Abstract
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.
Highlights
An occluded front is often formed during the development of a stronger cyclone
2b). ◦ C and 500-hPa relative showing a frontal coverage of cloud top temperatures humidity >70% within the warm front cloud belts was expanded to the north and northeast, and the warm and moist air on the northwest side was wrapped around the cyclone center, showing that a back-bent occluded front was formed at this time (Figure 2b)
The snowstorm event occurred in the divergence area of the upper-level jets
Summary
An occluded front is often formed during the development of a stronger cyclone. Operational forecasts show that some extratropical cyclones producing snowstorms over northeastern China in Atmosphere 2020, 11, 1272; doi:10.3390/atmos11121272 www.mdpi.com/journal/atmosphere. A strong snowstorm event that occurred on 3–5 March 2007 stemmed from a cyclone, whose warm front stretched westward and southward and twined in a spiral around the cyclone’s center to form an occluded front during the period when the cyclone rapidly deepened and moved northward from the Yangtze River Basin This cyclone resulted in snowstorms and storm surge disasters in eight Provinces over northeastern and northern China, with total accumulative snowfall at 32 stations in Lioaning Province exceeding. Understanding is limited with respect to the evolution, frontal structure and associated snowfall mechanism in a cyclone characterized by a back-bent warm front and wrap-up process, which leads to inaccurate fine-scale forecasting of snow.
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