Classification Characteristics And Model Forecast Evaluation Of Short-time Heavy Precipitation In Eastern Gansu Province

Short-time heavy precipitation is prone to disaster and difficult to forecast and warn in eastern Gansu Province.This study combines the scientific issues of short-time heavy precipitation forecasting and warning,and investigates the classification characteristics of synoptic-scale circulation situation for short-time heavy precipitation by using hourly precipitation data from encrypted stations,C-band doppler weather radar,NCEP/NCAR reanalysis,and numerical model forecasts from the actual forecasting operational needs.Through the analysis of the synoptic-scale circulation classification and its relevant conceptual models,the common characteristics of radar echoes of small and medium-scale systems in different circulation situations are summarized,and the spatial biases of numerical models for different types of heavy precipitation forecasts are quantified.Firstly,an innovative combination of objective cluster analysis and subjective synoptic verification is used to classify the synoptic-scale circulation situation,which is more objective than the previous over-reliance on manual subjective circulation classification in this region.The relevant conceptual models are constructed based on the classification results,which help to improve the understanding and operational application of forecasting and warning for short-time heavy precipitation in the region.Using hourly precipitation observations from encrypted stations starting from April to September of 2010-2021,53 regional short-time heavy precipitation cases are selected.Based on the reanalysis data,a combination of objective cluster analysis and subjective synoptic verification are used to classify the synoptic-scale circulation situation of these selected precipitation cases,and it is found that the synoptic-scale circulation situation of regional short-time heavy precipitation in eastern Gansu Province can be classified into three major types: eastward-moving plateau trough,marginal sub-high type(southwest flow type,two high shear type)and northwest flow type.The relevant conceptual models for the three types of weather forms are constructed:(1)When the eastward-moving plateau trough type occurs,the western Pacific subtropical high moves southward,and the short-wave trough in the westerly wind belt superimposes on the plateau trough during the eastward-southward,guiding the cold air from the north and meeting the warm and wet airflow from the southwest in front of the high altitude trough over the eastern part of the northwest China,and the baroclinic fronts are generated and strongly converge to form widespread short-time heavy precipitation.(2)The marginal sub-high type can be subdivided into the southwest flow type and the two high shear type,of which the southwest flow type is mainly characterized by the western ridge of the western Pacific subtropical high west of 110°E and the ridge line near 30°N.The eastern part of the northwest China is influenced by the stronger southwest airflow at the periphery of the western Pacific subtropical high,and the cold air is active,forming a "west low and east high" distribution,the baroclinic front is stronger than the eastward-moving plateau trough type,and the scope and intensity of short-time heavy precipitation is relatively larger.The two high shear type is the northward lifting of the western Pacific subtropical high ridge line to near 33°N-35°N,accompanying with a nearly north-south shear line and the Qinghai-Tibet high pressure from the eastern part of northwest China to Sichuan.There are often deep southerly winds near or to the east of the shear line,which often brings hotter and more wet and unstable environmental conditions than the southwest flow type.The cold air activity area moves northward and mainly intruded by the lower troposphere,causing a larger range of short-time heavy precipitation.(3)When the northwest flow type occurs,the area of eastern Gansu Province is influenced by the northwest airflow in front of the middle and upper troposphere ridge,and the northwest airflow and temperature trough form a strong dry and cold advection,superimposed on the lower relative warm and humid airflow or temperature warm ridge,which becomes the dominant factor of the instability mechanism of this type of short-time heavy precipitation.Besides,the short-time heavy precipitation fall area is relatively scattered.In order to establish the interface between the classification of synoptic-scale circulation and the common features of radar echoes involved in small and medium-scale convective systems,reflecting the basic features and key characteristics of small and medium-scale radar echoes in the three types of synoptic-scale circulation situations,radar and satellite data are further used to condense the common features of scale,morphological structure and their evolutionary propagation of small and medium-scale systems in the three circulation situations.(1)The radar echoes of eastward-moving plateau trough are often mixed band echoes of NNE-SSW direction in the middle ? scale,which are more consistent with the direction and position of the low-level jet axis to the right of the 700 h Pa shear line,mostly formed by low-level jet or low-level jet with infiltrating weak cold air.The propagation direction of the band echoes is mainly influenced by the low-level southerly winds and along the NNE-SSW direction,leading to the "train effect" in the southeastern Gansu Province.The echoes feature a low center of mass,warm cloud precipitation,and high precipitation efficiency.(2)The radar echoes of marginal sub-high type are often narrow band echoes of medium-? scale in the NE-SW direction,which are located near the ground front slightly ahead of the 700 h Pa shear line.The narrow band echoes are often generated when the cold air invades the central Gansu Province,and the northwest airflow in the lower layer of its rear troposphere has a much greater influence on its movement than the southerly wind-guided airflow in the middle layer,and the echoes move rapidly from the northwest to the southeast.The strongest echoes weaken during the movement and the center of mass decreases.As it moves to the southeastern Gansu Province,it gradually evolves into a radar echo signature with an eastward-moving plateau trough type.However,it moves faster to the southeast than the eastward-moving plateau trough.(3)The radar echoes of the northwest flow type are more scattered and highly localized,mainly medium-? scale block echoes,which belong to deep convection.The block echoes are stable and less dynamic,with a short life cycle in the form of a single body generating and eliminating,or multiple single bodies organizing,merging and strengthening,with a duration of less than 2hours.In addition to the analysis and research on the mechanism of its occurrence and evolution,the application and evaluation of numerical model precipitation forecasting is also an important way to improve the capability and level of forecasting and warning of short-time heavy precipitation.Finally,the CRA(Contiguous Rain Area)spatial verification method is used to evaluate the spatial bias of the ECMWF and CMA-GFS numerical models for heavy precipitation forecasts under three types of circulation situations,and to provide quantitative verification results on the location,extent and intensity of the heavy precipitation fall areas,which are of greater interest in forecasting.The study finds that the CMA-GFS and ECMWF models have the largest morphological errors in precipitation forecasts,followed by displacement errors,and intensity errors,with the forecasted heavy precipitation fall areas all being located farther west and north than the actual conditions.The area,mean rainfall intensity and maximum precipitation of the three types of heavy precipitation forecast by CMA-GFS are all small,especially for the southwest flow type and the two high shear type.In terms of spatial bias,the fall area of eastward-moving plateau trough type is westward and northward shift,the southwest flow type is westward shift,and the deviation of the two high shear type is not obvious.The ECMWF forecast of maximum precipitation for the eastward-moving plateau trough type is small,with the forecast area changing from small to large as the precipitation scale increases.The ECMWF forecast for the southwest flow type is overestimated,with slightly stronger average rainfall and smaller maximum precipitation.The ECMWF has a small forecast area for the two high shear type,but well captures the average and maximum rainfall forecasts.In terms of spatial bias,the ECMWF forecasts a west-northwestward bias for the eastward-moving plateau trough and southwest flow type,and a northward bias for the two high shear type.