PEM-West A: Meteorological Overview

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Title PEM-West A: Meteorological overview

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Journal Journal of Geophysical Research: Atmospheres, 101(D1)

ISSN 0148-0227

Authors Bachmeier, A Scott Newell, Reginald E Shipham, Mark C et al.

Publication Date 1996-01-20

DOI 10.1029/95jd02799

License https://creativecommons.org/licenses/by/4.0/ 4.0

Peer reviewed

eScholarship.org Powered by the California Digital Library University of California JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D1, PAGES 1655-1677, JANUARY 20, 1996

PEM-West A: Meteorologicaloverview A. ScottBachmeier, • Reginald EoNewell, 2 Mark C. Shipham, 3Yong Zhu, 2 DonaldR. Blake,4 and Edward V. Browell3

Abstract.Phase A of the NASA PacificExploratory Mission in thewestern North Pacific(PEM- WestA) regionwas conducted during September-October 1991. The backgroundmeteorology of easternAsia andthe western North Pacific region during the PEM-WestA studyis described.Mean large-scaleflow patternsare discussed along with transientsynoptic scale features (e.g., midlatitude cyclones,anticyclones, and frontal systems) responsible for long-rangetransport of tracespecies overthe studyregion. Synoptic summaries are givenfor eachof the 18 dataflights, together with selectedexamples of meteorologicalprocesses that gave rise to someof thechanges observed in the measuredtrace gases. Examples of large-scaleozone features observed above and below the DC-8 flight altitudeby an onboardlidar systemare alsorelated to meteorologicalprocesses such as stratospheric-troposphericexchange and upward transport of air from the boundarylayer.

1. Introduction 2. Mean Large-ScaleTropospheric Flow Patterns The broad objectivesof the NationalAeronautics and Space The meanlarge-scale tropospheric flow patterns during PEM- Administration's(NASA) PacificExploratory Mission in the west- WestA canbe characterized using 12 hourlygridded meteorologi- ern North Pacific (PEM-West)region are to studythe chemical cal fieldsobtained from the U.S. NationalMeteorological Center processesand long-rangetransport of tracegas speciesover the (NMC) operationalanalyses. The meanstreamline fields, aver- PacificOcean and to estimatethe humanimpact on chemistryof agedover the entire PEM-West A studyperiod (September 15 to the tropospherein thisregion. Specifically, the majorobjectives of October22, 1991) are schematicallyrepresented at the surface PEM-Westare to understandthe factorsinfluencing the budgetsof (Figure1), 850 hPa(-1.5 km,Figure 2), 500 hPa(--5.5 km, Fig- ozoneand sulfur.The overallexperimental design of PEM-West ure3), and200 hPa(--12 kin, Figure 4). encompassedtwo intensiveairborne field studiespositioned in DuringPEM-West A thelower-tropospheric flow characterized time suchthat contrastingmeteorological regimes in the western by the streamlinefields at the surfaceand 850 hPa(Figures 1 NorthPacific could be sampled.The firatphase, PEM-West A, was and2) wasinfluenced by largesemipermanent anticyclones over conductedduring September-October 1991, a periodin which the the western and eastern North Pacific as well as a less marked anti- lower troposphericairflow was dominatedby flow from the mid- cycloneover China. The cyclonicflow featuresfound over the Pacific regions.The phaseB was conductedin March 1994, a PhilippineSea and the western equatorial Pacific are not semiper- period characterized by maximum outflow from the Asian manentbut are manifestations of several transient tropical cyclones continent. which often reached their maximum intensities over those areas. DuringPEM-West A, a largesuite of atmospherictrace gases Note thatat the surface.three regions of convergingstreamlines was measuredby instrumentsaboard the NASA DC-8 aircraft. A definethe boundaries among the followingprincipal air streams: total of 18 flightswere conductedover the North Pacificbasin with (1) continental(Asian) air andmarine (western Pacific or South operationalflights at YokotaAir ForceBase (Japan), Hong Kong, ChinaSea) air, (2) westernand easternPacific marine air, and and Guam.An overviewof the instrumentation,flight tracks,and (3) northernhemisphere and southernhemisphere marine air. collaboratingprograms is given in an accompanyingpaper by Thesethree boundaries are also clearly identified by Newellet al. Hoell et al. [thisissue]. The purposeof thispaper is to providean [thisissue (d)] usingthe divergentwind components computed overviewof the meteorologicalsetting for the PEM-West A mis- from thevelocity potential at 1000 hPa. sion,including a flight-by-flightdiscussion of the meteorological The meanmidtroposphere flow characterizedby the 500-hPa factorsinfluencing the transport and chemistry of the tracespecies windsshown in Figure3 exhibittwo semipermanentsubtropical observed aboard the DC-8. anticyclonecenters over southeasternAsia and the westernNorth Pacific.Two north-south oriented trough axes indicated by dashed 1LockheedEngineering and Sciences Company, Hampton, VA 23666. linesare locatedjust off the eastcoast of Asia andover the central 2MassachusettsInstitute ofTechnology, Cambridge, MA 02138. Pacific.Westerly flow off the Asian continentdominated north of 3AtmosphericSciences Division, NASA Langley Research Center, about25øN. The weakcyclonic center evident over the western Hampton,VA 23681-0001. 4Universityof California, Irvine, CA 92717. equatorialPacific (near 15øN, 150øE) is again related to theregion of intensificationand recurvature of tropicalcyclones which occur Copyright1996 by the AmericanGeophysical Union. frequentlyduring the fall season(see discussion below). In the uppertroposphere (200 hPa, Figure 4) anticyclonecenters were Papernumber 95JD02799. againfound over southeastern Asia and the western North Pacific, 0148-0227/96/95JD-02799505.00 with trough axes off the Asian east coast and over the central

1655 1656 BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW

Mean Surface Winds

i•i•i•!!•!!i!ii•ii •ii,•i!•i•iii,ii,•?•i!i!•i!i!i•!•:•ii•!• •i•ii!,i!•i•i!• ?...... •,•,•,•,•,•!•i•!,•!...... : ...... j...:•...... :•...... •'•iii•i!i•:!:.•:ii!:• i•ii!•i!:i!•?i i

,, ...... i.;

20øS 100øE 140øE 180 ø 140øW

Figure 1. Schematicof the meansurface wind streamlinefield, derivedfrom 12 hourly(NMC) grid pointanalyses, averagedover the entire PEM-West A period(September 15 to October22, 1991).Anticyclonic (A) andcyclonic (C) featuresof interestare highlighted.

Pacific.Continental flow dominatednorth of about25øN, similar Biomassburning annually occurs over parts of westernand cen- to that at 500 hPa. tral Indonesiaprior to andduring the September-October period. A Severalprevious studies have been made of theoutflow of con- factorthat may havecontributed to enhancedbiomass burning dur- tinental material over the North Pacific. Particularly noteworthy ing PEM-WestA was the El Nino/SouthernOscillation (ENSO) are contributionsby Merrill et al. [1989] who discussedmeteoro- eventwhich was underway, which inducedan eastwardshift of logicalinterpretation of surface-measuredaerosol material col- major convectionleaving portions of the Indonesiaregion with lectedfrom Pacificisland stations, and by Prosperoet al. [1989] below-normalrainfall [Janowiak,1993]. Media reportsindicated who foundthat 40-70% of atmosphericnitrate collected at North that over70,000 hectareshad burned,including 30,000 hectaresin Pacificisland stations originated from continentalsources. These southernBorneo alone. This burningproduced a massivesmoke data were part of the Sea-AirExchange Program (SEAREX) pall acrossthat regionwhich was oftenevident on visibleGMS whichwas carriedout in 1981-1983.Savoie et al., [thisissue] dis- satelliteimagery (Figure 5). The smokeusually appearedto be cussthe compositionof aerosolsamples collected during the thickestbetween 5øS-5øN and 100øE-115øE and sometimes spread PEM-WestA experimentat sevenPacific islands stations, includ- acrossthe Philippinesand adjacent areas of the SouthChina Sea. ingHong Kong. Merrill [this issue] discusses air parcel trajectories The meanlower-tropospheric flow illustratedin Figures1 and 2 for PEM-West A separately. suggeststhat some of thispersistent smoke could have been trans-

Mean 850 hPa Winds

Figure2. Schematicof themean 850 hPawind streamline field, derived from 12 hourly(NMC) gridpoint analyses, averagedover the entire PEM-West A period(September 15 to October22, 1991).Anticyclonic (A) andcyclonic (C) featuresof interestare highlighted. BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1657

Mean 500 hPa Winds 60ON

40ON

20ON

20os ...... !...... :...... 100øE 140øE 180 ø 140ø1N

Figure 3. Schematicof the mean500 hPawind streamlinefield, derivedfrom 12 hourly(NMC) grid pointanalyses, averagedover the entire PEM-West A period(September 15 to October22, 1991). Anticyclonic(A) andcyclonic (C) featuresof interestare highlighted. portednorthward and/or northeastward within the boundarylayer trailingcold fronts formed the boundarybetween continental air of into portionsof the study region.Some evidenceof biomass- midlatitude(Asian) origin and marine air of either midlatitudeor burning plumes encounteredduring flight 16 is discussedin tropical(Pacific) origin. The meanpositions of the two primary section 6. cold frontal zones (subjectivelyaveraged from an ensembleof 12 hourly NMC frontal analyses)and the southernmostfrontal locationsare illustratedin Figure 6, togetherwith the mean tracks 3. Transient Synoptic Scale Features taken by migratory cyclone and anticyclonecenters. Cyclones In the PEM-Weststudy region, the major transientsynoptic tendedto trackeastward and, eventually, poleward as they crossed scalefeatures affecting long-range transport into and throughout the Pacific.Anticyclones tended to form over northwesternChina, the studyarea are migratorycyclones and their associatedfrontal then move eastward or southeastward in the wake of the transient systemsand tropicalcyclones. During PEM-West A, migratory cyclones. midlatitudecyclones would frequently form (an averageof oneper The analysisreported by Gregoryet al. [thisissue] suggests two week) andintensify along the east coast of Asia or overthe interior basictransport pathways of agedmarine air into the PEM-West A of the continent.This coastalarea is a region of preferential studyarea: (1) "northmarine" air massesmoving eastward, south- cycloneformation due to the stronglower-tropospheric tempera- ward, and then westwardaround the peripheryof the Pacific sub- ture gradientin the vicinity of the warm Kuroshioocean current tropicalanticyclone; and (2) "southmarine" air massesfrom the [Carlson,1991]. As thesecyclones moved across the Pacific,their southeasterlytrade wind flow or from the southernhemisphere.

Mean 200 hPa Winds

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•:•?::"...... :::•:•?:•:•½...... •:• ...... ----:: ...... •...... •...... C--• ...... r-%- ...... :•......

...... 2oos •• • ••l 100øE 140øE 180 ø 140øW

Figure 4. Schematicof themean 200 hPawind streamlinefield, derivedfrom 12 hourly(NMC) grid pointanalyses, averagedover the entire PEM-West A period(September 15 to October22, 1991).Anticyclonic (A) andcyclonic (C) featuresof interestare highlighted. 1658 BACHMEIER ET AL.- PEM-WEST A, METEOROLOGICAL OVERVIEW

ß.:;::--ß..- ..

95øN"' 100øN ' ' 1'05ø-N' ' 110ø. 115øN 120øN 125øN

Figure 5. GMS visiblesector showing the vastsmoke pall (hazy areas)surrounding Borneo on October8, 1991.

With the Pacificanticyclone and Asian cold front in their mean October1991 was a periodof near-normalactivity with 10 tropical positions,one wouldexpect a transportof mainly agedNorth cyclonesreported over thesetwo basinswith sevenreaching Pacificmarine air intothe study area. A weakerPacific anticyclone typhoonstrength. Six tropicalcyclones formed during the actual in tandemwith a strongerAsian anticyclonewould tend to move experimentperiod. One of these(Luke) reached tropical storm the cold front to its southernmostposition, bringing continental strength(maximum sustained wind speeds ofat least 17 m s-l), flow to a largerportion of the samplingregion. Tropical cyclones three(Nat, Orchid, and Pat) were classified as typhoons (winds of recurvingclose to the eastcoast of Asia also induceda stronger at least33 m s-1), and two (Mireille and Ruth) reached "super- continentalflow along the western portion of thestorm circulation, typhoon"intensity (winds ofat least 67 m s-l). Figure 8 depicts the owing to the increasedpressure gradient between the cyclone "besttrack" paths of thesesix storms[Rudolf and Guard, 1992], and the Asian anticyclonewhich often followed in its wake. andTable I liststheir dates of formation/dissipationand maximum Talbotet al. [this issue]also stress that at higheraltitudes conti- surfacewind speeds. nentaloutflow can originatemuch farther upstream than eastern A three-stormoutbreak occurred in mid-September,with Asia. The reasonfor this is illustratedin Figure 7a which shows stormsLuke, Mireille, and Nat. Luke and Mireille recurvednorth the latitudinaldistribution of the polarjet (PJ) and the subtropical of 20øN,affecting Japan as theyaccelerated northeastward. The jet (STJ) and figure7b showsa meridionalcross section of the erraticpath of Natwas due to the fact that Luke and Mireille effec- meanzonal wind illustratingthe higherwind speedsencountered tivelyeroded the western Pacific subtropical ridge, weakening the with increasingaltitude. steeringflow overthe SouthChina and Philippine Seas. Orchid and Pat formedon October1, and their circulationsinteracted with oneanother as the stormsmatured. Ruth did not fully developuntil 4. Tropical CycloneActivity afterDC-8 haddeparted Guam and Wake Island and therefore did The frequencyof tropicalcyclones over the westernNorth notdirectly affect any of theflights. Pacificand SouthChina Sea normallyreaches a peak in August, Oneflight was specifically devoted to thesampling of boundary with Septemberand Octoberbeing the secondand fourth most layerinflow and upper tropospheric outflow in thevicinity of a activemonths [Royal Observatory, Hong Kong(ROHK), 1993]. typhoon.Flight 9 investigatedTyphoon Mireille southwestof Climatologically,the September-October period produces 10 trop- Japanon September 27, sampling the inflow of thesoutheast quad- ical cyclones,six of whichreach typhoon intensity. September to rant of the circulationand then the outflow at the top of the eye BACHMEIER ET AL.' PEM-WEST A, METEOROLOGICAL OVERVIEW 1659

16ow

180 60N

50N

40N

160E 30N

20N

10N

MeanCold Frontal • MeanAnticyclone Position Paths

Southernmost Extent of Mean Cyclone Paths Cold Fronts

Figure 6. Mean positionof the transientmidlatitude cold frontalsystems (solid cold front symbols)as well as the extremesouthward and eastward positions of thefronts (dotted lines) during PEM-West A; meantracks of migratory cyclones(C) andanticyclones (A) are alsodepicted.

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Figure7(a). Mean NMC 200 hPa wind isotach analysis (ms-l), along with the mean axes of the polar jet (PJ) and subtropicaljet (STJ) streams. 1660 BACHMEIER ET AL.' PEM-WEST A, METEOROLOGICAL OVERVIEW

lOO -5'

15o

200 35 250 300 0 5 10 25 400

500 600 700 850 lOOO 1O.ON 20. ON 30 .ON 40. ON 50.0N 60. ON 120.0E 120.0E 120.0E 120.0E 120.0E 120.0E Figure7(b). Meridional cross section ofmean zonal wind (m s -1) for the PEM-West A period, based on (ECMWF) grid point analyses.

region.An in-depthanalyses of the transportof tracegases by in the vicinity of suchjet streams(near and polewardof the Mireilleis discussedby Newellet al. [thisissue (a)]. In addition,jet axis)is well documented[Danielsen, 1980; Danielsen and duringthe transit flight from Hong Kong to Guamon October8 Hipskin&1980; Bachmeier et al., 1994],with high ozone mixing (flight 14) the upperlevels of the southernquadrant of Typhoon ratiosand potential vorticity (PV) beingclassic indicators of air of Orchid were sampled. stratosphericorigin. Overthe westernNorth Pacific, significant portions of the tro- 5. JapanJet and Stratospheric/Tropospheric pospheredisplayed characteristics of stratosphericallyinfluenced Exchange air,especially at higherlatitudes and altitudes. Browell et al. [this issue]discuss stratospherically influenced cases observed during Figure7a showsthe mean wind isotachs at 200 hPa,along with PEM-WestA flightsusing the airborne differential absorption lidar the locationsof the mean axes of the polar and subtropicaljet (DIAL) aboardthe DC-8. On thebasis of theDIAL observations, streams.The branchof the polarjet exitingthe eastcoast of Asia stratosphericair injectedinto the uppertroposphere gradually (the"Japan jet") often contained wind speeds of50 m s -1, occa- descends into themiddle and lower troposphere and is frequently sionallyexceeding 80m s-1. Stratospheric-tropospheric exchangeconfined within subsidence inversions which are maintained within the deepsemipermanent Pacific subtropical anticyclone. Pem-E-A Severalspecific examples of stratospheric/troposhericexchange Tropical Cyclone Paths eventsare discussedin conjunctionwith the synopticsummaries below.

"ji,.iiii...... -/ ...... i!...... /- •8o• 6. Flight-by-Flight SynopticSummaries 7ON :::::::::::::::::::::::::: ThePEM-West A missionconsisted of 21 flights.The firstthree were test flights;the remaining18 consistedof eightsurvey or ©!!:•!•i•I .,...... ,,,,.' "'}½ ...... Ma?0œ transit flights, four local flightsfrom YokotaAir ForceBase in M ,?...... ,/' Japan,two localflights from Hong Kong,three local flights from Guam,and one local flight from Hawaii. This section will provide a synopticsummary for eachflight beginningwith flight 4. For ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::....•I•,:.:.;%•::.:..:.,,, • , ,,- ,,, ,,, • ?- aa0œselected flights, examples are included of therelationship between thechemical measurements and the meteorological features.

Flight 4; September16, 1991 (1704-2251UT) iiis}s::s::s::!::i::i::i::i::i::i::i::i::i::i::•is::•is!s::s::iii[i[i': ii•i•iii::i}isss:Sii::i::i::s' t ', • .' ' ' ß " '', Flight 4 was the initial surveyflight from Moffett Field, ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::...... •i,, ...... •.. }•-•' • • • • • ... as0œ California to Anchorage,Alaska. In thelower troposphere a high- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::•.• •.'._ • ,,,, ..'..' '.... pressuresystem dominated western North America and adjacent 20N •s•s:::::::::::::::::::::::::::::::::::::::::::::• ". ..-- .. .. portionsof the easternNorth Pacific. An extensivearea of low aa0E a20E a30E aa0E pressurecovered the Aleutiansand Alaska peninsula region, with Figu• 8. "Besttrack" paths of tropicalcyclones during the PEM- an associatedfrontal boundary stretching from the Gulf of Alaska southwestward toward the Hawaiian Islands. In the middle to WestA period;L, tropicalsto• Luke;M, supertyphoonMireille; N, Typh•n Nat; O, TyphoonOrchid; P, TyphoonPat; R, super- uppertroposphere a strongridge was centeredalong the northern typh•n Ruth.The square (labeled F9) denotedwhere Mireille was Californiacoast, with the ridgeaxis running northward into west- interceptedduring flight 9 (basedon datafrom Rudolf and Guard, ern Canadaand the Oaken Territory. A broadupper trough of low [1992]). pressurewas locatedover the North Pacific,with closedlow cen- BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1661

70N

60N

50N

40N

30N

Figure9. Potential vorticity (10 -? K m2 kg -I s-l) andwind vectors (m s-l) for0000 UT, September 17,1991, based onECMWF data. Thick solid line represents DC-$ track.Stratospheric air wasentered near 56øN, 148øW.

ters over the easternBering Sea and near the Kamchatkapenin- storm Luke was centered at 25.5øN, 130.7øE at 0000 UT on sula.These features were quite similarto the mean flow patterns September18 and was movingslowly northward(Figure 11). A shownin Figures1-4. The axisof the polarjet streamat 300 hPa broadtrough of low pressureat 500 hPa remainedacross the North waslocated along 40ø-45øN across the western and central North Pacific, as the individual lows centeredover the Kamchatkapenin- Pacific.The jet then turnedsharply northward over the Gulf of sulaand the Bering Seaweakened and beganto fill. A strongwest- Alaska.High valuesof potentialvorticity at 400 hPa(,-, 6 km) sug- erly flow off the Asian continentdominated over the North Pacific gestedthat stratospheric air waspresent at flight level oversouth- in the 35ø-50øNlatitude belt. The axis of the polarjet streamat westernAlaska. A map of this high potentialvorticity region, 300 hPa was orientedwest to eastalong about 38øN acrossthe basedon ECMWF dataon the 315 K surface,is shownin Figure 9 western and central North Pacific. Over the eastern North Pacific alongwith the flight track.Note thatthe DC-8 flighttrack turned thejet turnednorlheastward toward the Gulf of Alaska. westwardat about55øN, 146øWinto the high potentialvorticity Patchylow- and middle-levelclouds (stratocumulus, altocumu- region.The DC-8 flew for about25 min at about11.3 km in the lus, altostratus)were evidentacross much of the BeringSea and in stratosphericair thenstarted a gradualdescent at about2120 UT, the vicinity of the Kamchatkalow. High pressureoff the eastcoast reachingabout 400 m at 2147 UT. A timeseries of SO2,CO, CH4, of Japankept that regiongenerally clear. Extensive layered cloudi- 03, andflight altitude is shownin Figure10. The elevated0 3 and nessassociated with tropicalstorm Luke coveredmuch of southern reducedCO andCH 4 areclear indicators of stratosphericair. Note Japan. thatelevated concentrations of SO2 werealso observed during this During this flight and on flights 9 and 14, dimethly sulfide encounterwith stratosphericair. Analysisby Thorntonet al. [this (DMS) mixing ratioswere observedin the free troposphere(115 issue]attributed this to theJune 1991 eruption of MountPinatubo, poundsper trillion volume (pptv)) comparable to thoseobserved in whichinjected huge quantities of sulfurinto the stratosphere. the boundarylayer (,--160pptv). The free tropospheremeasure- Extensivelow-level clouds(stratus, stratocumulus) were evi- ment was made at 2100 UT at 54øN, 171øW, and an altitude of dent alongcoastal California and the surroundingeastern North 8 km, betweenthe Kamchatkaand Bering Sea low-pressureareas Pacific.A smallpatch of middle-and high-level clouds (altocumu- ("FT" in Fig-ure 11). The boundary layer measurementwas lus, cirrus)was locatedjust off the northernCalifornia coast. A obtainedat 2230 UT, near 53øN, 174øE at an altitude of 365 m bandof layeredcloudiness associated with the frontalboundary ("BL" in Figure11). The aircraftdown-looking video showed con- coveredmuch of the Gulf of Alaska. Otherwise,no significant vectiveclouds reaching ahnost up to the aircraftflight level, sug- cloudfeatures were apparentalong the flight region. gestinga mechanismfor the verticaltransport of boundarylayer air to the 8 km altitude. The other two occasions where this event Flight 5; September17-18, 1991 (1857-0328UT) occurredwere in TyphoonsMireille (e.g., flight 9) [see Newell Flight5 was the secondleg of the surveyflight from Moffett et al., this issue(a), Table3] and Orchid(flight 14, notedbelow), Field, California, to YokotaAFB, Japan,via Anchorage,Alaska. wherethere was alsoopportunity for rapidvertical mixing. The Aleutian surfacelow was weakeningover the Bering Sea, while anotherlow wasmoving eastward from the Kamchatkapen- Flight 6; September22, 1991 (0224-0947UT) insula.High pressuredominated from the east coastof Japan Flight 6 was a local flight from YokotaAir ForceBase (AFB). acrossthe western North Pacific. A quasi-stationaryfrontal bound- The flight profile consistedof flights at severaldifferent altitudes arywas oriented west to east just to the south of Japanbetween 30 ø in the samevertical plane. Such a profile is characterizedhere as a and 35øN, separatinga drier modifiedcontinental air massto "wall profile." the north from a moist marine air mass to the south. Tropical After tropicalstorm Luke movednortheastward across Japan on 1662 BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW

TIME SERIES OF S0e, CO, CH4, 0s, & HEIGHT MISSION 04, 16 SEP., 1991

750

500

250

o 17 18 19 20 21 22 23 15o

lOO

o 17 18 19 20 21 22 23 19oo

-Q 18oo Q- Q-

•- 17oo

1600 17 18 19 20 21 22 23 300

-Q 200

•o lOO o o 17 18 19 20 21 22 23 12 E lO ¾ 8 • 6 o• 4 m 2 -1- o 17 18 19 20 21 22 23 UTC (hour)

Figure10. Timesseries of SO2 (pptv),CO (ppbv),CH4, 0 3 (ppbv)and DC-8 altitude(km) for mission4 on Septem- ber 16, 1991.

September19, a cyclonemoved from Manchuriaand Korea east- at 300 hPawas located between 35 ø and40 ø acrossChina, then ward overthe northernSea of Japan.A cold front trailingfrom this turnednortheastward across Japan. The jet wasmost intensive over cyclone stretchedfrom just east of Japan southwestwardtoward thecentral North Pacific, with wind speeds in excessof 70 m per Taiwan. This frontal boundarywas the leading edge of a weak second. surge of continental air from northern China. The southward progressof the cold front was hindered by the presenceof Flight 7; September24, 1991 (0211-0915UT) TyphoonsNat (21.6øN,124.1øE) and Mireille (15.5øN,134.5øE) over the PhilippineSea (seeFigure 12a at 1000 hPa).The frontal Flight7 wasconfigured as a wall profile,east and northeast of cloudband was positioned just to the southand east of Japan,but YokotaAFB. A weakcyclone was located off the southerntip of cloudinesswas lessextensive in the regionunder the subtropical theKamchatka peninsula, with an associatedcold front stretching ridgeaxis (south of 25ø-30øN). southwestward.Behind the cold front an anticyclonehad spread The subtropicalridge aloft hadbeen intensifying to the southof fromthe Sea of Japaneastward across Japan and over the adjacent Japanfollowing the exit of tropicalstorm Luke. The ridgeaxis was watersof thewestern North Pacific. This frontal boundary was still situatedbetween 20 ø and30øN (see Figure 13a at 300 hPa)from definingthe leading edge of theweak surge of continentalair from southern China eastnortheastward across the North Pacific. A northernChina. Typhoon Mireille waslocated at 20.3øN,128.9øE westerlyflow off the Asiancontinent prevailed to the northof the at 1200UT onSeptember 24 (Figure12b). The Pacific subtropical ridge axis, while a south-southeasterlyflow was presentover the ridgeaxis aloft was along 30øN (Figure 13b) with a strongwest- flightregion south of theridge axis. The axisof thepolar jet stream erly flow off the Asian continentevident north of about35øN. The BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1663

Figure 11. SchematicNMC surfaceanalysis for 0000 UT, September18, 1991. High- (H) andlow-pressure (L) cen- tersare denoted,along with coldfronts and trough lines (standard symbols). FT andBL are positionsof (DMS) sam- plesin the free troposphereand boundary layer, respectively, as discussedin text.

intensityand aerial coverageof the Pacific subtropicalridge had strongwesterly flow off the Asian continentwas still evident north beenweakening during the pastseveral days, especially along the of about35øN. The axis of thepolar jet streamat 300hPa stretched westernperiphery, with the approachof Typhoon Mireille. The from36øN over China to 40%45øNnortheast of Japan.Layered axisof the polarjet streamat 300 hPawas locatedbetween 35 ø cloudinessassociated with thefront covered much of Japan.South and40øN across China, Korea, and Japan, then turned northeastof 30øN,the region under the subtropical ridge axis was generally ward over the western Pacific. Middle- and high-level clouds clean (altocumulus,altostratus, cirrus) covered southern Japan, but the flight regionto the eastand northeast was generallyclean Flight 9; September27, 1991 (0105-0739 UT) Thewall profilewas made along 147øE, nearly perpendicular to Flight9 was specificallydesigned to studytrace gas transport the flow alongthe 320K surfaceshown in Figure 14. A regionof associatedwith TyphoonMireille. A largeanticyclone was situated highpotential vorticity was intercepted near 36øN, and a crosssec- eastof Japan(Figure 12d). TyphoonMireille was over the East tion (Plate 1) showsa directconnection with the stratosphere.In ChinaSea (29.8øN, 127.6øE at 0000UT, September27) andwas situozone values near 100 poundsper billion volume(ppbv) were acceleratingrapidly toward the northeast.With the approachof measuredat about9 km, and the uppertropospheric ozone mea- Mireille the trailingedge of theold coldfrontal boundary (which suredby the airborneDIAL [seeBrowell et al., this issue,Plate 3] had becomestationary to the southof Japan)was pushednorth- was likewisehigh. wardacross Japan, bringing a maritimetropical air massto all but the northernhalf of Japan. Flight 8; September 25-26, 1991 (1656-0017 UT) The subtropicalridge had amplified slightly and built northward Flight 8 occurredsoutheast of Yokotaduring a night-to-day just to the eastof Japan(Figure 13d). In the midtropospherethe transition.Another weak cold front had pushed from Mongolia and deepcyclonic circulation associated with TyphoonMireille was Manchuriasoutheastward across Japan. The trailingedge of this just southwestof Japan.The eye andextensive spiral bands and frontalboundary was orientedwest to east at around32ø-33øN, cirrusoutflow of TyphoonMireille were very prominent and cov- just southof Japan.An anticyclonewhich followed the front was eredmuch of Japan,the Sea of Japan,and the Yellow Sea. From positionedalong the east coast of Japan(Figure 12c). satelliteanimation the bestoutflow aloft appearedto be from the The Pacificsubtropical ridge was intensifying,with the ridge northwesternquadrant, although outflow from the southeastern axisstill just to the southof 30øNover the flightregion (Fig- quadrantwas alsowell defined.A satelliteimage and a discussion ure13c). Typhoon Mireille was beginning to penetratethe western of the traceconstituents for this caseand their interpretationis peripheryof the ridge as it recurvedand moved northward.A givenby Newell et al. [thisissue (a)]. 1664 BACHMEIER ET AL.' PEM-WEST A, METEOROLOGICAL OVERVIEW

FLIGHT 10- Oct. 1, 1991 FLIGHT 6- Sep. 22, 1991 60N 60N

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 100E 120E 140E 160E FLIGHT 7- Sep. 24, 1991 FLIGHT 11 - Oct. 2, 1991 60N 60N

40N

20N • 20N

EQ 100E 120E 140E 160E 100E 120E 140E 160E

FLIGHT 8- Se 3. 25-26, 1991 60N FLIGHT 12 - Oct. 4, 1991 60N

40N

20N

EQ 100E 120E 140E 160E

FLIGHT 9- Sep. 27, 1991 60N FLIGHT 13 - Oct. 6, 1991 60N

40N

EQ 100E 120E 140E 160E 100E 120E 140E 160E

Figure12. Streamlinesfor flights6-21, drawn from NMC analysesat 1000hPa. Anticyclone (A) andcyclone (C) centersof interestare highlighted. Tropical cyclone centers are denoted by thestandard symbol. Flight track is indi- catedby a thick solid line. BACHMEIER ET AL.' PEM-WEST A, METEOROLOGICAL OVERVIEW 1665

FLIGHT14 - Oct.8, 1991 60N FLIGHT 18- Oct. 17-18 iON

40N

-..

EQ 120E 140E 160E 100E 120E 140E 160E FLIGHT 19- Oct. 18-19, 1991 FLIGHT 15- Oct. 10-11 1991 •ON •0N

tON

EQ 160E 180E 160W 100E 120E 140E =

FLIGHT 20- Oct. 20, 1991 60N FLIGHT 16 - Oct. 13, 1991 60N

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 170W 150W 130W FLIGHT 21 - Oct. 21-22, 1991 FLIGHT 17- Oct. 14-15, 1991 60N 60N

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 170W 150W 130W

Figure1.2. (continued) 1666 BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW

FLIGHT 10 - Oct. 1, 1991 FLIGHT 6- Sep. 22, 1991 60N 60N

40N

20N

EQ 100E 120E 140E 160E 100E 120E 140E 160E FLIGHT 7- Sep. 24, 1991 FLIGHT 11 - Oct. 2, 1991 60N 60N

40N

20N

EQ 100E 120E 140E 160E FLIGHT 8- Sep. 25-26, 1991 FLIGHT 12 - Oct. 4, 1991 60N 60N

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 100E 120E 140E 160E FLIGHT 9- Sep. 27, 1991 FLIGHT 13 - Oct. 6, 1991 60N 60N

40N

20N

EQ EQ 100E 120E 140E 160E 100E 120E 140E 160E

Figure13. Streamlines forflights 6-21, drawn from NMC analyses at300 hPa. Anticyclone (A)and cyclone (C) centersof interest are highlighted. Tropical cyclone centers are denoted bythe standard symbol. Flight track is indicated by a thick solid line. BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1667

FLIGHT 18- Oct. 17-18, 1991 FLIGHT 14 - Oct. 8, 1991 60N 60N

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 120E 140E 160E FLIGHT 15- Oct. 10-11, 1991 FLIGHT 19- Oct. 18-19, 1991 60N 60N

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 160E 180E 160W FLIGHT 16- Oct. 13, 1991 FLIGHT 20 - Oct. 20, 1991 60N 60N

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 170W 150W 130W FLIGHT 17-Oct. 14-15, 1991 FLIGHT 21 - Oct. 21-22, 1991 60N 60N

:::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::.:.:.:.:.:.:.:.:.:.:.. :.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.• :....:.:.:

40N 40N

20N 20N

EQ EQ 100E 120E 140E 160E 170w 150w 130w

Figure13. (continued) 1668 BACHMEIER ET AL.' PEM-WEST A, METEOROLOGICAL OVERVIEW

16 '-" ":"::::"?".':'.':.':'i..:::.'"'.':".:- ..... -. '..... '.... ""

... .-

. .:

14 '----

12 ------....-.35o...... 345 ...... '......

2 30.0N 147.0E

2.0 4.0 6.0 8.0 10 15 4O 6O 9O 120

I PV I (10'7 deg.m 2 / (kg.s) Plate1. Meridional cross section ofpotential vorticity (10 -7 K m2 kg -I S-1) from ECMWF analysis data at 0000 UT, September24, 1991.Solid line is potentialtemperature (øK), dashed line is temperature(øC).

e eN •1 TRI; _ TION

OZONE (PPBV) 0 20 4.0 60 80 1 O0

200 220 240 300 320 UT

19- IlP'I'3tPT6 19 18 t7 <: 16 16 15 • 14,' 14 13 ::::) 12 12 • II 11 <( 1o 10 9 8 21.64 1 9.93 18.73 1 8.29 18.4.5 N LAT. III I i I i I i i I I i i I i i & i i I i i . ß ,, '"' '''' 124-.70E LON. 7O Plate2. Crosssection of ozonemixing ratio (ppbv) from (.DIAL) during flight 14 onOctober 8, 1991. BACHMEIERET AL.: PEMoWESTA, METEOROLOGICALOVERVIEW 1669

60N

50N

40N

30N

20N 12•

Figure14. Potential vorticity (10 -7 K m2 kg -] s-]) andwind vectors (m s -]) onthe 320 K surfacefrom ECMWF analysisdata at 0000 UT, September24, 1991. Thick solidline representsthe DC-8 flight track.

Flight 10; October1, 1991 (0244-0935UT) and was weakening rapidly as it moved northeastward(Fig- ure 12f). Satellite imagery loops indicated a low-level offshore Flight 10 was the first leg of the transit/surveyflight from flow of continentalair along the South China Coast west of Nat. YokotaAFB to Hong Kong,via Okinawa,Japan. An anticyclone Aloft, the subtropicalridge was still prominentacross the western had moved eastward from northern China across Korea and the NorthPacific between 20 ø and30øN. However, the western periph- Seaof Japanand was situatedeast of Japan.A cyclonewhich ery of the ridgewas weakeningas tropicalstorm Nat movedinland developednear Taiwan had moved northeastward and was located acrossthe SouthChina Coast(Figure 13f). over southwesternJapan. Severe tropical storm Nat was tracking Cloudinessassociated with the remnantsof tropicalstorm Nat slowlynorthward across the South China Sea (Figure 12e). coveredmuch of southeastChina and adjacentportions of the Tai- The subtropicalridge had become well establishedacross the wan Strait. In addition,clouds remaining from the previousdays' westernNorth Pacific between 20 ø and30øN. The cycloniccircu- convective cluster east of Taiwan had drifted northeastward over lationof Nat wasevident even at 300 hPajust off the SouthChina the East China Sea. New convectiondeveloped during the day off Coast(Figure 13e). A weakshort-wave trough of low pressurewas the southerntip of Taiwan. propagatingeastnortheastward from central China across the Yel- lowSea and over the Sea of Japan. A very localized 30 m s-] jet Flight 12; October 4, 1991 (0151-1015 UT) streak was associated with this wave. A centraldense overcast obscured the eye of Nat, which was Flight 12 was a local fligh! from Hong Kong which includeda justoff theSouth China Coast between Hong Kong and the south- wall profile eastof Taiwan. A large anticyclonehad beenbuilding ern tip of Taiwan(Figure 15). Outflowaloft from the northern over north central China and was beginning to move southeast- quadrantof Nat was beingdrawn north-northeastward into the ward. The leadingedge of this surgeof continentalair had moved short-wavetrough over the East China Sea. A well-organizedarea off the southernand easternChina coastsand was passingsouth- of convectionhad formedjust east of Taiwan,aided by strong eastwardacross the islandof Taiwan(Figure 12g). The main por- upperlevel diffiuence over that region. tion of the subtropicalridge aloft had moved eastward to the On thesouthbound leg fromYokota, DIAL recordeda layerof centralNorth Pacific. A weak off-continentflow existedat high elevatedozone between 4.5 and 7 km extendingover at least altitudes along much of the southernand easternChina coasts 600km along the flight track (26ø-31øN, 141ø-137øE). It was asso- (Figure13g). ciatedwith potential vorticity values of4-6 x 10-7 K m2 kg4 sq Cloudinessassociated with the remnantsof tropicalstorm Nat withinthe 320-330 K potentialtemperature layer (Figure 16). The had drifted slowly northeastwardand was moving into the East windsshown in Figure16 indicatethat the flow within thislayer China Sea and the northern Taiwan Strait. Embedded thunder- was from the southwest. storms were evident along the eastern China coast. Scattered patchesof low- and middle-levelclouds (cumulus, stratocumulus, Flight 11; October2, 1991 (0002-0222UT) altocumulus)covered the SouthChina Sea and the southernend of Flight11 wasthe secondleg of the transit/surveyflight from the Taiwan Strait. A multilevel wall profile, in which flights are YokotaAFB to Hong Kongvia Okinawa,Japan. A cyclonehad made at severalaltitudes along the sametrack, was flown just to the east of Taiwan. movednortheastward across southern Japan and was locatedjust off the eastcoast of Japan.A cold front trailedsouthwestward Flight 13; October 6, 1991 (0119-0815 UT) fromthis cyclone toward northern Taiwan. An anticyclonebehind the front was situated over the Yellow Sea and the East China Sea. Flight 13 was an intensiveflight northeastof Hong Kong. A Tropicalstorm Nat hadmade landfall along the South China Coast large anticyclonewas centeredover centralChina. Tropicalstorm 1670 BACHMEIER ET AL.' PEM-WEST A, METEOROLOGICAL OVERVIEW

Hong Kong

120øE 125øE

Figure 15. GMS infraredsatellite image for 0532 UT, October1, 1991.

40N

30N

20N

10N

EQ 101 160E Figure16. Potential vorticity (10-7 K m 2 kg -1 s q) and wind vectors (ms q) on the 325 K surface from ECMWF analysisdata at 0000 UT, October 1, 1991.Thick solid line represents DC-8 flight track. BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1671

200 •,•'

0 • 5o , o o 2 3 4 5 6 7 8 9 10 11

140

120

100

900

800

700

600

,.• 60 CL 4O o •)o o

12 / • 10 ,• 8 .• 6 0'• 4 .,,. m 2 -1- o 2 3 4 5 6 7 8 9 10 11 UTC (hour)

Figure 17. Time seriesof SO2 (pptv),DMS (pptv),CO (ppbv),CH 4 (ppbv),0 3 (ppbv),and DC-8 altitude(km) for flight 14 on October8, 1991.

Orchid(18.5øN, 134.0øE at 0000 UT, October6) intensifiedto TyphoonOrchid (10.4øN, 130o4øE at 0000 UT, October8) and typhoonstrength during the day as it moved slowly westward.A TyphoonPat (17.3 ø, 151.3øEat 0000UT, October8) werebegin- quasi-stationaryfront, orientedsouthwest-northeast from southern ning to move northwestwardand accelerate. The boundary Taiwan to just southof Japan,was the boundarybetween the conti- betweenthe continentalair massmoving eastward and southward nentalair massmoving eastward and southwardoff the Asian con- off the Asian continentand the cyclonicflow of marineair around tinent and the marine air mass over the western and central North TyphoonOrchid was becomingdiffuse and difficult to defineas a Pacific (Figure 12h). The large cyclonic circulationof Typhoon frontal feature(Figure 12i). Orchid was well definedover the Philippine Sea and was begin- An anticyclone aloft along the southern China coast was ning to draw low-level air from Japanand Korea southwardinto responsiblefor weak off-continentflow over that region (Fig- the system. ure 13i). The deepcyclonic circulations associated with Typhoons The subtropicalanticyclone covered a largeportion of thewest- Orchid and Pat were erodingthe well-definedridge of high pres- em and centralNorth Pacific (Figure 13h). In addition,a weaker surewhich had persisted over the central North Pacificduring the anticycloniccirculation was evident along the southernChina pl•viou• IIIUIltIl. coast. Off-continentalflow aloft was weak along the southern Patchesof low- and middle-levelclouds (cumulus, stratocumu- China coast,but the westerlyflow off the easternChina coastwas lus, altocumulus)were over the SouthChina Sea and to the south strongernorth of 30øN.High-level cirrus outflow from Typhoon of Taiwan. Extensivecloudiness associated with TyphoonsOrchid Orchidhad reachedthe areajust southeastof Okinawa. and Pat was prominentover much of the westernNorth Pacific, with clustersof organizedconvection located between the two Flight 14; October 8, 1991 (0148-1002UT) storms.Based on the motion of cirrusclouds on satelliteimagery Flight 14 was a transit/surveyflight from Hong Kong to Guam. loops,the outflowaloft appearedto be along the southernquad- During thisflight the outflowfrom TyphoonOrchid was sampled. rantsof both typhoons. An anticycloneremained over centralChina but was weakening. This flight encounteredseveral major synoptic systems, includ- 1672 BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW

ing passingwithin 300 km of the centerof TyphoonOrchid. As normalascent rate of a rawinsondeballoon. At thisrate the whole wasthe case for TyphoonMireille [Newellet al., thisissue], DMS air samplescollected in pressurizedcanisters could be collected was higher in the upper troposphere(44 pptv at 8.5 km, near with sufficientfrequency to showthe fine scalestructure on verti- 18.5øN,129øE, at 0350 UT) than in the boundary layer (34 pptv at calprofiles of ethaneand fluorocarbon F-12 (Figure 18). A sharp 400m, near17.5øN, 128.6øE, at 0730UT, seeFigure 17). There layer,about 0.5 kmthick, occurs at about1 kmaltitude in ethane, wasalso a sharpboundary revealed by CO, CH4,03, andSO 2 at whileF-12 (with its long lifetime) is well mixed and gives a good about18.5øN, 126.0øE (0330 UT) thatcorrelated with thewind indicationof thereliability of thesampling. These species appear analysis,in the sensethat the rotationalwind componentbefore to havedrifted into the flight region from the east as shown by tra- this time appearedto havecome from the northand was of conti- jectoriesand wind crosssections (not shown). The profileshown nentalorigin, whereas after this time the flow was part of thecircu- in Figure18 is typicalof thelayered structure found in thetropics latingvortex and appears to haveoriginated further south (Figure [Newellet al., thisissue]. 13i). The presenceof high CO, CH4, and 03 togetherargues againststratospheric air. Anotherinteresting observation later in Flight 17; October14-15, 1991 (1810-0208UT) the flight is the low 0 3 values(> 40 ppbv)in the 11- to 16-km regionas seenon DIAL (Plate2) andthe correspondinglow PV Flight 17 was a local flight from Guam to the northwest.An (2-4x 10-7 K m2 kg -1 s -1, see Plate 3). Low 03 in tandem with low anticyclonelocated over the central North Pacific and a weak PV suggeststhat boundary layer air hadbeen fairly recently lifted equatorialcyclone south of Guamwere responsible for a low-level intothe upper troposphere by deepconvection. easterlyflow across the Mariana Islands region (Figure 121). An anticyclonicflow regimepersisted aloft over the tropicalwestern Flight 15; October 10-11, 1991 (2351-0729 UT) Pacific,with light and variable flow over much of theflight region (Figure131). Flight 15 was a localflight southeast of Guam.As Typhoons A large well-organizedcluster of convectionwas active to Orchidand Pat beganmoving northeastward across the western the east and northeastof Guam (in the region 15ø-20øN, NorthPacific, a weakanticyclone began building over the region 145ø-155øE).Of particularinterest was a layerof low ozone southof Guam(Figure 12j). Thesefeatures were responsible for a (> 20 ppbv)at 11-12km (Plate4) wheremixing ratios were simi- low-levelwesterly to southwesterlyflow in thevicinity of Guam lar to thosein theboundary layer. Winds at thelevel of thislayer and northward,but a low-leveleasterly to northeasterlyflow werefrom the east-northeast,as wasthe motionof precipitation existedsouth of about10øS (south of theanticyclonic shear axis). echoesdetected by weatherradar at GuamAFB. It thereforeseems Weak southerlycross-equatorial flow was evidentfrom 150ø to reasonablethat this convectionto the northeastmay havebeen 160øE at the surface. responsiblefor theformation of theelevated low-ozone layer. With the circulationsassociated with TyphoonsOrchid and Pat to thenorth of 25øNan upper level anticyclone had become estab- Flight 18; October 17-18, 1991 (2343-0337UT) lishedbetween 10 ø and15øN, leaving a lightand variable flow aloftover Guam and the surrounding region (Figure 13j). A pro- Flight 18 was the transit/surveyflight from Guam to Wake nouncedclear region associated with the anticyclonicshear axis Island.An extensiveanticyclone occupied much of the western wasevident southwest of Guam.Cirrus outflow from Typhoon Pat and centralNorth Pacific, and an elongatedcyclone was located was streamingsouthward and westwardover the Mariana Islands southof theMariana Islands. These two features were responsible andadjacent areas to thesoutheast. Scattered areas of deepconvec- for an easterlyto northeasterlyflow acrossmuch of the flight tion southeastof Guamdiminished as the day progressed.The region(Figure 12m). In the uppertroposphere a cyclonic shear DIAL instrumentshowed that low valuesof ozonemixing ratio axis was orientedsouthwest-northeast just northof the flight were sometimesfound up to high altitudesin the troposphereregion, producing a southwesterlyflow betweenGuam and Wake (-12 km),often in theform of layerswhich could have been pro- Island(Figure 13m). ducedby convectiveprocesses, as discussedelsewhere [Newell et An extensive area of cloudiness with embedded convection al., thisissue]. coveredthe region immediatelysouth of Wake Island, between 15ø-20øN,160ø-170øE. Upper level diffiuence coupled with low- Flight 16; October 13, 1991 (0039-0813 UT) levelconvergence over that region created a favorableenvironment Flight 16 was a local flight from Guam to the southwest.A for the formation of this cloudiness.Otherwise, the remainder of weak ridge anticycloneremained over Guam and the Mariana the flight region betweenGuam and Wake Island was free from Islands.The anticyclonicshear axis was orientedsouthwest to any significantcloud features. northeastacross that region, with westerly to southwesterlyflow to thenorth and easterly to northeasterlyflow to thesouth of theaxis Flight 19; October18-19, 1991 (2202-0338 UT) (Figure12k). Weaksoutherly cross-equatorial flow wasagain Flight 19 was the transit/surveyflight from Wake Islandto foundat the surfaceto the southand southeast of Guam(140 ø Honolulu,Hawaii. The dominantsynoptic features were two anti- to160øE) and also to the southwest of Guam (120ø-130øE). Anti- cyclones,one over the westernNorth Pacific, and the other over cyclonicflow aloft was dominant over the tropical western Pacific. the easternNorth Pacific. A troughof low pressurewas located The 300-hPaflow was light and variableacross the region betweenthese two highs, with an associated weak cold frontal sys- (Figure13k). Areasof organizedconvection associated with the tem oriented southwest-northeast across the central North Pacific. Inter-TropicalConvergence Zone (ITCZ) generallywere found Thesouthernmost extent of thefront was near 25øN, or just north between5ø-10øN, 140ø-160øE. Except for cumulusclouds below of theflight region. Another weak trough of lowpressure (the rem- thetrade wind inversion, it was generally clear in thevicinity of nantsof a decayingfrontal system) was located just northwest of Guamand southwestward toward the Philippines. theHawaiian Islands. Low-level flow was generally easterly to Theaircraft performed a slow descent south of thePhilippines northeasterly between Wake Island and Hawaii (Figure 12n). from9.3 to 0.7 km during the period 0332-0411 UT, correspond- A broadupper level trough covered much of thecentral North ingto a vertical velocity ofabout 3.8 m s -1, somewhat lessthan the Pacific.As a result,cyclonic flow was found across the flight BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1673

16 --.------365. : .•...•...-360 ...... 70. 14

...... -50. 12 -• 5o

...... 35. 10

...... 335. '------.----

6 ----- 330. -- ,-5..---- ......

. r 325,, '...... -.... - ...... 320. 5,----.:•,...... -- .___ . .... 4 •- ...... 315. 10. -- -- 310. .... 24.0N 22.8N 21.6N 20.4N 19.2N 18.0N 112.0E 113.6E 115.2E 116.8E 118.4E 120.0E

2.0 4.0 6.0 8.0 10 15 40 60 90 120

I PVI (10'7 d©g.m 2i (kg.s) Plate3. Cross-sectionof potential vorticity (10 -7 K m2 kg-] s-l),potential temperature (solid, øK), and temperature (dashed,øC) at 0000UT, October8, 1991.

OZONE DISTRIBUTION

OZONE (PPB¾) 0 20 40 60 80 100

1900 191 o UT a ß ,. ! !: : ß :• :: . I ß _a ß

19 19 18 18 _J 17 17 16 16 15 15 14. 14. 13 13 12 12 11 11

10 I0 9 9 8

15.00 15.00 N LAT. ' 'I'"'[,..... • :' ; • • : • ß ß 141.76 14,0.59 E LON. 93

Plate4. Crosssection of relativeaerosol scattering and ozone mixing ratio (ppbv) from DIAL duringflight 17 on October 14, 1991. 1674 BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW

OZONE DISTRIBUTION

OZONE (PPBV) 0 20 4-0 60 80 100

600 ! 620 164-0 1710 1 730 UT

17 17 16 16 J 15 15 14 14 13 13 12 12 11 11 10 10 9 8 8 7 7 6 6

18.83 2.0.56 19.4,5 19.16 20,51 N LAT. ;::I;*;;I;:;;I;;;;•:;-;I:;;•" -150.38 -1.55.69 -154.4-9-154-.87 -- 155.4,• LON. 114

Plate5. Crosssections of relativeaerosol scattering and ozone mixing ratio (ppbv) from DIAL duringflight 20 on October 20, 1991.

16 370. 365. 360.

14 .65. •-

_ 315. lO. ;

2 ...... • ...... • ...... ,.4, ...... •.:,31..•._•...0...... , .... 1...... •.-•__ !•' ,.,I.__•..... -• .... • 18.8N 19.1N 19.5N 19.8N 20.2N 20.5N 156.4W 156.2W 156.0W 155.8W 155.6E 155.4W

2.0 4.0 6.0 8.0 1•- 15 40 60 90 120

I PV1(10 '7 deg.m 21 (kg.s) Plate6. Cross section ofpotential vorticity (10 -7 K m2 kg -1 s-1), potential temperature (solid, øK), and temperature (dashed,øC) at 0000UT on October21, 1991. BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1675

Ethane + .... +

F-12 = =

2 _--...... !...... -+--'-• ' ...... ' .... :

_ - : : - - - • - +'" - --!--- - ß ' I ...... ' ...... :...... ',.L7_------:-----,----.-•F- _ - ' ...:--I --. -- : - _ -.- , ! - o ,,, i ...... i,,, , ,, i,, , 200 400 600 800 '1000' 1200 1400 1600

[pptv]

Figure18. Vertical profile of C2H6 (ethane)(pptv) and CC12F 2 (F-12)(pptv) near 4øN, 125øE from 0332 to 0411 UT on October13, 1991. Data providedby F. S. Rowlandand D. R. Blake, UC-Irvine.

region,with northto northeasterlyflow in the vicinity of Wake tinued to dominate across the eastern North Pacific and was Island, turningto westerlyand southwesterlyflow toward the responsiblefor a northeasterlylow-level flow acrossmuch of the HawaiianIslands (Figure 13n). flight region.A weak cycloniccirculation (the remnantsof tropical A small area of clouds with embedded convection was over the depressionMarty) had becomequasi-stationary over the region WakeIsland region. Layered clouds with convectionwere evident between15ø-25øN, 145ø-150øW (Figure 12p). A coldfront was alongthe trough just west of theHawaiian Islands. Along the flight movingsouthward along the west coast of theUnited States. trackwere patchylow- and middle-levelclouds. A north-southanticyclonic axis aloft was orientedalong 160øW,while a weakcutoff cyclonic circulation was located near Flight 20; October 20, 1991 (1347-1947 UT) 28øN,140øW. A strongshort-wave trough was propagating south- Flight20 wasa localflight from Hawaii designed to comparea eastwardalong the west coast of NorthAmerica (Figure 13p). On wide rangeof measurementsaboard the DC-8 with a similar suite this occasion a PV maximum to the northeast of Hawaii had been of ongoing measurementsat the National Oceanic and Atmo- evidentfor severaldays on isentropicPV mapssupplied to us in sphericAdministration (NOAA) observatoryon Mauna Loa. The the field by EuropeanCentre for Medium-RangeWeather Fore- measurementsat Mauna Loa werepart of MaunaLoa Observatory casts(ECMWF). A GOESwater vapor image obtained prior to PhotochemistryExperiment (MLOPEX 2), a collaboratingproject takeoff(not shown)depicted a pronounceddry band(indicating duringthe PEM-West A study.A largeanticyclone remained over strongsubsidence in the middleand upper troposphere) to the theeastern North Pacific, bringing an easterlyflow to theHawaiian northeastof Hawaii,and the flightwas programed to traversethis Islandsregion (Figure 12o). An anticyclonealoft was centered feature. Stratosphericair was interceptedat 10 km around overthe HawaiianIslands (Figure 13o). As a result,upper tropo- 2200UT (near30øN, 150øW), as indicated by in situozone mixing sphericwinds were light andvariable over the flightregion. The ratiosnear 100 ppbv; similar values (near 70 ppbv)occurred below flow patternsencountered during the flightwere furthercompli- theaircraft flight level, as detected by DIAL (Plate7). Thisinter- catedby the local island-inducedflow effects. pretationis furthersupported by a PV crosssection (Plate 8), The rawinsondeprofile at Hilo, Hawaii indicatedthat the base showinga protrusionof highPV intothe middle troposphere. Air- of the trade wind inversion was between 750 hPa and 800 hPa craftwinds showed that a strongflow from the north accompanied (2-2.5 km Mean Sea Level (MSL)). Satelliteimagery showed thesehigh 0 3 values,indicating a jet streakalong the western patchylow-level clouds (trade wind cumulus) to thenortheast of peripheryof thecutoff cyclonic feature (Figure 13p). MaunaLoa. This flightdocumented another good example of very low 0 3 (> 20 ppbv) in the uppertroposphere as measuredby DIAL (Plate5) which againcorresponded to a layer of low PV 7. Conclusions (> 1x 10-7 K m2 kg -1 s -1, see Plate 6). A brief descriptionof the synopticsituation for each of the PEM-WestA missionshas beenpresented. In addition,meteoro- Flight 21; October 21-22, 1991 (1932-0112 UT) logical data have been usedto give backgroundinformation for Flight 21 was the final transit/surveyflight from Honolulu, someof the more markedatmospheric trace constituent features Hawaii,to MoffettField, California. An extensiveanticyclone con- observed.In particular,in severalcases where stratosphere-to- 1676 BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW

OZONE DISTRIBUTION

OZONE (PPBV) O 20 40 60 80 !100

21 30 2150 2210 22'30 UT A_J...LLI_i_• •.l , ! ß it t a 1• ,,,. ß I l:,l,..,• ß ! a a i •1 ß ,•..t..i. i , a [,.,.

lO -10

9 9 8 8 7 7

6 5

4. 3 2

30.00 30.0o 29.96 30.78 N LAT ',':•:::1 - 153.03 - 14-9.99 - 14-7.06 - 14.4.4.5 E LON. 119

Plate 7. Crosssection of relativeaerosol scattering and ozonemixing ratio (ppbv) from DIAL duringflight 21 on October 21, 1991.

6 ------40. -- .... -

30.0N •.0N 30.0N •.N •.N 30.0N 12.0E

2.0 4.0 6.0 8.0 10 15 40 ":: 60 90 120

I PVI (10'7 deg.m 2 i (kg.s) Plate8. Crosssection of potentialvorticity (10 -7 K m2 kg-! s-I),potential temperature (solid, øK), and temperature (dashed,øC) for 0000UT onOctober 22, 1991. BACHMEIER ET AL.: PEM-WEST A, METEOROLOGICAL OVERVIEW 1677 troposphereexchange was suspected,comparisons were made Danielsen,E. F., andR. S. Hipskind,Stratospheric-tropospheric exchange between the DIAL ozone and PV crosssections, which were found at polarlatitudes in summer,J. Geophys.Res., 85, 392-400,1980. Gregory,G. L., A. S. Bachmeier,D. R. Blake,B. G. Heikes,D.C. to supportthe explanation.Three caseswhere boundary layer air Thornton,J. D. Bradshaw,and Y. Kondo,Chemical signatures of aged was foundwithin the middleto uppertroposphere have been iden- Pacificmarine air: Mixed layer andfree troposphereas measureddur- tified by virtue of DIAL low ozonetogether with low PV. Two ing PEM-WestA, J. Geophys.Res., this issue. morecases were foundfrom the flightsthrough Typhoons Mireille Hoell,J. M., D. D. Davis,S.C. Liu, R. Newell,M. Shipham,H. Akimoto, J. McNeal, R. J. Bendura, and J. W. Drewry, Pacific Exploratory and Orchid, where the identificationof boundarylayer air was Mission-WestA (PEM-West A): September-October 1991,J. Geophys. madefrom in situ observationsof DMS at nearly the samemixing Res., this issue. ratiosin the boundarylayer and in the uppertroposphere. From a Janowiak,J. E., The globalclimate for September-November1991: Warm totalflying time above 6 km of 60 hours,5.6 hours(about 9%) had (ENSO)episode conditions strengthen, J. Clim., 6, 1616-1638,1993. valuesof DMS greaterthan one quarterof the boundarylayer val- Merrill, J. T., Trajectoryresults and interpretationfor PEM-West A, J. Geophys.Res., this issue. ueson the sameflight. This is a roughmeasure of the fractionof Merrill, J. T., M. Uematsu,and R. Bleck, Meteorologicalanalyses of long time the boundarylayer was in communicationwith the uppertro- rangetransport of mineralaerosol over the NorthPacific, J. Geophys. posphere;some distortionexists becauseflights were often Res., 94, 8584-8598, 1989. plannedto avoidactive storm areas. Several of theseconstituent Newell,R. E., et al., Atmosphericsampling of supertyphoonMireille with featuresare treatedin detail in papersthat follow in this volume, the NASA DC-8 aircrafton September27, 1991 duringPEM-West A, J. Geophys.Res., this issue (a). andthe topicof layeringis alsoexplored further. Newell, R. E., Z.-X. Wu, Y. Zhu, W. Hu, E. V. Browell, G. L. Gregory, G. W. Sachse,J. E. CollinsJr., K. K. Kelly, andS.C. Liu, Verticalfine- Acknowledgment.The authorswould like to recognizethe invaluable scaleatmospheric structure measured from the NASA DC-8 during assistanceand kind hospitalityof the staffof the Hong KongRoyal Obser- PEM-WestA, J. Geophys.Res., this issue (b). vatory.Thanks are alsoextended to FrankMewszel, who spentmany long Newell,R. E., Y. Zhu,E. V. Browell,W. G. Read,and J. W. Waters,Walker hoursgetting the GMS groundstation operational. In the field we were circulationand upper tropospheric water vapor, J. Geophys.Res., this helpedby thereception of dailyfaxes from ECMWF containingisentropic issue(c). PV mapsand forecast trajectories for eachmission. ECMWF alsoprovided Newell, R. E., Y. Zhu, E. V. Browell, S. Ismail, W. Read, J. Waters, their modelgrid point analysisdata after the mission.John Merrill pro- K. Kelly,and S. Liu, Uppertropospheric water vapor and cirrus: Com- vided isentropictrajectories during and after the mission.Air Forcefore- parisonof DC-8 observations,preliminary UARS MLS measurements, castersat Honolulu, Guam, and Yokota were helpful in contributingtheir andmeteorological analyses, J. Geophys.Res., this issue (d). local knowledgeand forecasting advice. At Guamwe receivedmuch prac- Prospero,J. M., M. Uematsu,and D. L. Savoie.1989. Mineral aerosol tical aid from C. P. Guard,Frank Wells, andMark Lander;their knowledge transportto thePacific Ocean, In J.P.Riley, R. Chester,and R. A. Duce, and experiencecovered the wholePacific. During the flightswe received eds., Chemical Oceanography,Vol. 10-SEAREX: The Sea/Air substantialcooperation from theflight deck crews and mission managers so ExchangeProgram. Academic Press, London, England, pp. 187-218. that the samplingcould be adjustedto the meteorologicalsituation, often in RoyalObservatory, Hong Kong, (ROHK), Tropical Cyclones in 1991,Gov. close to real time. Donald Blake and SherwoodRowland (Universityof Publ. Cent., 1993. California-Irvine)provided NMHC data for Figure 18, Donald Thornton Rudolf, D. K, and C. P. Guard, 1991 Annual TropicalCyclone Report, and Alan Bandy (Drexel Univ.) providedsulfur componentsof Figure 10 238 pp.,Joint Typhoon Warning Cent., Guam, 1992. and Plate 3, Gerald Gregoryand BruceAnderson (NASA Langley)pro- Talbot, R. W., et al., Chemicalcharacteristics of continentaloutflow vided03 datain Figures12 and17, andGlen Sachseand James Collins Jr. from Asia to the troposphereover the westernPacific Ocean during (NASA Langley)provided CO and CH4 data in Figures10 and 17. Susan September-October1991: Results from PEM-West A, J. Geophys.Res., Waltersprepared the final versionsof manyof the figures. this issue. Thornton,D.C., A. R. Bandy,B. W. Blomquist,D. D. Davis,and R. W. Talbot,Sulfur dioxide as a sourceof CN in the uppertroposhere of the References PacificOcean, J. Geophys.Res., this issue.

Arimoto, R., et al., Relationshipsamong aerosolconstituents from Asia andthe North Pacificduring PEM-West A, J. Geophys.Res., this issue. A. S. Bachmeier(corresponding author), Lockheed Engineering and Bachmeier,A. S., M. C. Shipham,E. V. Browell, W. B. Grant, and J. M. SciencesCompany, 144 Research Drive, Hampton, VA 23666 Klassa,Stratospheric/tropospheric exchange affecting the northernwet- R. E. Newell and Y. Zhu, MassachusettsInstitute of Technology, landsregion of Canadaduring summer1990, J. Geophys.Res., 99, Cambridge,MA 02138 1793-1804, 1994. E. V. Browelland M. C. Shipham,Atmospheric Sciences Division, Browell, E. V., et al., Large-scaleair masscharacteristics observed over the NASA LangleyResearch Center, Hampton, VA 23681-0001 westernPacific during the summertime,J. Geophys.Res., this issue. D. R. Blake,University of California,Irvine, CA 92717 Carlson,T. N., The evolutionof cyclones,in Midlatitude WeatherSystems, pp. 221-264,Harper Collins Acad., London, 1991. Danielsen,E. F., Stratosphericsource for unexpectedlylarge values of ozone measuredover the Pacific during GAMETAG, August 1977, (ReceivedAugust 8, 1995;revised September 10, 1995; J. Geophys.Res., 85, 401-412, 1980. acceptedSeptember 10, 1995.)