2018年8月22日宁夏北部局地浓雾天气形成机制分析

利用宁夏972个地面自动站的10 m风场、2 m温度及露点温度，银川、大武口、平罗、贺兰4个常规地面观测站的能见度、温度及相对湿度逐时观测资料和欧洲中期天气预报中心（ECMWF）ERA-Interim逐6 h再分析资料（0.125°×0.125°），对2018年8月22日宁夏北部局地突发浓雾天气过程的环流形势、逆温结构及其热力、动力条件和形成维持机制进行分析。结果表明：8月22日宁夏北部局地浓雾发生在地面冷高压、高空暖脊及近地面微风的静稳天气条件下；较为深厚的暖平流和近地面冷空气侵入所形成的稳定大气层结，是大雾形成的必要条件；弱上升运动使混合层扩展至840 hPa左右，是该地区雾的发展和维持的重要原因。在本次过程中，深厚的弱水汽辐合较水汽自身的饱和与否更为重要，是大武口站浓雾形成的关键性因素。浓雾呈现平流-辐射雾特征，地面热通量在过程前期促进浓雾的发生，后期对浓雾起到抑制作用。

Formation mechanism of local thick fog in the northern Ningxia Hui Autonomous Region on August 22,2018

Based on the 10 m wind field,2 m temperature,and dew point temperature of 972 automatic ground stations in Ningxia Hui Autonomous Region,the visibility,hourly temperature,and relative humidity of Yinchuan,Shizuishan,Pingluo,and Helan stations,and 6-hourly reanalysis data with a resolution of 0.125°×0.125°from the ECMWF ERA-Interim(European Centre for Medium-Range Weather Forecasting,ECMWF Reanalysis-Interim),the circulation pattern,inversion structure,thermal and dynamic conditions,and formation and maintenance mechanisms of a local sudden dense fog weather process in the northern Ningxia Hui Autonomous Region on August 22,2018,were analyzed.The results show that the favorable conditions of the surface cold high pressure,the warm high ridge,and the surface layer breeze contribute to the occurrence of this process.The stable atmospheric stratification formed by the invasion of cold air near the ground and deeply warm advection is a necessary condition for the formation of this fog process.For the development and maintenance of the local fog,the weak ascending movement extending the mixing layer to about 840 hPa is an important reason.In this process,the convergence of deep and weak water vapor is more important than the saturation of water vapor itself,which is the key factor for the formation of fog at Dawukou station.The process has advection-radiation fog characteristics,the surface heat flux promotes the occurrence of the dense fog in the early stage of the process,but inhibits it in the later stage.