A Comprehensive Analysis Of The Extreme Snowfall Process Of A Sudden Cold Wave In Early Spring In Ningxia

Using the conventionally observational data,hourly observation data of automatic meteorological station,radar data,satellite data and the ECWMF 6-hourly reanalysis data,based methods of combining conventional weather analysis with comprehensive physical quantities such as potential vorticity on the isentropic surface of 315 K and frontogenesis function,a disastrous sudden cold wave and extreme snowstorm process in Ningxia from 2 to 3 April,2016 was comprehensive analysised in terms of weather systems evolution,the sources and paths of cold airs,frontogenesis and secondary circulation,precipitation types and relevant forecast indicators.The results indicate that:（1）This process was classified as the type of high-level small trough moving eastward and merging.The surface cold anticyclone split and the main body moved southward rapidly with cold front,leading to the outbreak of cold wave and heavy snow behind the cold front.It is difficult to predict accurately by using conventionally meteorological observational data because the stable atmospheric circulation in the early stage of the process,and the abrupt weather system in the later period.（2）The 315 K isentropic potential vorticity can be used as an effective tool for analyzing and predicting the short-term local cold wave and snowstorm process in spring.First,the isentropic potential vorticity（IPV）can clearly trace the source and path of cold air.The cold air in the tropospheric from Aral Sea moved southeasterly,then merged with the cold air in the middle troposphere in Qinghai-Tibet plateau and thus got enhanced.The cold air in lower-stratosphere from east of Novaya Zemlya moved eastward slowly,then turned southward from Lake Baikal at a higher speed and supplemented the direct cold air mentioned above,causing the cold wave in Ningxia.Second,the great anomaly value areas of IPV can express the evolution of small trough quantitatively and clearly and had better forecast instruction significance than the conventional meteorological data.Third,the temporal evolution of IPV coincided with the evolution of cold wave,and the cold air activity can be indicated 6 hours ahead.The cold wave and the snow fall zones agreed with the area of values above 1.0PVU very well,and the zones of severe temperature drop agreed with that of 0.8PVU.IPV can be used as a key forecast indicator for fine quantitative forecast in Ningxia.（3）Under the effective configuration of upper jet,the westerly small trough and mesoscale convergence line drove the frontogenesis in the middle and lower troposphere,which was mainly affected by wind shear deformation,horizontal divergence and latitudinal distribution of maximum upward motion.The large value region of the frontogenesis function coincided with the position and evolution of frontal zone.The frontogenesis function can quantitatively indicate the intensity of frontal zone and predict the occurrence,enhancement and weakening of cold wave,rain and snow through its numerical evolution.It can be used as a comprehensive physical indicator for frontal analysis and related frontal weather diagnosis.The precipitation area was located below the ascending branch of the jet-front secondary circulation,which consistented with the high gradient area of the frontogenesis function tilt term（F4）and the ascending maximum area.The precipitation,the gradient of F4 and the ascending motion reached their maximum at the same time.（4）There were two main types of water vapor conveyor belts.First,water vapor in the Bay of Bengal（700⁶00h Pa）was transported to the northern part of Ningxia（39°N）through Yunnan and Sichuan Province,which was the main source of water vapor.Second,water vapor in the Bohai Bay（850h Pa）was transported westward to Ningxia with a small contribution.（5）This precipitation process was mainly caused by the low trough-cold front stratiform cloud system.Radar echoes were patchy,accompanied by strong echo centers of 30⁴0d Bz,which advanced from north to South and maintained for about 5 hours in the north-central part of Ningxia,resulting in short-term heavy precipitation.The intensity of precipitation was closely related to the strength and duration of echo center.The structure characteristics of TBB low value area（TBB<-30°C）can better indicate the existence of shortwave trough,vortex and cold front,and the low value area of TBB（-42^-51°C）corresponded well with the strong precipitation area.（6）If T₈₅₀≤0°C,T₇₀₀≤-4°C or H₀≤160dagpm,the north-central part of Ningxia will change from rain to sleet or snow.Combined with the surface temperature,the sleet and snow can be further determined.If the ground temperature is below-1°C,the main precipitation type is snow;if the ground temperature is between 0¹°C,the main precipitation type is sleet.Radar reflectance factor can also reflect the rapid change of precipitation types.When the echo intensity is 30³5d Bz,the main precipitation type is rain;with the decrease of the echo intensity,the rain turns into sleet;when the echo intensity is 20²5d Bz,the main precipitation type is snow.