| The snowstorm in extratropical cyclones usually is characterized by wide range of influence,large amount of precipitation,and complex precipitation phases.The bias of operational quantitative precipitation forecasting for snowstorm is mainly related to insufficient understanding of synoptic and mesoscale precipitation systems within extratropical cyclones.In this paper,the characteristics of warm conveyor belts in extratropical cyclones causing snowstorms in Northeastern China are firstly analyzed,and then the structure and development mechanism of mesoscale precipitation system in typical snowstorm cases are studied.The role of warm conveyor belts(WCBs)on the development of cyclones and precipitation are investigated for snowstorms in northeastern China during 2006–2021.Main results show that:(1)Rapidly intensifying occluded cyclones typically have strong WCBs,the interaction between low-level potential vorticity(PV)and upper-level PV in occluding stage eventually forms a PV tower throughout the troposphere.Non-occluded cyclones also have strong WCBs but lack of upper-level PV forcing.The development of these cyclones is relatively moderate.Weakly intensifying occluded cyclones are driven by upper-level PV,their WCBs and precipitation are not obvious,and the cyclone deepening rate is the smallest.(2)The low-level inflow of WCBs locates in the center and warm sector of cyclones,forming a commalike structure after the cyclone occlusion,and the outflow locates northwest and downstream of cyclones.(3)The PV increases significantly due to the release of latent heat during the uplift of WCBs.Cyclones with strong WCBs cause more precipitation,and the distribution of rainfall is closely related to the location of WCBs.In order to analyze the mesoscale characteristics of winter season precipitation within extratropical cyclones,the coverage of GPM-DPR observations on snowstorm cases over Northeastern China is counted,and two cases with relatively complete observations are selected for research.GPM observed three precipitation areas during the two snowstorms on November08–09,2018 and November 07–08,2021,which had differences in many aspects such as the precipitation rate,phase,and three-dimensional radar echo structure.A bright band of radar echo could be seen near the freezing layer of mesoscale rainband;The radar echoes of heavy rainfall were greater than 40 d BZ and extended from ground to the freezing layer;The lowlevel reflectivity of warm front snowfall and snowband was less than 30 d BZ,with no freezing layer nor bright band.The distribution characteristics of precipitation particle concentration and radius indicated that the forming mechanism of strong radar echoes in these mesoscale precipitation systems were different.Affected by the Huanghuai cyclone,heavy rainfall and snowfall occurred in Liaoning and Heilongjiang respectively during 18–20 November 2020,and different mesoscale rainfall and snowfall echoes were observed by ground radar.Using the ensemble-based sensitivity analysis,the key synoptic-scale features influencing the mesoscale rainfall and snowfall are investigated.The global ensemble could forecast the occurrence of heavy precipitation 60 hours in advance,and the forecast of snowfall was better than that of the rainfall.The heavy rainfall was relatively not sensitive to the upper-level jet,mid-level westerly trough,low-level vortex,and surface cyclone,but was closely related to the low-level jets and especially the boundary layer jet over the sea.Whereas the amount of snowfall near warm front had strong correlations with various weather systems in the upper and lower troposphere. |
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