Abstract:

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.

Key words: Thick fog, Inversion layer, Cold advection, Weak vapor convergence, Surface heat flux