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Convective boundary layer evolution to 4 km asl over high-alpine terrain: airborne lidar observations.. Colbeck, I.; Kalberer, M.; Nyeki, S.; Steyn, Douw G.; Lugauer, M.; Furger, M.; De Wekker, Stephan F. J.; Kossmann, M.; Gaggeler, H. W.; Baltensperger, U.; Wirth, M.; Weingartner, E. 2000

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GEOPHYSICALRESEARCHLETTERS,VOL. 27, NO. 5, PAGES689-692,MARCH 1, 2000  Convective BoundaryLayerEvolutionto 4 km asloverHigh-  AlpineTerrain:Airborne LidarObservations in theAlps  SNyeki, laM.Kalberer, 1I.Colbeck, •S.DeWekker, 3M.Furger, •H.W.G•iggeler, • '  1  M.Kossmann, 4M.Lugauer, 1D.Steyn, 3E.Weingarmer, 1M.Wirth s,and U.Baltensperger Abstract.Mountain ranges haveimportant influences on the structure andcomposition of theconvective boundary layer(CBL) andfree troposphere (FD. Evolution of the summer CBL, measured overtheEuropean Alpsusingairborne lidar,wasclearly observed to attaina near-uniform heightupto4.2 kmaslbyearly afternoon.A climatologyof in-situ high-alpineaerosol measurements suggeststhat such substantialgrowth,  corresponding to- 0.3ofthemid-latitude tropopause height, often occurs during summer months. Subsequent nocturnal collapse of  (4.5- 12km)above theJFJinthemoming andaftemoon [Nyeki et al., 1999]indicated a uniform winddirection fromthenorth-west  (305- 315 ø)witha moderate wind strength (7- 15ms-•). The middleandupperFF wasstablystratified, wherez10/Az - 4.1 K km-• (0 = potential temperature). Although profiles below4.5 km werenot availablein the innerAlps, 1200UTC radiosonde datawindward of theAlps(Payerne, Switzerland; Lyon,France; Munich,Germany) indicated a CBL topcapped by a subsidence inversionat 2000- 2500 m, while leewardprofiles(Milan and  Udine,Italy)showed thesubsidence toreachto3000m.  theCBL wasestimated toresultin theventing of- 0.8_+0.3 (SO4)  Ggday -•intoaFFresidual layer, leeward oftheAlps. 3. Results and Discussion 1. Introduction  Lidarmeasurements consisted of a morningandafternoon flight  (Figure l) overtheJFJmassif at8 km,oriented parallel and Atmospheric structure in thelowertroposphere overhigh-alpine pattern totheregional mountain divide(NE-SW).Sixsimilar terrain is stillpoorlydefined [Blumen, 1990],despite theimpor- orthogonal  (labeled P3)wereflownduringthedayandareshownin tance of theEuropean Alps,Rockymountains, Andes andTibetan transects 2, whereatmospheric structure is depicted bytheaerosol plateau onsynoptic andglobal-scale circulation oftheatmosphereFigure ratio,B (i.e.(Rayleigh+ aerosolscatter)/Rayleigh [Barry,1992].Exchange of airmasses from the convective backscatter Transect P3represents thesharp transition in topography boundary layer(CBL)intothefreetroposphere (FF),orventing, is scatter)). foothills intheEmmental region (upto- 2000 aidedbymountain ranges [Kossmann etal., 1999]andmaythus fromthepre-alpine theJFJandthesurrounding massif (3000- 4000m), affectthespatial andtemporal distribution of aerosol properties. m),through HenceCBL depthis a fundamental variable in boundary layer (BL)research. Whileairborne aerosol lidaris beingincreasingly used toinvestigate CBLevolution, most investigations havesofar onlyconsidered flatterrain [Cooper andEichinger, 1994],coastal mountainous regions [McElroy andSmith,1986]or pre-alpine terrain[McElroyand Smith,1991].In orderto studyCBL evolution overhigh-alpine terrain,an airborne campaign was conducted overtheJungfraujoch (JFJ)research station (46.55øN, 7.98øE;3580 m; Switzerland). Favorableconditions for an 46.70 observational casestudy wereencountered onJuly30, 1997,when a nadir-pointing aerosol lidar(wavelength)• = 532nm)aboard the German Aerospace Establishment (DLR)Falcon20 jet aircraft [Kiemle et al., 1995]obtained a dataset withhightemporal and 46.60 spatial resolution ofatmospheric structure below8 km. 46.50  2. MeteorologicalConditions Synoptic conditions weresuch thatahigh-pressure ridgeextended from Scandinaviato southernFrance. Airmass back-trajectory  46.40  analyses usingthe 48-hourmesoscale modelof the Swiss Meteorological Institute suggested anoriginfromoverthenoah  ,,." i",,  Atlantic.Airborne measurements from stackedverticalprofiles  46.30  -  46.20  '  ' .---•----,  PaulScherrer Institute, Villigen,Switzerland.  University ofEssex, Colchester Essex, England. University ofBritish Columbia, Vancouver, Canada. University of Canterbury, Christchurch, NewZealand DLR,Oberpfaffenhofen, Germany.  7.60  i  7.70  '  t  7.80  '  i  7.90  ,  8.00  •  8.10  '  ,  8.20  I  8.30  '  8.40  Copyright2000 by theAmericanGeophysical Union.  Figure1. Nominal lidarpattem overtheJFJregion at 8000m. Topographic intervals at 1000m, darkest = 3000- 4000m.  Papernumber1999GL010928.  m), JFJ= Jungfraujoch station (3580m), FSA= Finsteraarhom  0094-8276/00/1999GL010928505.00  Abbreviations: I = Interlaken town(563 m), L = Lauberhom (2472  (4274m),andE = Eggishom (2927m). 689  690  NYEKI  ET AL.: CBL EVOLUTION  OVER THE ALPS' AIRBORNE  LIDAR OBSERVATIONS  • 2 ½•i"'"" •  1  •  0 A)0800 LST  3  ,  !  0•  0  • B)0920LST  C) 1010 LST  ,  li  0i D)1415 LST ,  .r-•.  4  1.00  3  1.08 1.17  1.27  1.48 •  1.37 [ 1.60  0 E)1535 LST l"Y", 1.73 • 1.87 5 • •.;.""•.,q•¾•, ' ?•'• ,-•;•.•-•z'•., •'•. • •.• • .... o  z4  •  Z6 3.  •  •.0  .•' ,•,:-'• <,  -h .•.  ,v•x•  ' *'  ,..•'  •,  :  ',  *". •,• •  F) 1620 LST  Lat. (øN) Dist. (kin)  46.40  46 50  ß•  -30  -25  -20 Rhone  Valley  ß  E  -10  46.70  46.60 ,  -15  -5  Aletsch  Glacier  ,  0  5  ß  ß  JFJ  L  10  20  15  i  Figure 2. Lidartmnsects (transectP3) illustratingCBL temporalevolutionovertheJFJmassifiDetailssameas in Figure1.  NYEKI ET AL.: CBL EVOLUTION OVER THE ALPS: AIRBORNE LIDAR OBSERVATIONS and down over the Aletschglacier towardsthe inner Swiss and Italian Alps. Pre-alpine landuse consists largely of urban, agriculturaland forested areas, which changesto snow and glaciatedcover abovethe snowlineat -- 2500 m in late summer. Transecttimes at 0800 LST (= UTC + 1), 0920, 1010, 1415, 1535  and 1620 in Figure 2, correspond to averageBL heightsof- 2.5 km, 2.7, 2.8, 3.9, 4.0 and 4.0, respectively.In the early morning (Figure2A-C), the growingCBL is hard to distinguisht¾omthe residuallayer, with the latter most probablyforming the upper boundaryof the BL. However,CBL growthoccumfairy rapidly thereafter,until the last two transectsillustratea quasi-stationary averageCBL height of- 4.0 km, which is substantialas it corresponds to - 0.3 of thetropopause heightat - 12.5km. A numberof dynamicalfeaturesare evidentin Figure2 and include:Cloud formationabovesouth-facingslopes(e.g. Fig. 2D-F, 46.59øN), cloud-formingthermalsthat reachto the CBL top (e.g. Figure2E, 46.73øN),and maximumgrowthof the CBL to - 3.1 km agi (abovegroundlevel)abovesomevalleyswhich is comparableto strongCBL growthover flat terrain.It shouldbe notedthatenhancements in B are partlydue to aerosolhumidity growthabovea relativehumidity- 85% and not only to increased aerosolconcentrations from naturalor anthropogenic activity. Among the interestingaspectsin Figure2, two main t•aturesare discussed in greaterdetail:Uniformityor "levelness" of the CBL top, andkatabaticflow of FF air downthe JFJmassif. Transectsin Figure 2 exhibit no significantterrain-following behaviorof the CBL top,at leastovera spatialscaleof 20 - 25 km. Similar observations of CBL levelnesswere recentlymade in an investigation over the BlackForestregion(< 1500 m) of southern Germany[Kossmannet al., 1998,Kalthoffet al., 1998].However, terrain-followingbehaviorwas observedto occur during the morning,andonly changedin the afternoonwhenthe CBL depth exceededthe characteristic scaleof the obstacle,given by Z -  (HL)ø'5, where H andL arethecharacteristic height andlength of themountain,respectively [Kaithoffetal., 1998].Whenappliedto the JFJ massif (H •- 2500 m and L-• 40 km), this resultsin Z-  10  km, and is -• 3 timesthe maximumobservedCBL depthof 3100 m over the Rhone valley (Figure 2D-F, 46.41øN). While elementaryconceptsexist for the descriptionof CBL levelness over smooth hilly terrain below the snowline [Stull, 19921, conceptsfor high-alpinemountainous terrainand the influenceof highsnow/icealbedoonCBL evolutionhaveyetto bedeveloped. Characterizationof the CBL top largely depends on the horizontalscaleof investigation. Over scalessimilarto transectP3 (- 45 - 50 km), an inclination of the CBL top towards the Emmentairegionis seenin Figure2. The residuallayerover the Rhonevalleylies - 700 m higherthanover the Emmentairegion at 0800 LST, and is a featurethat remainsthroughoutthe day, reducingto a heightdifferentialof- 400 m by 1620 LST. This  691  8O 7O 6O  • 50 •  40•  40  m•30 20  20 10  \ •_• ....  v  0  • i  .....  6  _.1 I .-o- Norfh(N-NW) i  .....  • July 30,1997  12  i'''  18  24  .......  0  6  Time (LST)  .L,-T 12  18  [o 24  Time (LST)  Figure 4. JFJdiurnalcycleof the medianaerosolsurfacearea concentration(St,). Anticyclonicconditionsfor June- August (1988- 1997) for the followingcategories: (A) North and West, and(B) South,Eastandfirdifferent.Horizontalline indicatesmean FF conditionson July30, 1997. may partly be explainedby the differentaveragealtitudeboth windward (500 - 2000 m) and leeward (2500 - 3500 m) of the mountaindivide.On a scalelargerthan 100 km, resultsin Figure2 and radiosondeprofiles suggestthat the CBL top follows the large-scale topography of theAlps. Severalevents,in whichcleansingflowsof FF air penetrateinto theCBL are seenat varioustimesandlocationsin Figure2. Highresolutionanalysisof transects in Figure2A-B revealsthe flow of FT air down the Aietschglacierin a surfacelayer50 - 80 magi. Similarglacier windsup to 100 magi havebeenobservedon the Pastemeglacierin Austria[van den Broeke,1997]. However,a more significantfeatureis the largekatabaticintrusionof FT air ontothe Aletschglacierand down the northfaceof the JFJmassif in Figure2D, which persistedto a lesserextenttbr at leasttwo houmuntil the last transect.Furtherevidenceof the spatialextent of the intrusionis seenat 1520 LST in transect04 (Figure3), orientedorthogonalto transectP3. Based on these resultsand other iidar transects(not shown), only one large intrusionwas observedto occur,whosehorizontalspatialextent(-• 15 x 25 km) corresponded approximately to thatof terrainabovethe snowline. These katabaticintrusionsarise from the cold air directly above ice/snowsurfacesresultingin the sinkingof FT air. Aerosoland meteorologicalrecords at the JFJ (discussedbelow) and the Eggishom automatic station (Fig, I; point E) confirm this intrusion, but also indicate that such occurrences are not common.  Evidenceof substantial CBL growthand a diurnalvariationin CBL heightalsoappeamin the long-term,in-situJFJandairborne aerosol  records.  Measurements  of  the  aerosol  surface  area  concentration(S^) have been conductedsince 1988 at the JFJ and  intermittentlyfrom 1988 - 1994 at Coile Gnifetti (4452 m; 45.90øN, 7.87øE, Switzerland) [Baltenspergeret al., 1997; Lugaueret al., 1998]. Figure4 exhibitsthedistinctdiurnalcycleat the JFJ from 1988 - 1997 during summermonths,and indicates the arrival of CBL airmassesat 1200 - 1300 LST, reachinga maximum  at 1800 LST and a minimum  after 0200 - 0300 LST.  The time periodduringwhich FF conditionsprevailis therefore provisionallydefinedhereas 0300 - 0900 LST, and the remaining period as BL conditions.A similar diurnal cycle, beginning2 houmlater undersimilarsynopticconditions,alsooccumat Coile Gnifetti and suggestsCBL growth to above4500 m, but would requireconfirmationwith for instancean aerosolairborneiidar. While the onset of CBL conditions is evident in the aerosol record,  Figure 3. Lidar transect04 (1520 LST), illustratingkatabatic flow down the JFJ massifi  the time at which the CBL "collapses" and a new nocturnalBL formsis moredifficult to estimate.Due to the presenceof residual CBL layerswindwardof the JFJ,whichattaina maximumheight of 3800 m overtheEmmentalregion(Figure2E-F), FF conditions areonlyrestoredoncetheselayershavebeenadvectedaway.  692  NYEKI ET AL.: CBL EVOLUTION OVER THE ALPS: AIRBORNE LIDAR OBSERVATIONS  Transectsin Figure2 indicatethat the transportmechanismof airmasses to theJFJdependsmainlyon CBL growth,ratherthan on local thermally-induced winds.Althoughsomeevidencefor thesewindsare seenin Figure2E-F, on the northernflank of the JFJ massif(46.56 - 46.58øN) as a local increasein B, they are expectedto be less significantdue to the aspectand snow/icecoverof theincline[Baltensperger et al., 1997]. Long-termresultshavealsobeenanalyzedaccordingto the synoptic Alpine WeatherStatistics(AWS) scheme[Schiiepp,1979]. Figure4A and B illustratesthat winds in the i) North and West categoriesduringFF conditionsexhibita lower meanvalueof SA  (18.5lam2cm3;STPconditions unless stated) thaninii) theSouth, EastandIndt.'fferent categories (40.5lam2cm-3). Thehigher mean elevationandarealextentof the innerAlps (2500 - 3500 m) south of theJFJmassif,in comparison to theEmmentalregion,resultsin this behavior[Lugaueret al., 1998]. The JFJ diumal variationin SAon July30 resembles andcorresponds to theAWS anticyclonic North categoryin Figure 4. A sharpincreasewas observedto beginat--1300 LST, andagreeswith lidartransects whichindicate growth of the CBL to JFJ altitudebetween 1010 and 1415 LST (Figure2C-D). Airbornemeasurements in the lowertroposphere  (4.5- 5.5km)indicated thatSA= 8.7- 9.0lam2cm-3during both morningandaftemoonverticaldescents overtheJFJ[Nyekiet al., 1999],illustratingthattheCBL top wasbelow4.5 km. An alternativemethodto determineCBL heightis throughinsitu JFJ aerosoland column-integrated aerosoloptical depth (AOD) measurements on July30. Reasonable assumptions include a well-mixed  CBL above the JFJ and that the increase in AOD  duringthe daytimeis due to CBL aerosols.Usingthe 1620 LST lidar transect(Figure 2F), for which the CBL is more fullydevelopedand homogeneousthan other transects,an AODderiveddepthof 370 + 80 m aglgivesa totalCBL depthof 3950 + 80 m andcompareswell with thelidarvalueof 4000 - 4100 m. A final issueof importanceconcernsthe relevanceof the JFJ data recordon mountainventingof CBL airmasses into the FF. Recent evidence suggeststhat once pollutants have been transported to high elevations,horizontaladvectionthenplaysan important rolein theformationof FF residuallayers[Kossmann et al., 1999]. This is alsoobservedherein radiosonde profiles.The water vapor mixing ratio (r) profile from the Milan 0000 UTC radiosounding on July31, exhibiteda layerof enhanced r above theCBL up to 4 km, a featurenot seenin thewindward1200UTC Payemeradiosounding on July 30. Similar findingswere also observed for locations windward(MunichandLyon)andleeward (Udine)of the Alps,andsuggest thatthesourceof thishumidity wasverticaltransport in theAlps. Basedon thisevidenceandtheJFJaerosolrecord,a simplebox  above.As thisestimatefor aerosolexportto the FF via residual layersis basedon a numberof assumptions (point measurements at theJFJ,well-mixedCBL), interpretation shouldbe restricted to the specificmeteorological conditionsencountered on July 30, 1997.Applicability of ourresultson a spatialalpine-widescaleis suggested by radiosoundings, while on a temporalscale,further airbornelidar data duringdifferentseasons and meteorological situationsare required. However, as a first-order estimate, mountain-induced ventingmay only be an importantmechanism undercertainsynoptic conditions duringsummermonths.  Acknowledgments.Our thanksare extendedto the DLR and JFJ  research foundation. AOD dataandbacktrajectories weresupplied by Dr. Heimoand Mr. Schneiter, SMI. This work was kindlyfundedthrough NERC,BUWAL, andtheEU STAAARTEandSwissGAW programs. References Baltensperger, U., et al., Aerosolclimatologyat the high-alpinesite Jungfraujoch, Switzerland, J. Geophys.Res.,102, 19707-19715,1997. Barry,R. G., MountainWeatherandClimate,Routledge, London,1992. Blumen,W., ed., Atmospheric Processes over ComplexTerrain, AMS, Boston, ! 990.  Chin, M. and D. J. Jacob,Anthropogenic and naturalcontributions to tropospheric sulfate:A globalmodelanalysis. J. Geophys. Res.,101, 18691-18699, 1996.  Cooper,D. I., andW. E., Eichinger, Structure of theatmosphere in an urbanplanetaryboundarylayerfrom lidarandradiosonde observations, J. Geophys.Res.,99, 22937-22948, 1994.  Kalthoff, N., H.-J.Binder, M. Kossmann, R. Vrgtlin,U. Corsmeier, F. Fiedler, andH. Schlager, Temporal evolution andspatial variation of theboundary layerovercomplex terrain, Atnu•s. Environ., 32, 1179-1194, 1998. Kiemle,C., M. K•istner, andG. Ehret,The convective boundary layer structure fromlidarandradiosonde measurements duringtheEFEDA '91 campaign,J. Atmos.Ocean.Technol.,12, 771-782, 1995,  Kossmann, M., et al., Aspects of theconvective boundary layerstructure overcomplexterrain,Atmos.Environ.,32, 1323-1348,1998.  Kossmann, M., et al., Observations of handover processes betweenthe atmospheric boundary layerandthefreetroposphere overmountainous terrain,Contr.Atmos.Phys.,72,329-350, 1999.  Lugauer, M., et al., Aerosol transport to thehighalpinesitesJungfraujoch (3454m asl)andColleGnifetti(4452m asl),Tellus,50 (B),76-92,1998. McElroy,J. L., and T. B. Smith,Vertical pollutantdistributions and  boundary layerstructure observed by airborne lidarnearthecomplex southernCaliforniacoastline,Atmos.Environ.,20, 1555-1566, 1986.  McElroy,J. L., and T. B. Smith,Lidar descriptions of mixing-layer thickness characteristics in a complexterrain/coastal environment, J. Appl. Met., 30, 585-597, 1991.  Nyeki S. et al., Condensation Nuclei (CN) and UltrafineCN Number  Concentrations in theFreeTroposphere to 12km:A casestudyoverthe Jungfraujoch high-alpine research station,Geophys.Res.Lett., 26, 2195-2198, 1999.  model was used to estimate the CBL aerosol mass concentration  Schiiepp,M., Klimatologie der Schweiz,Beilagezu denAnnalen1978, SwissMeteorological Institute,1979.  ventedintothelowertroposphere, leewardof theAlps.The aero-  Stull,R. B.,Procs. 6thAMSConfMountain Meteorology, Portland, J92-J94, 1992.  sol sulfate mass concentration(M) in the accumulationmode Van den Broeke, M. R., Structure and diurnal variation of the range(d = 0.1 - 1.0 lam)hasbeenshownto correlatewell with SA atmospheric boundarylayerovera mid-latitude glacierin summer, Bound.Lay. Met. 83, 183-205, 1997. at the JFJ(seeBaltensperger et al., 1997).Hence,it is estimated  thatM = 3.30+ 0.33(SO4) lagm-3during CBLconditions onJuly M. Furger, H. W. G•iggeler, M. Kalberer, M. Lugauer, S. 30, 1997(Fig.4A), whichcompares well withthecorresponding U. Baltensperger, Nyeki,andE. Weingartner, PaulScherrer Institute, CH-5232Villigen, Switzerland. (emai!:nyeki@psi.ch) the residualaerosollayerappearing leewardof the Alps were I. Colbeck,Institutefor Environmental Research, University of Essex, estimatedfrom: i) radiosoundings (depth= 2000 + 100 m), ii) Colchester/Essex, CO4 3SQ,England. windwardbreadthof the Alps (width = 630 + 50 km) and iii) S. De Wekker,and D. Steyn,Dept. Geography, Universityof British Columbia,217-1984WestMall, VancouverBC, V6T 1Z2,Canada. durationof CBL conditions andthe averagewindspeed (to give  AWSNorthvalue ofM = 2.90+ 0.30(SO4) lagm-'•.Dimensions of  Dept.Geography, University of Canterbury, PrivateBag length = 197+ 28km).Results gave0.8+ 0.3(SO4) Ggday -•for M. Kossmann, Christchurch,New Zealand. July30,andcompares with18.6(SO4) Ggday -I (yearly average) M. 4800, Wirth,Institute for Atmospheric Physics, DLR, Oberpfaffenhofen, D-  for CBL airmass exchange with the FF overEurope[Chinand  82234Wessling, Germany.  Jacob, 1996]. These translateinto a vertical flux oI -- 6.5 + 2.7  kgkm-2day -I overtheAlpsanda yearly average valueof -- 1 kgkm-2day -I forEurope using thehorizontal dimensions given  (Received July13, 1999;Revised October 21, 1999; AcceptedNovember19, 1999)  

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