiotemporal distribution and microphysical features of snowfall. The results show that 7 h prior to the snowfall, northeasterlies below 900 hPa (called 'backflow') swept the northeast plain of China and the Bohai Sea and reached the North China Plain, under the joint influence of an inverted trough over the Yellow River bend and a high pressure in the Northeast plain. A shallow near-surface mesoscale convergence line formed in the North China Plain under the blocking effect of Taihang mountain. The convergence line corresponds to the heavy snowfall area. At 850 hPa, southeasterly flows around the Southwest Vortex prevailed over the snowstorm area. Northeasterly winds below 800 m were observed in Shijiazhuang about 1 h before the snowfall, and temperature below 1 km height dropped rapidly to −5—−1℃, forming a "cold pad". An Low-Level Jet (LLJ) near 700 hPa over the heavy snowfall area appeared 2 h prior to the snowfall, and the LLJ became thicker and extended down to 2 km. The warm and moist air below the LLJ was forced to climb along the "cold pad", triggering snowfall. The maximum wind speed of LLJ (19 m/s) and the peak value of LLJ index (about 8) coincide with occurrence of the heavy snowfall greater than 1 mm/h. At the same time, the maximum center of ascending motion and water vapor transport is located near 700 hPa. Snow particles are 0.35—0.55 mm in diameter. There is a positive linear correlation between snow intensity and particle number concentration. The snow-producing cloud layer is located at 1.3—5.5 km height. The lower layer (below 3 km; about −10℃) is ice-snow mixing layer and the upper layer (3—5.5 km) is ice-snow layer. Relative humidity in the ice-snow layer is positively correlated with snow particle concentration and snowfall intensity. The fitting relationship between ground reflectivity factor (Z) and snowfall intensity (R) detected by disdrometer is Z=149.85R1.14. These results provide a reference for better forecasting of heavy snowfall in North China.
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图 1 (a) 华北地形 (色阶)、风廓线雷达 (“×”) 和 (b) 主要观测设备分布:石家庄市风廓线雷达与微波辐射计 (加号)、雨滴谱仪 (三角),石家庄 (加号)、邢台 (圆圈) 和邯郸 (圆圈) GPS水汽含量探测仪 (图a中蓝色实线为黄河,红色字体BJ、SH分别为北京和上海,虚线矩形为图b显示范围;图b中的黑色实线为京津冀各市边界,红色矩形为研究的暴雪关键区;图a和b中的橙色实线为石家庄市界)
Figure 1. Topography in North China (shaded),wind profilers ("×") and distribution of major instruments:wind profiler and microwave radiometer (cross),disdrometer (triangle) in SJZ,and GPS stations in SJZ (cross),XT (circle) and HD (circle) (blue line represents the Yellow River,the red letters "BJ" and "SH" denote Beijing and Shanghai,black rectangle in Figure a indicates the area shown in Figure b,the regions enclosed in black solid lines denote the cities of Beijing-Tianjin-Hebei,orange thick lines in (a) and (b) denote the boundary of SJZ city,red rectangle in (b) outlines the key region of heavy snowfall)
图 2 (a) 2020年1月5日07时—6日04时 (北京时) 累计降雪量空间分布,以及 (b) 石家庄和邢台南宫 (图(a)中分别标记“三角”和“圆圈”) 5日11时至6日01时逐时降雪量 (红色方框为暴雪关键区,黑色方框为暴雪上游区域)
Figure 2. Accumulated snowfall (a) from 07:00 BT 5 January to 04:00 BT 6 January and (b) hourly snowfall at SJZ (triangle) station and NG (circle) station from 11:00 BT 5 January to 01:00 BT 6 January (red rectangle outlines the key region of heavy snowfall and black rectangle outlines the upstream region of the key region)
图 3 1月5日08时 (a) 地面气压场 (黑实线,单位:hPa)、温度场 (色阶)、风场 (红色风矢) 和地面锋面 (粗黑实线),(b) 925 hPa高度场 (黑实线,单位:dagpm)、风场和温度场 (色阶),(c) 850 hPa高度场 (黑实线,单位:dagpm)、风场和温度场 (色阶),(d) 700 hPa高度场 (黑实线,单位:dagpm)、风场和水汽通量 (色阶,单位:g/(cm·hPa·s),(e) 500 hPa高度场 (黑实线,单位:dagpm)、风场 (色阶),(f) 200 hPa高度场 (黑实线,单位:dagpm)、散度场 (色阶,单位:10−5 s−1)、水平风速 (绿色等值线,单位:m/s) 和急流核 (风向杆,风速大于60 m/s)(红色矩形为暴雪关键区,黑色圆点为石家庄,红色圆点为北京,(d) 中的红色双实线为700 hPa切变线)
Figure 3. Synoptic analysis at 08:00 BT 5 January (a. surface pressure (black solid line,unit:hPa),temperature (shaded),wind (red vector) and surface front (thick black solid line),b. 925 hPa geopotential height (black solid line,unit:dagpm),wind and temperature (shaded),c. 850 hPa geopotential height (black solid line,unit:dagpm),wind and temperature (shaded),d. 700 hPa geopotential height (black solid line,unit:dagpm),horizontal wind and water vapor flux (shaded,unit:g/cm·hPa·s),e. 500 hPa geopotential height (black solid line,unit:dagpm) and horizontal wind (shaded),f. 200 hPa geopotential height (black solid line,unit:dagpm),divergence (orange shaded,unit:10−5 s−1) and horizontal wind (green contour,unit:m/s);red rectangle denotes the key region of heavy snowfall;red and black dots denote Beijing and SJZ,respectively)
图 4 ERA5再分析资料显示5日11时 (a) 1000 hPa、(c) 850 hPa水平风和假相当位温θse (等值线,单位:K) 水平分布以及沿 (b) AB线、(d) CD线的θse垂直剖面和剖面内风矢量 (a、c图中的红色矩形和b、d图中两条黑色竖线之间为暴雪区,a、c图中黑色粗实线和b图中紫色虚线为锋面位置;b、d图中黑点表示上升运动处,黑色阴影表示地高度)
Figure 4. Spatial distributions of horizontal wind and pseudo-equivalent potential temperature (θse, contour,K) at 1000 hPa (a) and 850 hPa (c),vertical cross sections of θse and composite in-plan flow vectors along the line AB (b) and CD (d) from ERA5 reanalysis at 11:00 BT 5 January (red rectangles (a,c) and black lines (b,d) outline the key region of heavy snowfall,the fronts below 850 hPa is denoted by the purple dotted line (b) and thick black lines (a、c),black dots represent ascending motion,black shadings represent terrain height)
图 5 1月5日04—16时 (a—d) 间隔4 h的地面自动气象站10 m风场 (色阶表示地形高度,风向杆表示10 m水平风,蓝色粗虚线为地面辐合线,暴雪关键区和暴雪上游区域同图2)
Figure 5. 10 m winds collected at dense automatic weather stations from 04:00 to 16:00 BT 5 January (a—d,shaded area indicates the terrain height,wind bars indicate 10 m horizontal winds,blue thick dotted lines indicate surface convergence line,red rectangle represents the key region of heavy snowfall and black rectangle outlines the upstream region of the key region,which are same as in Fig. 2)
图 6 (a) 石家庄风廓线雷达观测的逐30 min水平风时间-高度分布和微波辐射计监测的地面 (红线)、500 m (橙线) 和1000 m (绿线) 气温逐分钟变化 (蓝色实线为12、14、16、18 m/s风速等值线,两条黑色竖线表示石家庄降雪开始和结束的时间),以及 (b) 石家庄及周边6部风廓线雷达观测的5日11时水平风垂直分布 (蓝色风矢表示风速≥12 m/s)
Figure 6. (a) Time-height distribution of 30 min horizontal winds (barb) observed by the wind profiler,and time series of temperature at 0,500 and 1000 m heights (red,orange,green line) from the microwave radiometer in SJZ (blue contours denote the speed of horizontal wind of 12,14,16,18 m/s and two vertical lines indicate the beginning and ending time of snowfall in SJZ),(b) vertical distribution of horizontal wind from six wind profilers near SJZ at 11:00 BT 5 January (blue barbs represent horizontal wind speed greater than 12 m/s)
图 7 1月5日02时—6日07时石家庄相对湿度高于90% (阴影)、水平风、温度 (红色实线,单位:℃)、水汽通量 (浅蓝色实线,单位:g/(cm·hPa·s)) 和垂直速度 (蓝色实线,单位:10−3 m/s) 剖面 (黑色竖线表示石家庄降雪开始和结束时间)
Figure 7. Time-height cross section of relative humidity higher than 90% (shaded),horizontal wind barbs,temperature (red solid line,unit:℃ ),water vapor flux (light blue solid line,unit:g/(cm·hPa·s)) and vertical velocity (blue solid line,unit:10−3 m/s) in SJZ from 02:00 BT 5 January to 07:00 BT 6 January (two vertical lines denote the beginning and ending time of snowfall in SJZ)
图 10 1月5日08时至6日08时石家庄微波辐射计观测的 (a) 积分水汽含量 (IWV,曲线) 和石家庄小时降雪量 (柱)、(b) 0—8 km温度 (虚线从上至下对应5、4、3 km高度,实线为云底高度) 和 (c) 相对湿度 (虚线从上至下对应5、4、3 km高度,实线为云底高度) 廓线的时间演变
Figure 10. Temporal evolution of (a) integrated water vapor (IWV,solid line) and hourly snowfall in SJZ (column),(b) 0—8 km temperature and (c) relative humidity observed by microwave radiometer from 08:00 BT 5 January to 08:00 BT 6 January (black dotted lines in (b,c) indicate 5,4,3 km respectively,and the black solid lines represent the cloud base height)
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