Circulation Characteristics And Case Study Of Extratropical Cyclone Snowstorm Process In Northeast Chin

In Northeastern China,extratropical cyclone is a significant weather system that contributes to snowstorm,particularly when it is accompanied by atmospheric river that brings both heavy rain and snow.However,it has always been challenging to accurately forecast snowstorms caused by extratropical cyclones.Therefore,evaluating the characteristics of the atmospheric rivers in snowstorms caused by extratropical cyclones was the first step in this paper,which was followed by a case study of a typical snowstorm caused by Huanghuai cyclone affecting North China and Northeastern China on November 17-20,2020 to examine the occurrence and evolution of the mesoscale snowbands through diagnostic analysis and numerical simulation.The results of a statistical study on snowstorms in Northeastern China caused by extratropical cyclones with and without the support of atmospheric rivers(ARs)revealed,ARs occurred in 86 percent of snowstorms associated with extratropical cyclones,with 61%of ARs landing in Northeastern China.The frequency of ARs was the highest during the seasonal transition period.The frequency and intensity of ARs were high during the snowstorms involving the Southern and Huanghuai cyclones,and the associated precipitation was heavy.There were two branches of upper-level jets and strong low-level jets for the snowstorms with ARs.And a cold vortex was located at 500 h Pa,forming the interaction between the cut-off low and the ARs,which was conducive to heavy rainfall.An obvious warm tongue and strong frontal zones were located at 850 h Pa,which was favorable for water vapor transport and dynamic lifting with the support of ARs.And then it rained associated with the cold front and snowed associated with the warm front during snowstorms accompanied by ARs.Compared with snowstorms accompanied by ARs,there was one branch of upper-level jets and weak low-level jets for the snowstorms without ARs.And there was no cold vortex and a weak westerly trough at 500 h Pa.The frontal zones and low vortex were weak at 850 h Pa without the support of ARs.The upper-level divergence was weak and water vapor was poor,and it mostly snowed associated with the warm front in snowstorm events without ARs support.ARs not only contain strong water vapor transport,but also have good dynamic uplift conditions.Although the Huanghuai cyclone which bringed both rain and snow on November 17-20 was not strong,it was accompanied by an obvious atmospheric river.A mesoscale snowband A with a radar reflectivity of 25 d BZ or greater,about 350-km-long and 70-km-broad,originally developed near Harbin in the warm front cloud system of the Huanghuai cyclone and persisted for 8 hours until dissipating.There were new snowfall zones on the south edge of the mesoscale snowband,which advanced to the northwest and united with the dissipating mesoscale snowband A to form the rather wide and 7-hour-long mesoscale snowband B.These two mesoscale snowbands were likewise accurately predicted by the numerical model.According to the simulation results,the mesoscale snowband A was primarily created by the release of conditional instability by frontogenetic forcing.The inertial instability organized the snowband,and diabatic heating also contributed significantly to its formation.Before the formation of snowband A,the large and small scale terrain promoted the formation of banded inertial instability in the lower layer,leading to the organization of convection.And the frontogenetic forcing in the lower layer began to strengthen,and the conditional(symmetric)instability in the front area began to develop.When the snowband formed,the conditional instability layer continued to thicken,and part of the conditional(symmetric)instability was forced to release by the continuous enhancement of frontogenesis.The maturity of the snowband showed that the atmospheric stability began to increase and the frontogenetic forcing continued to increase,with large and meso scale terrain playing an important role during the maintenance of the snowband.The dissipation of the snowband was characterized by the weakening of frontogenetic forcing and diabatic heating,and the continuous enhancement of atmospheric stability.The conditional instability layer of continuous thickening and extending to the northeast during the activity of snowband A promoted the formation of the second snowband B.Diabatic heating and frontogenesis zone consistent with the trend of snowband B were also conducive to the development of snowband.