An extreme snowstorm event that occurred over Heilongjiang and Jilin Provinces on 24–26 November 2013 was related to a cyclone characterized by a back-bent occluded front structure. This study investigates the structure of the back-bent occluded front and snowfall mechanism using multiple observations and NCEP/NCAR 1° × 1° reanalysis data in concert with the HYSPLIT model. The main results show that the extreme event was more synoptically governed by the outbreak of the polar vortex and moisture anomaly of the East Sea. The cyclone occurred just ahead of the 500-hPa merged deep trough, and then developed under the effect of the positive vorticity advection ahead of the 500-hPa trough and intense divergence of the upper-level jet. The south-southwest wind strengthened obviously after the merger of the southern and northern branch troughs, which was the main reason behind the cyclone moving northward. The moisture mainly originated from the Sea of Japan, insofar as that dry and cold air in the lower troposphere over the western mainland moistened obviously as it turned southward and passed over the Bohai Sea and the Sea of Japan, supplying abundant moisture for the snowstorm event. The intensity of moisture transport depended on the location and intensity of the cyclone. When the cyclone developed, the dry air continuously intruded into the cyclone’s center, and made a conveyor belt of warm air wrap around it. The dry air gradually changed from descending to ascending motion as it moved ahead of the westerly trough, while the moist air in the northern part of the cyclone moved to the west and south and incorporated into the south of the cyclone center. Warm and moist air was lifted and arrived in the northwestern part of the cyclone after the occluded front’s formation. Frontogenesis within the comma head was enhanced evidently owing to the rotation and deformation. The convergence between the southeast and northeast winds resulted in intense frontogenesis, leading to the enhancement of the front-scale ascent. Strong ascent formed in the comma head of the cyclone, which resulted in intense snowfall.

On 12–13 February 2016, a record-breaking rain–snow event during the passage of an extratropical cyclone occurred in Shandong Province, China, in which the 24 h precipitation totals at 48 of 123 national meteorological stations in Shandong Province broke their historical records for the month of February, and a further 25 stations recorded their second-largest February totals. This paper investigates the evolution of the mesoscale precipitation areas and the mechanisms responsible for the formation, organization, and maintenance of the mesoscale precipitation areas, using FY-2G satellite data, Doppler radar observations, and a Weather Research and Forecasting (WRF) model numerical simulation at 4 km grid spacing. The main results show that the comma head cloud of the cyclone developed from four echo strips. Intense precipitation was related to the mesoscale elongated precipitation areas (EPAs) of reflectivity >30 dBZ within the stratiform clouds. The formation and development of the EPAs coincided with the activities of a low-level shear line and an associated increase in frontogenesis. The simulated EPAs occurred in an environment of conditional instability (CI), inertial instability (II), and conditional symmetric instability (CSI). In the initial stage of the elongated rainfall areas (ERAs), rainfall was initiated by the frontal forcing in the presence of elevated CI, and II was generated by upright convection. During the development stage of the ERAs, the CI was absent, and condensational heating was enhanced. II occurred in the absence of upright convection, and it seems likely that the presence of II is a diabatic signature of the precipitation itself. Upper-level II intensified the convective systems by enhancing outflow aloft, and II caused the ERAs to organize. Thus, II played an important role in the organization and maintenance of the ERAs. The frontogenesis provided the dynamic condition for the release of the instability. Enhanced CSI and II intensified slantwise convection, and combining with enhanced frontogenesis, intensified the ERAs. The echo, ascent, and frontogenesis in snowfall areas were weaker than those in rainfall areas.