The Dominant Circulation Pattern And Mesoscale Dynamic Process Of Extreme Precipitation In Spring In North China

The frequency variation of regional extreme precipitation under the warming climate is a hot and difficult issue in atmospheric science research in recent years.Spring precipitation in North China,especially extreme precipitation,is important to agricultural and urban economic development in China.Studies have showed that the large-scale circulation is a source of uncertainty in projections of regional extreme precipitation,and mid-latitude extreme precipitation events are often accompanied with frontal mesoscale convective systems(MCSs).Therefore,exploring the large-scale circulations related to spring extreme precipitation(SEP)in North China as well as the formation and maintenance of MCS accompanied with the fronts has important academic value and operational application prospect.In this work,daily dominant large scale circulation patterns as well as the key dynamic and thermaldynamic factors of SEP days in North China are studied using daily rainfall from 396 national operational weather stations and the fifth generation of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis during 1979-2019.The key dynamic processes of the formation and maintenance of MCS accompanied with fronts under the background of dominant circulation patterns are analyzed by using high spatio-temporal resolution observation,reanalysis data and the numerical simulation.Results show that:(1)During 1979–2019,the SEP days are signifantly correlated with the variation of annual total precipitation.The precipitation on SEP days accounts for the 63% of the total precipitation in spring.The number of SEP days shows large interannual variability but no significant trend in the study period.(2)Using the objective classification method of the obliquely rotated principal analysis in T-mode,the atmospheric circulations are classified into five different patterns based on the geopotential height at 700 h Pa.Three circulation patterns all have fronts and are associated with strong southerly wind,leading to 88% of SEP days in North China.The number of extreme precipitation days in spring is significantly correlated with the total number of days under the three dominant circulation patterns.By comparing the SEP days, dry days and the average circulation under the three circulation patterns,it is found that the strong southerly wind can provide water vapor transport and dynamic forcing for the frontal precipitation.The three dominant circulation patterns all have a geopotential height trough at 700 h Pa located in southwest China,a mid-high latitude trough located at the region north to North China,and a sea level high pressure along the coast of North China.The differences of different circulation patterns are the positions of mid-high latitude trough,corresponding to different locations of fronts and different areas with high frequencies of SEP events.The northern trough of the first dominant pattern is far to the east of North China.The cold air is mainly from the northeast and the north.This type is featured by the east–west-oriented front in North China and a shallow layer of relatively cold air east to Taihang Mountains showing the feature of backdoor fronts and cold air damming near the surface in North China Plain.The northern trough of the second dominant pattern extends from northeast China to the northern part of North China.The cold air affecting North China is mainly from the northwest.Accordingly,the front is along the Taihang Mountains and Yan Mountains.The trough and the front of the third dominant pattern are to the west of North China.The water vapor is lifted by the fronts and mountains,which leads to the highly frequent SEP events in line with the fronts and the mountains.Accordingly,the three circulation patterns are associated with high frequency of SEP spatial distributions in southern North China,northern North China and western North China,respectively.(3)The West Pacific pattern,influencing the circulation days of first and second dominant circulation patterns in spring,is found to be one of the possible climate viability modes related to the interannual variability of SEP days.The annual spring mean WP index,which is the mean WP index during March–May,is significantly positively correlated to the annual number of SEP days with the correlation coefficient being 0.42 and could explain 18% of the interannual variability of SEP days.The West Pacific pattern could influence the number of first and second circulation pattern days in spring.However,it could not significantly impact the northward transport of water vapor in the low level of troposphere.(4)Under the background of extremely strong water vapor transport through the lower layer from the southern boundary of North China,the precipitation centers of extreme precipitation cases,which are resulted from three dominant circulations,are all related to frontal MCS.All MCSs show the characteristics of elevated convection,occurring in the warm and wet air area region on the cold pad,accompanied by strong horizontal wind convergence.The fronts related to MCSs are different in three cases.In the case of first dominant circulation pattern,MCS occurrs along the back-door front paralleled with the Taihang Mountains,when the warm and moist air climbs the front.The rapid enhancement of the cold air from northeast below 1km behind the front made the front moving southward rapidly and the cold air dam becoming higher and stronger.At the same time,the MCS is enhanced and the convective center moving southward.The characteristics of fronts of the second and third dominant circulation patterns are both reflected in the warm and moist region on the cold pad in North China,which are characterized by the centers of horizontal equivalent potential temperature gradient greater than 4 K/ 100 km and strong horizontal wind convergence.However,the wind fields in the cold pad areas of the two cases are different.The MCS of the second dominant circulation pattern mainly occurrs at night,and the cold pad region is mainly dominated by north wind.The MCS of the third dominant circulation pattern occurred in the afternoon,and cold pad region is mainly dominated by south wind.Besides,the numerical simulation of the case under the first dominant circulation pattern shows that the ascending motion beneficial to MCS formation is mainly contributed by the resultant force of vertical pressure gradient force and buoyancy resultant force.Diagnostic analysis shows that the large value of the resultant force is distributed in front of the warm and wet air climbing along the front,where is close to the region with large horizontal or vertical equivalent potential temperature gradient.In this paper,the conceptual models of three dominant circulation patterns of SEP days in North China are established.The key thermodynamic factors of the large-scale circulation are the meridional water vapor transport in the lower troposphere through the southern boundary of North China.Based on the daily circulation patterns,the influence of climate variability mode on the interannual variation of SEP in North China is discussed,which could shed light on the study of the interannual variation mechanism of extreme precipitation days in mid-latitude region under the background of climate warming.At the same time,the case study reveals that the SEP days in North China ar related to MCS accompanied with the front.It is the first time to propose that the back-door front and cold air damming existed in the SEP day in North China under the background of the first dominant circulation pattern.This study also reveals the key mesoscale dynamic process of the SEP day in North China.