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(Qian