Evolution of ...Urumqi, China_Xia LI.pdf

Evolution of Meteorological Conditions during a Heavy Air PollutionEvent under the Influence of Shallow Foehn in Urumqi,ChinaXia LI*1,Keming ZHAO2,Shiyuan ZHONG3,Xiaojing YU4,Zhimin FENG5,Yuting ZHONG1,Ayitken MAULEN1,and Shuting LI11Institute of Desert Meteorology,China Meteorological Administration,Urumqi 830002,China2Xinjiang Meteorological Observatory,Urumqi 830002,China3Department of Geography,Environment and Spatial Sciences,Michigan State University,East Lansing 48823-5243,USA4LASG,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029,China5Urumqi Meteorological Satellite Ground Station,Urumqi 830011,China(Received 10 November 2021;revised 6 April 2022;accepted 11 April 2022)ABSTRACTThe air pollution in Urumqi which islocated on the northern slope of the Tianshan Mountains in northwestern China,isvery serious in winter.Of particular importance is the influence of terrain-induced shallow foehn,known locally aselevated southeasterly gale(ESEG).It usually modulates atmospheric boundary layer structure and wind field patterns andproduces favorable meteorological conditions conducive to hazardous air pollution.During 201317,Urumqi had anaverage of 50 d yr1 of heavy pollution(daily average PM2.5 concentration 150 g m3),of which 41 days were in winter.The majority (71.4%)of heavy pollution processes were associated with the shallow foehn.Based on microwaveradiometer,wind profiler,and surface observations,the surface meteorological fields and boundary layer evolution duringthe worst pollution episode in Urumqi during 1623 February 2013 are investigated.The results illustrate the significantrole of shallow foehn in the building,strengthening,and collapsing of temperature inversions.There were four wind fieldpatterns corresponding to four different phases during the whole pollution event.The most serious pollution phase featuredshallow foehn activity in the south of Urumqi city and the appearance of an intense inversion layer below 600 m.Intenseconvergence caused by foehn and mountainvalley winds was sustained during most of the phase,resulting in pollutantssinking downward to the lower boundary layer and accumulating around urban area.The key indicators of such eventsidentified in this study are highly correlated to particulate matter concentrations and could be used to predict heavypollution episodes in the feature.Key words:foehn,elevated southeasterly gale(ESEG),heavy air pollution,boundary layer structure,wind field pattern,complex terrainCitation:Li,X.,K.M.Zhao,S.Y.Zhong,X.J.Yu,Z.M.Feng,Y.T.Zhong,A.Maulen,and S.T.Li,2023:Evolution ofmeteorological conditions during a heavy air pollution event under the influence of shallow foehn in Urumqi,China.Adv.Atmos.Sci.,40(1),2943,https:/doi.org/10.1007/s00376-022-1422-x.Article Highlights:Heavy air pollution processes in Urumqi are often associated with shallow foehn.Shallow foehn influences inversion intensity and wind field patterns to be extremely conducive to heavy air pollution.Temperature at the top of the inversion layer and pressure differences across the Tianshan Mountains are key indicatorsfor air quality prediction.1.IntroductionFoehn is a name given to strong downslope winds thatare often warm and dry and usually occur on the lee side ofmountains when airflow crosses mountain tops (Seibert,1990).Strong foehn can bring risks to the safety of infrastruc-ture,transportation,etc.,and contribute to the spread of wild-fires(Fotini et al.,2012).Warm foehn can lead to early melt-ing of snow on mountains,which can affect skiing as wellas prompt an earlier start of the phonological cycle(Richnerand Hchler,2013;MacDonald,et al.,2018).Moreover,foehn winds have competing effects on air quality.On the pos-itive side,foehn flows can blow away local haze and*Corresponding author:Xia LIEmail:ADVANCES IN ATMOSPHERIC SCIENCES,VOL.40,JANUARY 2023,2943 Original Paper Institute of Atmospheric Physics/Chinese Academy of Sciences,and Science Press and Springer-Verlag GmbH Germany,part of Springer Nature 2023improve the air quality (Levin et al.,2005;Holla et al.,2015).Negatively,foehn,as a kind of flow,can transportair pollutants.The Altdorf region in Switzerland,on the leeside of the Alps,often suffers from an increase in ozone con-centration carried by foehn from high-altitude ozone-richareas (Baumann et al.,2001).Furthermore,air qualitybecomes poor in Munich of Germany(Hoinka and Rsler,1987)and in Calgary of Canada(Nkemdirim and Leggat,1978)due to temperature inversions associated with warmfoehn.In Inn Valley in Austria,there are three types of mecha-nisms related to the types of foehn that impact air pollutiontransport(Gohm et al.,2009).Thus,the effects of foehnwinds on air quality are complex.Foehn winds have been observed in the Taihang Moun-tains,Daxinganling Mountains,Tianshan Mountains(Pan,1994),Ailao Mountain(Zhang et al.,1993),and XialiangMountain(Ren,1998)in China.Many studies on foehn inChina have focused on the its formation mechanisms(Qiand Fu,1993;Meng et al.,1996;Ren,1998;Wang et al.,2012a),the weather and climatology(Zhang,2007;Wang etal.,2012b;Chen and Lu,2015;Luo et al.,2020)and the influ-ence of foehn on agriculture(Zhang et al.,1993;Zhang andZhang,1996)and skiing(Kong and Lian,2022).Now that itis understood how foehn winds influence climate,such asextreme heat waves in eastern China(Chen and Lu,2015),and local weather(Zhang,2007,Wang et al.,2012b),theyare suspected to also have impacts on air pollution.How-ever,the effects of foehn on air quality in China have so farnot been paid much attention(Yang et al.,2018).Urumqi,the capital city of Xinjiang Uyghur Autono-mous Region of China,has been known as one of the worlds top ten polluted cities since the late 1990s(Mamtimin andMeixner,2007).Apart from excessive anthropogenic pollu-tant emissions,the local topography and adverse meteorologi-cal conditions have also made contributions.Urumqi is situ-ated on the lee side of the northern Tianshan Mountains innorthwestern China(Fig.1a).Standing at the northern exitof the Middle Tianshan Mountains Valley (MTMV),Urumqi is surrounded on three sides by mountains of13005000 m above sea level(ASL),with only one openingon the north side to the Junggar Basin(Fig.1b).The urban ter-rain slopes from southeast to northwest with an average eleva-tion of 800 m ASL and a drop of 300400 m.Often in cold seasons,as cold air mass from the peripheryof the Mongolian high pressure returns to the south of theTianshan Mountains,a strong pressure gradient across theTianshan Mountains can build up,causing airflow to passthrough the MTMV and form a southeasterly gale(SEG)over Urumqi and its surrounding areas(Zhang et al.,1986;Meng et al.,1995).The SEGs,a kind of shallow foehn,canbe classified into two types:ground-based SEG(GBSEG)and elevated SEG(ESEG)(Li et al.,2012,2015,2020).Liet al.(2020)described the characteristics of these two typesof foehn winds in detail.In short,the GBSEGs can lead tostrong winds both in urban areas at the surface and in southernand western suburbs,especially in spring and autumn.ESEGs,which occur up to 128.5 d yr1 with a maximum fre-quency of 47 days in winter(Li et al.,2020),usually splitinto two layers upon landing on the southern suburbs ofUrumqi.The upper foehn layer spreads across the city to thenorthwest downwind region,while the lower foehn layer oscil-lates around the southern suburbs.Over the Urumqi urbanarea,winds are generally light or even calm(Li,2013).The impacts of SEGs on air quality in Urumqi are differ-ent depending on their occurrence seasons and other factors.In winter,the GBSEGs can clean out a heavily polluted airmass originally present over an urban area(Li,2006).How-ever,GBSEGs can sometimes result in dusty weather inUrumqi in the warmer half of the year,including summer.Observations taken during a GBSEG event on 3 April 2014show an astonishing PM10 concentration of 3720 g m3.Composite analyses of meteorological fields on 53 heavilypolluted days in winter demonstrate that the boundary layerover Urumqi is characterized by the ESEG,which is notonly favorable for maintaining a stable boundary layer,butalso conducive to the accumulation pollutants due to interac-tion with mountainvalley winds(Li,2013;Li et al.,2015).However,in-depth investigation into the effect of ESEG onatmospheric conditions has been lacking.Each heavy pollution weather process must exhibit a rela-tively long duration in order for the pollutants to graduallypile up and reach peak concentrations.At the same time,theboundary layer structure and surface meteorological fieldsmust also evolve step by step to eventually resemble unfavor-able meteorological conditions.The composite analyses(Liet al.,2015)illustrated the typical pattern of boundary layerstructure and surface fields related to heavy pollution inUrumqi,but they failed to explain the detailed situation in dif-ferent stages regarding the whole heavy pollution process,which is important information for forecasters to be able toidentify trends in air quality.Furthermore,is each heavy pollu-tion event accompanied by ESEG?Which factors are effec-tive for predicting air pollution in Urumqi?Up to now,thereis no in-depth analysis of the above questions.The aim of this paper is to present a comprehensiveanalysis of a heavy pollution event related to ESEGs thatoccurred in Urumqi during 1623 February 2013.The obser-vation sites and instrumentation are introduced in section 2.Section 3 describes the classification and statistics of heavypollution events in Urumqi from 2013 to 2017.Then,theinvestigation of both the three-dimensional boundary layerstructure and the surface meteorological fields under the influ-ence of shallow foehn is given in section 4,and the potentialforecast indicators for the development of heavy pollutionevents are shown in section 5.Finally,section 6 presents theconclusion.2.Methods2.1.Instruments and dataMore than 60 automatic weather stations were estab-lished around Urumqi in 2008(Fig.1b),providing hourlywind,temperature,pressure,humidity,and other meteorologi-30METEOROLOGICAL CONDITIONS ACCOMPANIED BY FOEHNVOLUME 40cal data for the city and the surrounding areas.At theUrumqi weather station (Fig.1b),a US MP-3000Amicrowave radiometer and a wind profiler(CFL-03)havebeen in operation since 2011(Yang et al.,2013;Li et al.,2016).The microwave radiometer can get atmospheric radia-tion brightness temperature through microwave signals emit-ting from the atmosphere.Historical radiosondes at sites repre-sentative of the observing location are used with a neural net-work to retrieve tropospheric profiles from the microwave,infrared,and surface meteorological measurements(Ware,2003).The retrieved temperature profiles at Urumqi stationare in good accordance with the radiosonde data(Li,2013).There were seven atmospheric environment monitoringstations in Urumqi in 2013,as marked by the blue dots inFig.1b.The air pollution data in Urumqi were obtainedfrom the Ministry of Environmental Protection of PeoplesRepublic of China.The hourly average concentration of a cer-tain type of pollutant in Urumqi is considered when observa-tions are available from more than three stations,and thedaily average concentrations are computed from more than18 hourly averaged values.2.2.Mixing layer algorithmVertical profiles of potential temperatures are obtainedfrom the temperature data of a microwave radiometer basedon the following equation(Zhang,2003):=T+(h0+z),where T is temperature at height z,is the dry adiabaticlapse rate of the free atmosphere(0.0098C m1),and h0 isthe local altitude(936 m ASL).The height at which the maxi-(b)(a)Fig.1.Topography of the Middle Tianshan Mountains(a)and location ofweather stations (black dots and red five-pointed star)and atmosphericenvironment monitoring stations(blue dots)in Urumqi and its surroundings(b).The black solid line roughly represents the border of the Urumqi urbanarea.Abbreviations for stations around Urumqi in(a):CJ=Changji;ANQ=Anninqu;GXQ=Gaoxinqu;WLB=Wulabo;SXG=Shuixigou;CWP=Chaiwopu;DBC=Dabancheng;DK=Dongkan.JANUARY 2023LI ET AL.31mum value of variability in the entire vertical profile of poten-tial temperature is located at a certain moment is taken asthe height of the mixing layer(Cimini et al.,2013).3.Classification and statistics of heavy airpollution processes3.1.Classification of heavy air pollution processesThe Air Quality Index(AQI),used to assess ambientair quality in China (CEA,2016),is classified into sixgrades:Grade I (050),Grade II (51100),Grade III(101150),Grade IV(151200),Grade V(201300),andGrade VI(300).For many years,the primary pollutant inUrumqi has been PM2.5,especially in winter(Li,2013).AnAQI 200 corresponds to a daily average PM2.5 concentration 150 g m3.In this paper,a daily average AQI 200defines a heavily polluted day.From 2013 through 2017,Urumqi experienced a total of 248 heavily polluted days(Table 1),with an annual average of 49.6 days,of which40.8 days were in winter(82.3%of the whole year),5.0 daysin spring(10.0%),0.6 days in summer(1.2%),and 3.2 daysin autumn(6.5%).Pollution in Urumqi is a result of both anthropogenicemissions related to human activities and natural emissionsfrom sources in the surrounding desert and bare land sur-faces.The timing of heavy pollution events caused by thesetwo types of emission sources rarely overlaps.Northern Xin-jiang,where Urumqi is located,is covered by snow in win-ter,and as a result,dusty weather is rare.However,tempera-ture inversions with light winds are prevalent in winter,which often leads to heavy pollution events.The incidenceof winter inversions over Urumqi is as high as 89%of thedays,and the average inversion layer thickness is 860 mabove ground level (AGL)(Liu et al.,2007;Li,2013).Some studies have linked air quality in Urumqi to the intensityof temperature inversions in this region (Li et al.,2007,2015).Shallow foehn,which frequently occurs in winter,fur-ther strengthens the intensity and increases the thickness ofthe inversion layer over the city,promoting the convergenceof airflows(Li,2013;Li et al.,2015).When cold air from the north enters Xinjiang duringthe warm half of the year(late spring through early fall),strong winds can lead to meso-to large-scale dust weather(Xu and Wang,2002;Wu,2003;Li et al.,2005;Shayiti etal.,2008),which is often referred to as northwesterly-induced dusty weather.In addition,the GBSEG originatingfrom the MTMV can entrain sand and dust along the way tothe urban area of Urumqi,leading to southeasterly-induceddust weather,also known as GBSEG-induced dusty weather(Wang and Zhang,2014).In contrast to these dust pollutionevents associated with strong winds,heavy pollution eventsalso occur under light winds and temperature inversionswith or without the occurrence of ESEG.3.2.Statistics of heavy air pollution processesA heavy pollution weather process is defined in thispaper as the period when the average daily AQI changesfrom a minimum value to a peak value greater