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Based on the conventional observation， the cloud top blackbody brightness temperature data of Fengyun satellite， and the European Centre for Medium-Range Weather Forecasts 0.25°×0.25° ERA5 reanalysis data from 2006 to 2021， we have conducted a statistical analysis on the heavy precipitation events caused by extratropical cyclones over northern and northeastern China during the cold season.The results show that： （1） Most of the extratropical cyclones occluded during the explosive development phase， but the development process is different， with the Shapiro-Keyser （SK-type） and the classical Norwegian （NW-type） cyclones each accounting for half.The SK-type cyclone has a deep trough at 500 hPa， strong baroclinicity， and steering airflow in the north-northeast-orientation， resulting in westerly cyclone path， and northward， widespread precipitation； the NW-type cyclone has a shallow trough at 500 hPa， with weak cold advection behind the trough， and the steering airflow in the east-northeast orientation， resulting in an easterly cyclone path， more to the south and more intense precipitation.（2） The atmospheric rivers of the NW-type cyclone are more intensethan those of the SK-type cyclone， and the corresponding heavy precipitation is extensive and more intense.With the development of the cyclones， the atmospheric rivers of the SK-type cyclone gradually turn to a north-south orientation， and there is a clear backward-turning feature on the north side of the atmospheric river， resulting in a warm front precipitation center of the SK-type cyclone located in the northwest quadrant of the cyclone， and the warm front precipitation center of the NW-type cyclone is close to the center of the cyclones.（3） The warm front frontogenesis in the northwest quadrant of the SK-type cyclone is significantly more intense than that of the NW-type cyclone， and the intense lifting forced by the front is conducive to heavy snowfall.（4） The SK-type cyclone has an intense potential vorticity structure in the shape of treble-clef at 300 hPa， while the NW-type cyclone has weaker potential vorticity in the upper levels.The stratospheric potential vorticity over the SK-type cyclone extends downwards， connecting with the low-level potential vorticity to form a potential vorticity tower， with a deep warm occluded structure nearby； while， the upper-level potential vorticity of the NW-type cyclone does not extend downward significantly， and there is no potential vorticity tower generated.The SK-type cyclone first develops in the middle troposphere， and then reachesthe surface， while the NW-type cyclone develops from the lower troposphere.

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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. [ABSTRACT FROM AUTHOR]