In order to deepen the understanding of extreme snowfall and reveal the possibility of anomalous influencing factors leading to extreme snowfall events, the extremes of two major snowfall weather processes in Shanxi Province on February 24, 2021 and from February 27 to March 1 (referred to as “Process I” and “Process II”, respectively) were analyzed by using the meteorological observations and reanalysis data. The results show that Process Ⅰ was a convective snowfall process, caused by the combined influence of a plateau trough, a surface trough and a return flow. The rapid climb of the strong southwest warm and wet jet on the “cold pad” and the symmetric instability together led to the rapid release of potential unstable energy, resulting in a concentrated precipitation range, a large snowfall intensity and a short duration. During this process, cold air quickly invaded and the precipitation phase changed from rain to snow quickly. Process Ⅱ, on the other hand, was characterized primarily by stability, which was influenced by an upper-level trough, a surface cyclone and an inverted trough. During the systematic invasion of cold air, an extreme snowfall event was formed, with a large area of precipitation and a prolonged duration, and the phase changes during this snowfall process were complex. Significant differences were found in the circulation patterns, moisture transport mechanisms, instability mechanisms, and vertical motion characteristics before the precipitation of two snowfall processes. However, compared to the climatological averages, both processes exhibited anomalously high local relative humidity, 700 hPa energy, and vertical upward motion, which was identified as one of the key reasons for the occurrence of extreme weather. The precipitation centers of both events were located in the downstream of the anomalous physical quantity centers 6 to 12 hours before precipitation, and the moisture transport and the thickening of the moist layer were also provided some indication for the precipitation starting time. Additionally, the transition of precipitation phase was closely related to the vertical distribution of temperature and frontal structure.

Based on conventional observation data，NCEP reanalysis data，satellite images and radar mosaic products，and combined with WRF mesoscale simulation，the severe convective weather which occurred in the south of Shanxi on 22 May，2013 was analyzed． The results show that during this weather process，lasting transport of positive vorticity advection over Hetao area played a significant role in formation，maintenance and development of the cut － off vortex on 500 hPa level． Both the low － level cold advection from the front and more stronger warm advection from the back made the vortex develop continually，and excited isolated cloud clusters nearby． The vertical structure with high － level divergence and low － level convergence above the cloud clusters made the upward motion strengthen continuously． Consequently，the isolated cloud clusters maintained and developed． These meso － and meso － scale clusters were the direct cause leading to this severe convective weather． The dry frontogenesis caused by surface continuous strengthening of water vapor from the sea at the back of high pressure system and the invading of dry air at the front of ridge，was an important triggering mechanism of the strong convective weather． From the radar mosaic products of composite reflectivity，this process resulted from the development and merging of convective cells． The evolution of these cells went through a process from convective cell，merging，ribbon echo，bow echo to weakening and vanishing ，and the whole process was divided into two stages，when the area，intensity and moving speed of echoes were different，the characteristics of convective weather were different too． Obvious differences among three types during this process existed in temperature and humidity profiles and environmental parameters，which could be served as judging the severe convective weathers．