Comparative Study On The Causes Of Typical Extreme Low Temperature Events

Under the background of global warming,the frequent occurrence of extreme low temperature events in China in recent years has brought severe effects on the sustainable,stable,and sound development of domestic economy.So it is necessary to further study the cause of typical extreme low temperature events.In this study,firstly,the extreme low temperature events in China are divided into two types: the extensive and persistent extreme cold events(EPECE)and the ordinary cold wave events(CWE)according to the duration of events.Then the evolution of circulation field during these two types of events is studied.Secondly,the combined anomalous characteristics of three large-scale circulation systems,i.e.,upper jet stream,blocking high,and Siberian high(SH),are studied using quantitative indices.Lastly,the evolution mechanism of the key circulation system is investigated from perspective of atmospheric low-frequency waves and transient waves during these two cases.The main conclusions are as follows..(1)Compared with the CWE events,the 500-hPa circulation field in Euraisa duringthe EPECE is characterized by the large tilting ridges and troughs,which are moreintensive and extensive.The ridge has been established over the Barents Sea12(-12)days before the blocking event and the SH is also intensified 8 days beforethe event.The intensity of these systems throughout the troposphere has beenslightly weakened but still maintained from 4 days prior to(-4)establiment ofevent and 4 days after(+4)the establishment day.On the other hand,the highatmosphere during the CWE is dominated by the ridge,without obvious precursorsignal.The tilting ridge and the SH significantly weakened and nearly disappearson +4 day.The continuous low temperature event in 2008 and the ‘Overlord level'cold wave event in 2016 are selected as the two typical cases on account of thesevere influence.It is found that,the precipitation is stronger,and the coolingduration is longer during the 2008 event,while the 2016 event occurred withstronger,wide-range cooling but less precipitation.Same as the EPECE events,during the 2008 event 500-hPa circulation field is dominated by large-scale tiltingridge.The lifetime of the blocking high is longer,but the strength is weaker thanthat during the 2016 event,along with the long distance between the main coldairmass and China,leading to the longer but weaker temperature reductioncompared with that in the 2016 event.The trough over the Middle Asia is activeand provides sufficient vapor transmission for the precipitation.The atmosphericcirculation in 2016 event is dominated by a strong ridge and the blocking high isextremely strong and developing rapidly.In the early stage,a large amount of coldairmass is accumulated by the mid-latitude transversal trough,leading to a sharpdrop in temperature lasting for a short time.(2)Then the combined anomalous characteristics of the large-scale system during thetwo events are investigated using the quantitative indices and the results show that,compared with the climatology,the jet core frequency of the subtropical jetstreams over the Middle East and East Asia in January increases 5 days in the2008 event,which increases slightly in 2016.And the jet strength in both events is10 m/s stronger.In 2008 event,the frequency of the Ural blocking high is 45%and the duration is persistent,and the Siberian High Index(SHI)fluctuates withthree peaks,20 hPa,14 hPa and 8 hPa,respectively.The blocking high is strongerwith 35% frequency,and the peak intensity of SHI is 37 hPa in 2016 event.Thetemperate jet in 2008 is stronger,with strengthened zonal stream and weakenedmeridional stream,and hence,the blocking high is weaker than that in 2016.However,the Ural blocking high has a longer lifetime,along with the fluctuationof the SHI,favorable for the transport of cold airmass into China.The easterly andstronger jet stream in Middle East and the active mid-latitude trough are favorablefor the vapor transport from the Bay of Bengal and the northeast Pacific intoChina,inducing the long-lasting precipitation and freezing rain.In the 2016 event,the blocking high abnormally strengthens.Dynamically modulated by the westerlyjet stream,the tilt of the blocking high ridge is northeast-southwest,transportingthe cold air southward,and hence the SH is abnormally strong,leading to a largearea of severe cooling.The cooling and precipitation during the 2016 event isweak due to the short duration of the blocking high,inactive mid-latitude troughand poor precipitation condition from the powerful cold air.(3)Using the low-frequency filtering method,the low-frequency characteristics ofcirculation system are studied from the perspective of atmospheric low-frequencywaves(30-60 days).The results show that the positive(negative)geopotentialheight anomalies in the unfiltered weather field in the 2008 event are consistentwith the low-frequency high(low)pressure anomalies in the low-frequencyweather map,and the positive center of zonal wind anomalies is also consistentwith the peak value of low-frequency wind field.The blocking high,theupper-level jet stream and the Arctic Oscillation(AO)show obvious 30-60 dayslow-frequency oscillation.The low-frequency high pressure center is ahead of the500-hPa geopotential height field,that is,the low-frequency high pressure signalcan be used for prediction.The 30-60-day low-frequency oscillation of thebackground circulation field in the beginning of 2016 is not obvious due to theshort duration and the different distribution of low-frequency systerms and thepath of southward transporting cold air.However,the low-frequency low pressurecenters provide a better reflection of the generation and evolution of polar vortex,and the low-frequency field can reflect the shift of the AO phase to some extent.When the cold wave breaks out,the AO is in its negative phase,the centers ofpolar vortex shows multipolar structure,and the strong cold air is accumulated,transported by the northerly flow in front of the blocking high ridge and the EastAsian trough into China and hence induces severe cooling.(4)Causes of the two events are investigated based on the blocking high lifetime,from aspect of the transport of transient eddy momentum and heat,theEliassen-Palm(E-P)flux,and Rossby wave breaking(RWB).The results showthat there is convergence of the transient wave [u'v'] and [v't'] at 55-65°N in thetwo events,favorable for the occurrence and the establishment of the blockinghigh.By contrast,on day 0 of the 2008 event,there is northward transport of [u'v']north and south of 60°N,but north side of [u'v'] transport is weaker than that onthe south side,indicating that there is convergence of [u'v'] at 60°N;there isconvergence of [v'T'] at 55°N.On day 0 of the 2016 event,[u'v'] is transportednorthward in the south of 65°N and southward in the north,indicating a strongerconvergence of momentum transport for transient waves at 65°N than that in 2008;and the convergence of [v'T'] is located at 60°N with a strong intensity.These areconsistent with the stronger and northward blocking high in this event.On day +3,there still exist transient momentum and heat transport in the region of theblocking high in 2008 event,leading to the persistence of the blocking high.Moreover,there is E-P flux convergence over the blocking high region during thetwo events.The westerly wind is decelerated in the convergence region,favorablefor the establishment of the blocking high.The E-P flux convergence center during the 2016 event is more northerly than that during the 2008 event,also consistent with the northward blocking high.The characteristics of RWB show that,there is anticyclonic RWB over Eurasia on day-2.On day 0 PV transport is enhanced significantly and the transient eddies influence the formation and maintenance of the blocking high through the anticyclonic RWB.RWB in the2016 event is stronger in intensity,more eastward in longitude and more northward in latitude,leading to an enhanced and northward shifted blocking high.