一次华北暴风雪过程中边界层中尺度扰动涡旋和水汽输送特征的分析

Analysis of Planetary Boundary Layer Mesoscale Disturbance Vortex and Moisture Transport during a Blizzard Process in North China

利用多部风廓线雷达和多普勒雷达、地面自动气象观测站、探空和卫星等多种监测资料,结合ERA5 0.25°×0.25°逐小时再分析资料,针对2020年2月14日华北暴风雪过程中边界层中尺度扰动涡旋(PMDV)开展分析,包括空间结构、形成维持机制及其对降雪的影响,同时关注多股气流在不同高度层的水汽输送和净流入等特征。结果表明:PMDV是在500 hPa冷涡前部、 850 hPa暖性倒槽内和近地层东北风“冷垫”之上的悬空涡旋,其厚度约为1.2 km,水平尺度为100~300 km,生命史达17 h。它最先在边界层内出现,而后向上伸展(顶部达2 km),最终在边界层内消失。PMDV的成因:一是强劲持久的偏东气流西进,遇太行山脉阻挡,发生逆时针转向;二是在涡旋初生地存在持久的锋生作用,且4 h后在850 hPa上出现完整的气旋性环流。PMDV的发展维持原因是暖平流输送造成减压、凝结潜热释放和弱锋生三者的共同作用。PMDV促进了研究区内的东南风急流、正涡度、垂直上升速度和水汽通量散度等物理量的增大或加强,从而影响了降雪的持续时间和强度。研究区内的水汽绝大部分源于850 hPa以下的三股气流,西南支气流携带的水汽虽最为深厚,但在700 hPa以上的净流入极少;东南支气流对水汽的贡献最大,占净流入总量的86.4%;东北支气流携带的水汽多集中于850 hPa层。

Based on the data of networked wind profiles and Doppler radars,ground automatic weather observation station,radiosonde and FY-4 satellite,combined with the ECMWF ERA5 1 h 0.25°×0.25° reanalysis data,this paper analyzes the spatial structure,formation,maintenance and function of PMDV that occurred in boundary layer to lower troposphere in North China during the blizzard weather on 14 February 2020.Besides,the moisture transported by multiple air flows and the net inflow are calculated and compared.The results indicate that PMDV was located in the front of the 500 hPa cold vortex,within the 850 hPa warm inverted trough and on the noutheast backflow.The PMDV was suspended vortex,with a horizontal range of 100~300 km and a vertical thickness of 1.2 km and a life span of 17 h.It first appeared in the boundary layer and then extended to higher altitudes(top at 2 km) and finally disappeared in the boundary layer.One of the reasons for formation of PMDV was that the strong and persistent east-northeast wind blew westward and then turned counterclockwise after being blocked by the Taihang Mountains running from northeast to northwest.Another reason was that the stronger frontogenesis appeared at PMDV primary position and it had maintained about 4 h ahead of complete cyclonic wind field circulation formed.The reason for maintenance of PMDV was under the common action of decompression(caused by warm advection),weaker frontogenesis(caused by dense temperature gradient) and release of latent heat of internal condensation.The PMDV promoted the strengthening of low-level jet and the increase of positive vorticity in the southeast of its northeast side(blizzard area),causing the upward movement,water vapor transport and aggregation in the study area to increase.Most moisture transported in the blizzard area was originated from multiple air currents below 850 hPa,among which the southeast branch contributed most,accounting for 86.4% of the total net inflow.The wet layer carried by the northeast branch airflow was concentrated in the 850 hPa layer,but the northwest airflow maintained dry all the time.Though the wet layer of southwest branch airflow was deep,the net inflow of water vapor above 700 hPa was very small.