Simulating the IHOP_2002 Fair-Weather CBL with the WRF-ARW

20 Pages·2010·6.6 MB·English

MARCH2010 LEMONE ET AL. 745
Simulating the IHOP_2002 Fair-Weather CBL with the WRF-ARW–Noah Modeling
System. Part II: Structures from a Few Kilometers to 100 km across
MARGARETA.LEMONE,FEICHEN,MUKULTEWARI,ANDJIMYDUDHIA
NationalCenterforAtmosphericResearch,*Boulder,Colorado
BARTGEERTSANDQUNMIAO
DepartmentofAtmosphericSciences,UniversityofWyoming,Laramie,Wyoming
RICHARDL.COULTER
ArgonneNationalLaboratory,Chicago,Illinois
ROBERTL.GROSSMAN
ColoradoResearchAssociates,Boulder,Colorado
(Manuscriptreceived13March2009,infinalform24September2009)
ABSTRACT
Fair-weatherdataalongtheMay–June2002InternationalHOProject(IHOP_2002)easterntrackandthe
2
nearbyArgonneBoundaryLayerExperiments(ABLE)facilityinsoutheastKansasarecomparedtonu-
mericalsimulationstogaininsightintohowthesurfaceinfluencesconvectiveboundarylayer(CBL)structure,
andtoevaluatethesuccessofthemodelingsysteminreplicatingtheobservedbehavior.Simulationsare
conductedfor4days,usingtheAdvancedResearchversionoftheWeatherResearchandForecasting(WRF)
modelcoupledtotheNoahlandsurfacemodel(LSM),initializedusingtheHigh-ResolutionLandData
AssimilationSystem(HRLDAS).Becausetheobservationsfocusonphenomenalessthan60kminscale,the
modelisrunwith1-kmgridspacing,offeringacriticallookathigh-resolutionmodelbehaviorinanenvi-
ronmentuncomplicatedbyprecipitation.
ThemodelreplicatesthetypeofCBLstructureonscalesfromafewkilometersto;100km,butsome
featuresatthekilometerscalesdependonthegridspacing.Mesoscale(tensofkilometers)circulationswere
clearlyevidenton2ofthe4days(30Mayand20June),clearlynotevidenton1day(22June),withthe
situation for the fourth day (17 June) ambiguous. Both observed and modeled surface-heterogeneity-
generatedmesoscalecirculationsareevidentfor30May.Ontheotherhand,20Junesatelliteimagesshow
north-northwest–south-southeast cloud streets (rolls) modulated longitudinally, presumably by tropo-
spheric gravity waves oriented normal to the roll axis, creating northeast–southwest ridges and valleys
spaced50–100kmapart.Modeledcloudstreetsshowedsimilarlongitudinalmodulation,withtheassoci-
atedtwo-dimensionalstructurehavingmaximumamplitudeabovetheCBLandnorelationshiptotheCBL
temperaturedistribution;althoughtherewerepatchesofmesoscaleverticalvelocitycorrelatedwithCBL
temperature.On22June,convectiverollswerethedominantstructureinbothmodelandobservations.
Forthe3daysforwhichsatelliteimagesshowcloudstreets,WRFproducesrollswiththerightorientationand
wavelength,whichgrowswithCBLdepth.ModeledrollstructuresappearedfortherangeofCBLdepthto
Obukhovlengthratios(2z/L)associatedwithrolls.However,sensitivitytestsshowthattherollwavelengthis
i
alsorelatedtothegridspacing,andthemodeledconvectionbecomesmorecellularwithsmallergridspacing.
*TheNationalCenterforAtmosphericResearchissponsoredbytheNationalScienceFoundation.
Correspondingauthoraddress:MargaretA.LeMone,NCARFoothillsLaboratory,3450MitchellLane,Boulder,CO80301.
E-mail:[email protected]
DOI:10.1175/2009MWR3004.1
(cid:2)2010AmericanMeteorologicalSociety
746 MONTHLY WEATHER REVIEW VOLUME138
1. Introduction
Thispaperisthesecondofatwo-partseriesthatuses
a combination of numerical simulations and observa-
tionstoexploretherelationshipofsurfaceheterogene-
ityandassociatedfluxes(W m22)ofsensibleheatHand
latentheat(LE),topotentialtemperatureQ(K),mixing
ratio Q (g kg21), depth, and convective structure on
scales from 1 to 100 km in the fair-weather convective
boundary layer (CBL), while evaluating the numerical
simulations. The numerical simulations use the Ad-
vanced Research Weather Research and Forecasting
(ARW-WRF) model (Skamarock et al. 2005), coupled
to the Noah land surface model (LSM), which was ini-
tialized using the National Center for Atmospheric Re-
search(NCAR)High-ResolutionLandDataAssimilation
System (HRLDAS; Chen et al. 2007). The data were
collectedinsoutheastKansasusingaircraft,surfaceflux
towers, and three radar wind profilers, during May–
June, 2002, as part of the International H O Project
2
(IHOP_2002; Weckwerth et al. 2004, LeMone et al.
2007). The goal of IHOP_2002 was to improve pre-
dictionofconvectiveprecipitationinnumericalweather
prediction (NWP) models by improving the measure-
ment and use of water vapor data, and by improving
representation of the evolution of water vapor in nu-
merical weather prediction models. Surface processes
were emphasized because of their importance in the
initiationandevolutionofprecipitatingconvection.
InLeMoneetal.(2010,hereafterPartI),wefocused
oncomparisonsofobservedandsimulatedsurfacefluxes,
CBLthermodynamics,andCBLdepth,andfoundthat
d TheNoahLSMproducedaneast–westgradientinH
andLEconsistentwiththemodeledsoilmoisturepat-
tern, but not the observed variation associated with
landcover,duetoshortcomingsintheinputland-use
FIG.1.Easterntrack,instrumentation,andlanduse.(top)In-
andland-covertable. strumentationsuperimposedonterraincontours(interval520m)
d WhilemodeledmeanLEvaluesandhorizontalvaria- intheWalnutRiverwatershedinsoutheastKansas.Thicklines:
tionintheCBLdepthcomparedreasonablywellwith outlineofwatershed,theWalnutRiver,anditstributaries;within
thewatershed,shading5grassland;noshading5mostlycropland.
observations,HandCBLdepthweretoohigh.Thisis
Thickdashedline:easternflighttrack.Partialtriangle(finesolid
likely due to too low a value of the coefficient C in
lines) connects the three ABLE radar wind profilers at Oxford
Zilitinkevich’sformulationusedinrelatingtheroughness (OXF; 37.278N, 97.108W); Beaumont (BEA; 37.638N, 96.548W),
lengthsforheatandmomentumintheNoahLSM,with andWhitewater(notshown,butontheNWvertexofthetriangleat
too-lowsimulatedsoilmoistureplayingasecondaryrole. 37.848N,96.198Wor63kmNNW(azimuth3508)ofOXF.(bottom)
Centralandeasterntracks(dot–dashlines),superimposedonland
Here,weevaluatetheARW-WRF–Noahmodelingsys- use/land cover: light gray 5 grasslands; dark gray 5 cropland;
tem’s ability to simulate convective circulations in the white5urban;black5openwater.Numbersandsymbolsinboth
framescorrespondtosurfacefluxsites.
fair-weather CBL on scales from a few kilometers to
;100 km. The region (the vicinity of the triangle and
‘‘eastern track’’ in Fig. 1) is characterized by a mix of easternsideoftheWalnutRiverwatershedsoutheastof
mostly grassland (green) and winter wheat (senescent, Wichita,Kansas,andintothewatershedtotheeast.
harvested by mid-June), with trees bordering many Theeffectsofsurfaceheterogeneityonfair-weather
fields and waterways. The track extends across the CBL structure have been the subject of numerous
MARCH2010 LEMONE ET AL. 747
TABLE1.BoundarylayerKingAirflightsandenvironmentalconditionsalongtheeasterntrack(zi5CBLdepth).
No.of No.of No.of
legslow legsmiddle legshigh Avgwind
(avgheight (avgheight (avgheight for‘‘low’’legs Clouds(airborne
range, range, range, Daysafter (direction/speed, z 1830UTC observers’notes,
i
Date mAGL) mAGL) mAGL) lastrain* (8/ms21) (m) satelliteimages)
30May2002 8(62–73) 6(523–688) 3/5.5* 159/3.9 900 Ci;isolatedsmallCu;
hazeatBLtop
17Jun2002 6(60–79) 6(135–271) 6(574–743) 1.7 201/7.7 1240 ScatteredCuhumulis
instreets
20Jun2002 5(62–66) 6(151–408) 5(545–765) 4.7 162/5.3 1250 ScatteredCuhumulis
instreets;Ci
22Jun2002 10(58–67) 6(752–897) 6.7 179/9.4 1260 ScatteredCuhumulis
instreets
*30May,3daysafterlight(5–10mm)rainon27May;5.5daysaftera5–10-mmevent;5.5daysafter9–20mm,6daysafter18–85-mm
rainfall;othersignificanteventended0345UTC16Jun2002,whichdepositedaround25mmofrainoverabouta5-hperiod.
modeling studies, some of which incorporate observa- were hindered by the need for periodic boundary con-
tionaldata,andafewobservationalstudies.Mesoscale ditions.Moreover,CBLstructurecanbeinfluencedby
models have been applied to demonstrate the potential interactionwithtroposphericgravitywaves(e.g.,Clark
importanceofCBLmesoscalecirculations(;10–100 km) et al. 1986; Balaji et al. 1993), requiring inclusion of
generatedbysurfaceheterogeneityandtounderstandthe a deeper domain than ordinarily used in LES. These
scalesfavoredforsuchcirculations(e.g.,ChenandAvissar factors, alongwith the convenience of having an NWP
1994; Baidya Roy and Avissar 2000; Baidya Roy et al. modelwithareasonablyrobustsurfacemodel,ledtothe
2003). Reen et al. (2006) and Desai et al. (2006) com- choice to run ARW-WRF run at 1-km resolution. We
binedobservationsandamesoscalemodeltolookatthe recognize the potential problems created by ‘‘double
effects of heterogeneous surfaces on CBL structure in counting’’associatedwithpartiallyresolvingCBL-scale
real situations. Larger and faster computers have en- motions and using a CBL scheme, but note the de-
ablednumerouslarge-eddysimulations(LESs)tostudy velopingliteratureofhigh-resolutionARW-WRFruns
the structure and evolution of mesoscale CBL circula- thatappearsuccessful,evenatveryhighresolution(e.g.,
tions(e.g.,AvissarandSchmidt1998;LetzelandRaasch Davis et al. 2008, Miao and Chen 2008). Furthermore,
2003; Patton et al. 2005; Kang and Davis 2008). How- actual comparisons to data such as done here can help
ever,observationaldocumentationofmesoscalecircula- usunderstandthelimitationsofstretchingARW-WRF
tionsistypicallysketchy(e.g.,Mahrtetal.1994;LeMone beyonditsexpectedcapabilities.
etal.2002;Kangetal.2007;Tayloretal.2007),withthe Thepaperisoutlinedasfollows.Datacollectionand
significant exception of circulations along the dryline analysisarediscussedinsection2,withmodelrunsand
that lead to precipitating convection (Ogura and Chen analysis of results summarized in section 3. Section 4
1977andmanysubsequentpapers,recognizingthatfac- compares the observed and modeled CBL structure
torsotherthansurfaceheterogeneityarealsoimportant fromkilometer-scaletomesoscale(;10–100 km),draw-
forthedryline).MoretraditionalLESshaveexamined ingfromsomesensitivityrunstohelpinterpretthemodel
CBL-depthscale(;fewkilometers,hereafterreferredto CBL convective structure. The results are summarized
askilometerscale)structurefordecades(e.g.,Deardorff insection5.
1972; Moeng and Sullivan 1994), and there has been
limitedattentiontokilometer-scalestructuregenerated
2. Datacollectionandanalysis
in high-resolution mesoscale model simulations (Trier
etal.2004;MiaoandChen2008). Thefourfair-weatherdaysexaminedhadscattered
OurgoaltocompareCBLstructureandevolutionat cloudsandwindfromapproximatelysouth-southwest
the scales observed requires high-resolution runs. The to southeast (Table 1). Winds at 65 m above ground
choice of an LES to simulate real-world situations is level(AGL)varyfrom3.9 m s21on30Mayto9.4 m s21
appealing,sincelargeCBLeddiesareresolved.Indeed on 22 June. Timing of the most recent rainfall ranged
ConzemiusandFedorovich(2008)usedLEStosimulate from 1.7 to 6.7 days previous. Figure 1 shows a higher
ofCBLdevelopmentnearthewesterntrack,andGorska densityofcrops(mainlywinterwheat)tothewestand
et al. (2008) used LES to simulate of mesoscale circu- mainly grasslands to the east, relative to the eastern
lationsalong theeastern track. However,these studies trackandforatleast100 kmtothesouth.Thebandof
748 MONTHLY WEATHER REVIEW VOLUME138
within 100 m horizontally. Aircraft altitude was based
on a King KRA5 radio altimeter for heights below
610 m;andanAPN159radaraltimeterforheightsabove
610 m.EstimatesofvirtualpotentialtemperatureQ are
y
based on water vapor densities from a LiCor 6262 gas
analyzer,1 flight-level air temperatures from a reverse-
flowplatinum-resistancethermometerbuiltattheUni-
versity of Wyoming, and pressure from a Rosemount
1201sensor.Dataweremadeavailableat25 Hzandas
1-saverages.
Statisticsonflight-levelwindandthermodynamicswere
based on the 1-s data. Aircraft-based divergences were
computed from the along-track wind component using
least squares best-fit lines to each flight track. Values at
specifictimescouldthenbecomputedfromaleastsquares
bestfitlinetothedivergencetimeseriesateachlevel.
To calculate the Obukhov length L, we use leg-
averaged flux measurements from the King Air fol-
lowingStrassbergetal.(2008),andfluxesfromthethree
towersalongtheeasterntrack,locatedongrasslandsites
atpositions7,8,and9inFig.1.Thesemeasurementsare
describedindetailinPartIandLeMoneetal.(2007).
Argonne National Laboratory’sAtmospheric Bound-
aryLayerExperiments(ABLE;ABLEisnolongerop-
erating;however,thedataareavailableathttp://gonzalo.
er.anl.gov/ABLE;Klazuraetal.2006)facilityradarwind
FIG.2.(top)Soiltypeand(bottom)landuse/landcoverforinner
profilerswereusedtoobtainCBLdepthandconvergence.
domain(d03;Fig.3)withstateboundaries(red),profilertriangle,
Thelocationsofthetwoprofilersusedforboundarylayer
andeasterntrack(solidblacklines)included.Soilsneartheeastern
track:85siltyclayloam,45siltloam;elsewhereinfigure:15 (BL)depth—BeaumontandOxford,Kansas—areshown
sand,25loamysand,35sandyloam,65loam,75sandyclay inFig.1.TheBLheightwasdefinedasthecenterheight
loam,95clayloam,105sandyclay,115siltyclay,125clay,135 of the gate just below the maximum signal-to-noise
organicmaterial,and145water.Forvegetation,neartheeastern
dropoffratewithheight(CoulterandHoldridge1998).
track:105grasslands,125croplands,135urban,and175water;
Forconvergenceestimates,theABLEWhitewaterpro-
other vegetation in figure: 4 5 deciduous broadleaf forests and
165barrenandsparselyvegetated. filerwascombinedwiththoseatBeaumontandOxford.
TheWhitewaterprofilerlies;63kmnorth-northwestof
winter wheat extends south to the Oklahoma–Kansas Oxford (;3508 azimuth) so that the three profiles form
border;southoftheborder,thewinterwheatbandex- anearlyequilateraltriangle.Theverticalresolutionofthe
tendssouthwestwardintoTexas.Thedominantsoiltype profilerdataisoftheorderof60 m.Radarwindprofilers
along the flight track is silty clay loam (Soil Survey caninprinciplemeasureverticalwinddirectly,butinthe
Geographic Database; more information is available CBL this measurement tends to be negatively biased
online at http://dbwww.essc.psu.edu/dbtop/doc/statsgo/ (Angevine1997).
statsgo_info.html; Fig. 2), consistent with the observed
near-surfacesoiltypeattheIHOP_2002surfacesites.
3. ARW-WRFruns
Asin Part I, aircraftdata arefromthe Universityof
Wyoming King Air, which flew straight and level legs ThemodelrunsaredescribedindetailinPartI,sowe
alongthe;46-kmlongeasterntrack(Fig.1).Whileat provideonlyessentialdetailhere.Ourcontrolrunsuse
least 5 passes were flown between 60 and 70 m AGL, ARW-WRF version 2.1.2 with the Yonsei University
there are data collected from at least 1 other altitude
range(Table1).Aircraft-relativewindsweremeasured
byaRosemount858AJ/1332differentialpressuregust- 1Thereisasmallbias(;1gm23)betweentheLiCor,andthe
Lymana,whichwasusedinPartIbecauseitwasreferencedto
probe system. Aircraft position and motion relative to
an EG and G chilled-mirror dewpointer. Since only horizontal
thegroundweremeasuredbyaHoneywellLaserefSM changes in Q are considered, the bias should not influence the
y
inertial navigation system and corrected using GPS to results.
MARCH2010 LEMONE ET AL. 749
runstestedtheeffectsofhorizontalresolutionusingdata
from22June;andseveralidealizedrunswereconducted
using data from 20 June to assess the impact of grid
spacing,surfaceproperties,andfiltering(Table2).Two
higher-resolution runs for 22 June were run using the
sameversionofARW-WRF(ARW-WRFV2.1.2)asthe
controlruns,withafourthdomainwith333-mgridspac-
ing(Fig. 3), with initial surface conditions interpolated
fromthe1-kmHRLDASresults.
Theidealizedrunsusedthepublic-releaseARW-WRF
version 3 (ARW-WRF V3), with all points in the do-
main initially assigned surface characteristics and me-
teorologicaldataforatypicalpointontheeasterntrack
(37.41888N,96.64728W),andindomainscenteredatthis
point(Fig.3).Initialsurfacepropertiesintheidealized
runs were varied horizontally by randomly perturbing
theinitialvolumetricsoilmoistureprofileand/orT,using
s
auniformdistributionwithrangesof60.02and60.01 K,
FIG. 3. Domains for ARW-WRF runs, with horizontal grid
respectively. We varied grid spacings for both the sur-
spacing. Outer domain (d01): 9km; intermediate domain (d02):
3km;innerdomain(d03):1km.Innermostdomainsforsensitivity face and atmosphere grid together and separately, ran
studies include the two 333-m grid spacing domains for 22 Jun simulations with and without the Knievel et al. (2007)
(representedbysolidrectanglesd04Eandd04W),andtherectangle filtertodampunresolvedmotions(,6Dx),andchanged
usedfortheidealizedrunsusing20Jundata(labeled‘‘ideal’’),which
thedomaindepthforsomeruns.Thegridpointsinthe
isdashedbecause(i)itvariesinsizeaccordingtogridspacing,and
innermostdomainvariedfrom2263226forthe200-m
(ii)therearenoouterdomainsfortheidealizedruns.
runsto1013101fortheremainingruns.Becauseinput
(YSU)PBLscheme (Honget al.2006),coupledtothe datawerebasedononlyonepoint,windshadtobede-
NoahLSM(ChenandDudhia2001;Eketal.2003).The terminedusinganoptioninARW-WRFthatmaintains
simulations, for the four fair-weather days in Table 1, thegeostrophicpartwithanimplicitpressuregradient,
start at 1200 UTC and run for 24 h, using the nested withonlytheageostrophiccomponentrespondingtothe
domains shown in Fig. 3. The vertical grid contains Coriolisacceleration.Table2willbediscussedingreater
30full-sigmalevelsfromthesurfaceto50hPa,ofwhich detailinsection4b.
the lowest 8 levels are below 1 km AGL (lowest level A final sensitivity run was conducted to assess the
28mAGL)inordertohavefinerresolutionintheCBL. effectoftheKnievelfilteronmesoscalestructure.Using
The horizontal grid spacing (number of points) of the the same model setup as for the control runs, we ran
threeouterdomainsare9 km(2373201),3km(2803 ARW-WRFV3for30MaywithandwithouttheKnievel
229), and 1km (391 3 289), respectively. Initial and filter.
boundary conditions are from the National Centers for
Environmental Prediction (NCEP) 6-hourly Eta Data 4. Results
Assimilation System (EDAS) on a 40-km grid. In the
a. Horizontalvariabilityinvirtualpotential
controlARW-WRFsimulation,theNoahLSMisinitial-
temperature
izedbyvolumetricsoilmoistureandtemperatureprofiles
and skin temperature obtained from HRLDAS (Chen As noted in Part I, Q increased westward along the
et al.2007), which isrun offline but on the same nested eastern track in both model results and observations.
WRF grid for an 18-month spinup period ending at the Thelargesteast–westdifferencewasfor30May,andthe
initializationtimeofeachARW-WRFsimulation(here, smallest for 22 June. These trends are consistent with
runsstart1January2001).Theinputdataatthesurface theeast–westdifferencesinQ ,whichextendsthrough
y
aredefaultvaluesexceptfortheland-usecharacteristics, theCBL(Table3).Thedifferencesbetweenthesetwo
which are based on Moderate resolution Imaging Spec- daysarealsoconsistentwiththemodeledsoilmoisture
troradiometer(MODIS)datafromtheBostonUniversity patterns, which show stronger contrast along the flight
Department of Geography (see online at http://www- trackon30Maythanfor22June(Fig.4).
modis.bu.edu/landcover/;Fig.2). Toseewhetherthesealong-trackQ changescanre-
y
We did several sensitivity runs to see whether the sultfromhorizontalheterogeneityalone(i.e.,nosynoptic-
modeledCBL-scaleconvectivestructurewasrobust.Two scaledifferential advection), we consider two idealized
750 MONTHLY WEATHER REVIEW VOLUME138
assland)andsoiltexture(siltyclayloam),andsurface,subsurface,usingrandomperturbations.22JunrunsuseARW-WRFV2.1.2.eferstotheKnieveletal.(2007)filterusedinARW-WRFtodamp hWRFlh/*2100UTCrollWRF1800UTC(km)Remarks ——Cellsn/a1.68Weakcellular1800UTC,weaklydefinedrolls2100UTCn/a1.68NoCBLconvectivestructuresn/a1.68NoCBLconvectivestructuresn/a1.68NoCBLconvectivestructuresn/a1.68NoCBLconvectivestructures n/a1.67Irregular,50-kmroll-likestructureatCBLtop,1800–2100UTC5.11.68Rolls,6.7kmat2100UTC3.11.66Rolls,4.8km2100UTC;2.61.69Rolls,4km2100UTC2.81.67Rolls,4.6km2100UTC1.81.67Cellularconv.by2100UTC;2.41.67Rolls,4kmat2100UTC1.41.67Rolls,cellstonorthn/a1.67Rolls1600UTCbutmostlycellularafterward1.81.67Rolls,largerat2100UTC h2.6forOXF,BEA,easterntrackWRF—Rollsandsomecells—Rollsandsomecells
vegetation(groisturevariedcated.‘‘Filt’’r hWRF1800UTC(km) —1.14 1.141.141.141.14 1.17 1.161.161.201.261.171.171.171.17 1.17 1.66—— tonorth.
T2.EffectsofresolutiononCBLstructure.AllidealizedrunsuseARW-WRFV3,withuniformABLE88Tandmeteorologicaldatafrom37.4188N,96.6472Won20June,withinitialand/orvolumetricsoilms555hY‘‘yes,’’N‘‘no,’’CBLdepthfromARW-WRF,averagedoverdomainunlessotherwiseindiWRFoutunresolvedmotions;‘‘Nest’’referstowhetherthefinestresolutionwasaresultofnesting. Rollspacingx)NoahgridAtmosphericgridDomain(directionRunSpacing(km)spacing(km)top(km)FiltNest1800UTC(km) 20JunidealizedrunswithsurfaceskintemperatureperturbationsT1mix1_0.2NFs10.25NY—T1NF1120NNn/a T11120YNn/aT22220YNn/aT44420YNn/aT10101020YNn/a20JunidealizedrunswithsurfaceskintemperatureandvolumetricsoilmoistureperturbationsTS55520YNn/a TS11120YN6.7TS1NF1120NN4.0TS0.50.50.520YN3.5TS0.5s0.50.55YN3.9TS0.5NFs0.50.55NN2.4TSmix1_0.5s10.55YN3.1TSmix1_0.33NFs10.3335NY1.8TSmix1_0.2NFs10.25NYn/a TSmix2_0.5sNF20.55NY2.422Jun,realdataControl1120NY4.3HiResE0.330.3320NY2HiResW0.330.3320NY2 l5x*Rollaspectratio,whererollspacingindirectiontimescosineofrollorientationwithrespectroll
MARCH2010 LEMONE ET AL. 751
TABLE3.Observedparametersrelatedtodevelopmentofmesoscalecirculations,interpolatedto1830UTCunlessotherwiseindicated.
70-mwind DQ 5[Q(W)2Q(E)] DQ rest
y y y y
(Avg) z (legcenter) hQi wu * at70m(Avg) ofBL(Avg) D (3) t /D (5)
i y y,0,c DQ Du Du
Date (ms21) (m) (K) (Kms21) (K) (K) (kmy ) (‘h)/(k‘m)
30May2002 3.9 900 305.0 0.11 0.7 0.6 86 1.4/19
17Jun2002 7.7 1240 303.6 0.13 0.4 0.4 122 1.6/44
20Jun2002 5.3 ;1250 306.7 0.10 0.3 0.3 83 2.1/40
22Jun2002 9.4 1260 306.6 0.11 0.2 0.2 90 3.1/107
*Thesubscript‘‘c’’representsthevalueatcooleasternendofthetrackat1830UTCcalculatedsothattheaverageofthe‘‘warm’’and
‘‘cool’’valuesequalthelegaverages.
trajectoriesthatstartoutwiththesameQ ,oneheaded whereSisthewindspeedalongthetrajectory.During
y
northward toward the western end of the flight track the morning hours, wu and z change together (cf.
y,0,c i
overdormantwinterwheatanddrysoils,andthesecond Figs. 7 and 8 in Part I), keeping their ratio in (1)–(3)
headed northward toward the eastern end over green roughlyconstant.Thisenablesustousenoontimevalues
grasses and wet soils, consistent with southerly winds forcrudeestimatesoft andD .
DQ DQ
passingoverthevegetationandsoilmoisturedepictedin Setting f 5 0.2 and appylying (3)vto the four IHOP_
Figs.1and4.Forsimplicity,weassumethatthesurface 2002days,withS,DhQ i,z,andwu fromTable3,we
y i y,0,c
virtual temperature fluxes wu along the trajectories find that D ranges from 83 to 122 km. From Figs. 1
y,0 DQ
arerelatedsuchthatwu 5 (11f)wu ,wherefis and 2, observyed and modeled upstream vegetation has
y,0,w y,0,c
a positive constant, the subscript w denotes the warm gradients over these scales. The modeled horizontal
westerntrajectory,andthesubscriptcdenotesthecool gradientinsoilmoisturefor30May(Fig.4)and17June
easterntrajectory. (not shown) are of sufficient scale; the soil moisture
Weassume2thatwu ,thevirtualpotentialtemper- patternsof20(notshown)and22June(Fig.4)aremore
y,zi
ature flux at CBL top, is related to the surface flux via irregular, but still exhibit a broad westward drying. A
wu ’ (cid:2)0.2wu .Neglectingdirect warmingbyradi- definitive calculation would have to account for fluxes,
y,zi y,0
ativeeffects,thealong-trajectorytimerateofchangeof BLdepths,andlandcoveralongtheactualtrajectories.
thevirtualpotentialtemperatureaveragedoverz,hQ i, Having a deeper BL for the western trajectory would
i y
isgivenby dilute the heating, and thus lengthen the time/distance
required to produce a given DhQ i. Moreover, since
dhQ i 1.2wu y
y ’ y,0. (1) terrainintheKansas–Oklahomaregionrisestowardthe
dt z
i west,constantsurfaceairtemperatureswouldmeanthat
Assuming the same z for both trajectories, and taking east–west convergence could increase the Q gradient,
i
thedifferencebetweendhQ i/dtforairalongthe‘‘warm’’ with upslope easterlies to the east lowering Q and
y
and‘‘cool’’trajectories: downslopewesterliestothewestraisingQ.
d(hQ i (cid:2)hQ i ) D(DhQ i) 1.2fwu
y w y c ’ y ’ y,0,c. (2) b. Convective-scalestructureandclouds
dt Dt z
DQ i
y On 3 of the 4 days, the ARW-WRF runs produced
Integrating(2)andrecallingthatDhQyiisinitiallyzero roll-type structures aligned with the CBL wind, with
(trajectoriesstartatthesamehQyi),weobtainthealong- structureandorientationsimilartothatdocumentedin
trajectorydistanceD requiredtodevelopahorizon- satelliteimages(Fig.5).Fromthefigure,themodelrep-
DQ
taltemperaturedifferenvcehDQyi: licatestheobservednorth–southcloudstreetson22June,
andsouth-southeast–north-northwestcloudstreetsthat
DhQ iz
D 5St ’S y i , (3) varylongitudinallyon20June.For17June,ARW-WRF
DQy DQy 1.2fwu produces south-southwest–north-northeast linear roll
y,0,c
structureswiththesameorientationofthecloudstreets
depictedinthesatelliteimage.Theradarcompositesin
2Conzemius and Fedorovich (2006) suggest the relationship Fig.6suggestrollalignmentcorrespondingtothecloud
wuy,zi ’ (cid:2)0.2wuy,0 applies if the shear at PBL top is small. streets. The observed and modeled roll wavelength,
Strassbergetal.(2008)foundthesheartobesmallfortheJune
about4 km,isconsistentwiththatassociatedwithCBL
days,and‘‘smalltomoderate’’for30May,withaz tosurfaceflux
i
rolls (;3z, e.g.,LeMone 1973,land values from Fig. 4
ratiolessthan0.2,sotherelationshipshouldapplyreasonablywell i
forthe4daysdiscussedhere. in Young et al. 2002). However, this result could be
752 MONTHLY WEATHER REVIEW VOLUME138
The presence of CBL rolls (as opposed to cellular
convection) corresponds to what occurs in nature. In
Figs.5and6,therollsoccurforvaluesof2z/Llessthan
i
around25,whereListheObukhovlength,asexpected
from previous work by Weckwerth et al. (1999) and
others. Similarly, ARW-WRF simulations by Miao
andChen(2008)showedrollsforsmaller2z/Lratiosand
i
random convection for larger ratios, and Me´so-NH
(Laforeetal.1998)simulationsbyLothonetal.(2007)
showed rolls transitioning to cells as 2z/L increased
i
beyond25.
For the observations, the ‘‘regional’’3 2z/L ratios
i
basedonaircraft46-kmaverages(Strassbergetal.2008)
seemtohaveabettercorrespondencetorollsthanthose
basedonthelocalfluxtowerdata(Table4).Asshown
byStrassbergetal.usingdatafrom22June,theseregional
valuescorrespondtoamomentumroughnesslengthz
0
of0.14 m,closetothevalueofz 50.12prescribedfor
0
grasslandinARW-WRF.Notethatthe2z/Lthreshold
i
forrolloccurrenceissignificantlyexceededfor20June
(Table 4) when we use u measured at the surface sta-
*
tions or use model fluxes with the local (0.02 cm) z
0
value;thus, additionalevidencefor theneedto usere-
gionalvalues.
Thegoodcorrespondencebetweenmodeledandob-
servedtypeofconvectivestructurewassurprising,given
the use of a PBL scheme designed for grid spacings
much larger than 1 km (e.g., Wyngaard 2004) and the
associateduseofgridboxeswithsmallerwidth-to-height
ratios(Wyngaardetal.1998).Totesttherobustnessof
theconvectiontypeandscaleintheARW-WRFsimu-
FIG.4.NoahLSMvolumetricsoilmoisturedistributionforlevel
lations, we ran a series of tests to assess the impact of
1(0–10cm)ininnerdomaind03(Fig.3)forcoupledrun,at(top)
1800UTC 30May and(bottom)1800UTC 22Jun 2002.White surfaceperturbationsandgridsizesonthecharacteris-
areasrepresentopenwater,thinblacklinesarestateboundaries, ticsoftheCBLconvection,summarizedinTable2.As
and thick black lines represent the profiler triangle and eastern seenfromthetable,thegrid spacingsfor boththesur-
track.
faceandtheatmospherewerevaried,sometimessepa-
rately.4 Volumetric soil moisture (surface temperature
fortuitous:forthese1-kmgridsimulations,4 kmisless T)wasperturbedrandomlyusingauniformdistribution
s
than the ;6–10Dx quoted in Skamarock (2004) as the witharangeof60.02(0.01 K)relativetothemean.On
lowerlimitforproperlyresolvingstructureandamplitude. some runs, T was perturbed and soil moisture left
s
TheARW-WRFdidnotproduceclear-cutconvective constant. We also varied the depth of the model and
patterns on 30 May, with patches of irregular cellular horizontaldiffusion(notshown).Eachrunisidentified
convection and artificial-looking linear structures 2Dx
wideneartheflighttrackandprofilertriangle(notwell
resolvedinFig.5a).TheWichitaWeatherSurveillance 3The ‘‘regional’’ fluxes, being based on momentum fluxes
computed from fluctuations relative to a 46-km flight-leg linear
Radar-1988Doppler(WSR-88D)radarpatterninFig.6
trend,includetheeffectsoftrees,buildings,etc.;whilethe‘‘local’’
alsoshowsacellularpattern.The2Dxfeaturesprobably
fluxes, being based on surface flux sites that are in open areas,
result from the shallow CBL depth (Table 1), which representthesurfacesampled(inthiscase,grasses).
reduces the expected convection scale (order CBL 4Thetwogridsizeswerealwayssuchthatnointerpolationwas
depth);andacombinationoflightwinds(Table1)and necessary.Forexample,forthelargergrid1km,thesmallergrid
mightbe500m,sothatthefour500-msquaresexactlyoverlapwith
the numerical diffusion scheme in ARW-WRF version
the1-kmsquares.Inthiscase,the1-kmgridsquarerespondstothe
2.1.2,whichislesseffectiveatreducing2Dxfeaturesin
average of the values for the four 500-m squares; and the four
lightwinds(Knieveletal.2007). 500-msquaresrespondtothe1-kmsquarevalues.
MARCH2010 LEMONE ET AL. 753
FIG.5.(a)CBLconvectivepatterns,asreflectedby(top)mid-CBL(level5,around
500m)verticalvelocityinARW-WRF(1800UTC)fordomaind03(1-kmgrid)and
(bottom)cloudpatternsinGOES-8(1809UTC)image,for(left)30Mayand(right)
17June.Modeldomaind03andeasterntrackareincludedforscaleandgeographic
reference.Valuesof2z/Lareforregional(gridscale)valuesofz forobservations
i 0
(ARW-WRF)fromTable4.(b)Asin(a),butforcloudfields(verticallyintegrated
liquidwater)on20and22JunefromGOES-8andARW-WRF.
754 MONTHLY WEATHER REVIEW VOLUME138
FIG.6.CBLconvectivepatternsat1830UTCrevealedbyclear-airreturn(dBZ)fromtheWSR-88Dradarsat
Wichita,KS(KICT),VanceAirForceBase,OK(KVNX),andTulsa,OK(bottomrightineachframe,centeroffthe
figure).Countylines,flighttrackandprofilertriangleareincludedforreference.Thestronger(weaker)returnsare
associatedwithclear-airmode(precipitationmode).
bywhatisperturbedandatwhatscale.Forexample,T1 Additional letters indicate the presence or absence of
has T at 1-km spacing and a 1-km atmosphere grid, theKnieveletal.(2007)filterandthesimulationdepth.
s
while TS1 has 1-km grids for T and soil moisture per- Thus, TSmix1_0.33NFs has 1-km soil moisture and T
s s
turbations and the atmosphere. When the surface and perturbationsand0.33-kmatmospheregridspacing,no
atmospheregridspacingdiffer,thesuffix‘‘mix’’isused. Knievelfilter(NF)andashallow(s)domain.
TABLE4.Criterion2zi/Lforconvective-rolloccurrencefrommodelandobservationsat1830UTC.
2z/L* 2z/L* 2L** 2L**
i i
observed observed model model z 2z/L** 2z/L**
i i i
(aircraftu ) (surfaceu ) (z 50.12m) (z 50.02m) mode1 model model
* * 0 0
Date Rolls? (m) (m) (m) (m)** (m) (z 50.12m) (z 50.02m)
0 0
30May2002 No 43 76 16 7 1350 81 199
17Jun2002 Yes 11 24 87 35 1325 15 38
20Jun2002 Yes 27 49 32 13 1340 41 100
22Jun2002 Yes 6 12.6 144 55 1750 12 32
*z datafromTable1,LcalculatedfromaircraftandsurfacefluxesprocessedasdescribedinStrassbergetal.(2008).
i
**UsingH,LE,observedT,P,andQ,Monin–Obukhovsimilaritytofind2L.z 50.12misgrasslandvalueforMODISlandusetable;
0
z 50.02misclosetotheobservedvalue.
0