A mesoscale convective system (MCS) embedded in a cold frontal cloud system over Beijing on July 16, 2018 was investigated using X-band dual-polarization radar observations from Fangshan Station and sounding data from the Beijing Observatory. This study focused on hydrometeor identification and microphysical characterization, yielding the following key findings. 1) The ice-phase hydromorphic species above the 0°C layer differed in different stages and regions of this MCS. From the stage of development to maturity to dissipation, the main hydromorphic species in the region with combined reflectance larger than 40 dBZ were graupel and snow, and the proportion of ice crystals was relatively low. In the region with combined reflectance between 20 and 40 dBZ, graupel, snow, and ice crystals were the main hydromorphic species. In these two regions, the proportion of graupel gradually decreased with time, while the proportion of snow and ice crystals gradually increased with time. In the region with combined reflectance smaller than 20 dBZ, the proportion of graupel was very low, and the main hydromorphic species were snow and ice crystals, with the proportion of snow gradually decreasing with time, and the proportion of ice crystals gradually increasing with time. 2) The area of the 20–40 dBZ echo region in this MCS played a dominant role in precipitation area. 3) At the beginning of convection, satellite brightness temperature observation had a better monitoring effect on precipitation. The application of X-band dual-polarization radar data to analyze the microphysical characteristics of MCS is beneficial to understand the microphysical processes occurring in the system, and can provide a useful reference for the revision of the microphysical parameterization scheme. 

In order to improve the accuracy of return flow snowfall forecast in Beijing area, the three return flow snowfall cases occurring in Beijing area in February 2019 are analyzed by using the ERA5/GDAS reanalysis data and the HYSPLIT back trajectory model for the meteorological analysis and characterization of the water vapor transport. The results show that, for all the three cases, the water vapor content in the easterly flow is limited regardless of the typical or atypical return flow, and significant snowfall can be produced when there is a cooperation of the southerly water vapor channel. In the typical return flow case (Feb. 14), the high backward return flow and the Loop Inversion Trough cooperate, and the water vapor transport channel is deeper, which is conducive to snow. In the two atypical return flow cases (Feb. 6 and 12), the water vapor transport is concentrated in the lower layers of the atmosphere (below 850 hPa). The case of Feb.12 has a significant contribution from the southerly water vapor transport, and the neglect of the southerly water vapor transport is one of the reasons for the underreporting of snowfall on that day. The arrival time of water vapor transported to Beijing area by the easterly and southerly vapor channels both basically correspond to the time of significant snowfall during the daytime, and can be used as a key factor for forecasting return flow snowfall.