Statistical Analysis of Heavy Precipitation Events Caused by Extra-tropical Cyclones of Different Occlusion Types during the Cold Season

Abstract

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.

Keywords

• extratropical cyclone
• rain-snow
• occluded front
• back-bent warm front
• composite analysis