Objectively Determined Fair-Weather NBL Features in ARW-WRF and Their Comparison to CASES ...

24 Pages·2014·4.19 MB·English

AUGUST2014 LEMONE ET AL. 2709
Objectively Determined Fair-Weather NBL Features in ARW-WRF and Their
Comparison to CASES-97 Observations
MARGARETA.LEMONE,MUKULTEWARI,FEICHEN,ANDJIMYDUDHIA
NationalCenterforAtmosphericResearch,*Boulder,Colorado
(Manuscriptreceived11November2013,infinalform27March2014)
ABSTRACT
Heightsofnocturnalboundarylayer(NBL)featuresaredeterminedusingverticalprofilesfromtheAd-
vancedResearchWeatherResearchandForecastingModel(ARW-WRF),andthencomparedtodataforthree
moderatelywindyfair-weathernightsduringtheApril–May1997Kansas-basedCooperativeAtmosphere–
SurfaceExchangeStudy(CASES-97)toevaluatethesuccessoffourPBLschemesinreplicatingobservations.
TheschemesareBougeault–LaCarrere(BouLac),Mellor–Yamada–Janji(cid:1)c(MYJ),quasi-normalscaleelimi-
nation (QNSE), and Yonsei University (YSU) versions 3.2 and 3.4.1. This study’s chosen objectively de-
terminedmodelNBLheighthestimateusesaturbulencekineticenergy(TKE)thresholdequalto5%TKE0 ,
max
whereTKE0isrelativetoitsbackground(freeatmosphere)value.TheYSU-andMYJ-determinedhcouldnot
beimprovedupon.Observedheightsofthevirtualtemperaturemaximumh andwindspeedmaximum
Tvmax
h ,andtheheightsh andh ,betweenwhichtheradiosondeslowsfrom;5to;3ms21asitrises
Smax 1wsonde 2wsonde
fromturbulenttononturbulentair,andthusbracketsh,wereusedforcomparisontomodelresults.Theob-
servationsrevealedageneralpattern:h increasedthroughthenight,andh andh convergedwith
Tvmax Tvmax Smax
time,andthetwomostlylaybetweenh andh afterseveralhours.Clearfailuretoadheretothis
1wsonde 2wsonde
patternandlargeexcursionsfromobservationsorotherPBLschemesrevealedexcessmixingforBouLacand
YSUversion3.2(butnotversion3.4.1)andexcessthermalmixingforQNSEunderwindyconditions.Observed
frictionvelocityu wasmuchsmallerthanmodelvalues,withdifferencesconsistentwiththeobservations
*
reflectinglocalskindragandthemodelreflectingregionalformdrag1skindrag.
1. Introduction Bougeault–LaCarrere(BouLac;BougeaultandLaCarrere
1989), and quasi-normal scale elimination (QNSE;
Theobjectivesofthispaperaretoevaluatetheheights
SukorianskyandGalperin2008;Sukorianskyetal.2005)
of significant nocturnal boundary layer (NBL) features
andusedatafromthreefair-weathernightswithmostly
using observed and model profiles and then use them to
moderate winds during the April–May 1997 Kansas-
evaluatefourplanetaryboundarylayer(PBL)schemesin
basedCooperativeAtmosphere–SurfaceExchangeStudy
high-resolution (1-km innermost grid) simulations using
(CASES-97; LeMone et al. 2000) field program. After
theAdvancedResearchversionoftheWeatherResearch
identifying a good criterion for objectively determining
and Forecasting Model (ARW-WRF, hereafter referred
NBL depth h, we explore PBL-scheme strengths and
to as WRF). As in its companion paper, LeMone et al.
shortcomingsbycomparingthebehaviorandmagnitudesof
(2013), which examined the convective boundary layer
h,h ,andh ,respectivelytheheightsofthevirtual
(CBL), we focus on the Yonsei University (YSU; Hong Tvmax Smax
et al. 2006), Mellor–Yamada–Janji(cid:1)c (MYJ; Janji(cid:1)c 2001), temperatureTyandwindspeedSmaxima,toobservations.
WhiletheCBLhasawell-definedmixed-layertop,the
NBL h can be difficult to identify. Numerous methods
havebeentried(Table1).Diagnosisofhusingmeanor
*TheNationalCenterforAtmosphericResearchissponsored
instantaneous vertical profiles has had mixed success.
bytheNationalScienceFoundation.
Using Doppler lidar data, Pichugina and Banta (2010)
foundastrongcorrespondenceofh tohasdefinedby
Smax
Correspondingauthoraddress:MargaretA.LeMone,National
profilesofthevarianceoftheradialvelocityforasubsetof
CenterforAtmosphericResearch,3090CenterGreenDr.,Boulder,
windprofiles(onemaximum,windinlowest200mgreater
CO80301.
E-mail:[email protected] than 5ms21), with minimum curvature in the S profiles
DOI:10.1175/MWR-D-13-00358.1
(cid:1)2014AmericanMeteorologicalSociety
2710 MONTHLY WEATHER REVIEW VOLUME142
TABLE1.SelectedpotentialcriteriaforNBLdepthhfromverticalprofiles.
Observations Model Comments/source
›T/›z50 ›Qy/›z510Kkm21 Bothnearlyequivalentto›Ty/›z50
Yamada(1979)
›S/›z50and ›S/›z50 Minimumcurvaturefoundsubjectively
minimumcurvature MelgarejoandDeardorff(1974)
Bantaetal.(2003)
PichuginaandBanta(2010)
Ri ;1.0 Ri 50.5 Valuesbasedontrialanderrorforsubsetofsoundings,
loc loc
MYJprofiles.
Bantaetal.(2003)
Dropinradiosonde ResultofTKEdropoffattopofNBL
riserate JohanssonandBergstrom(2005)
Ri(level1–h)5constant Constant50.2basedonsubsetofcases
VogelzangandHoltslag(1996)
YSU;Hongetal.(2006)
TKE50.101,0.2m2s22 ModificationsofMYJhcriterion
LeMoneetal.(2013)
TKE5fTKE0 ,1TKE Figure1pluscomparisonofTKEprofilestoKandflux
max b
f5constant profilesforLESandPBLschemes
Lenschowetal.(1988)
yielding even better results. However, in a large-eddy metoffice.com/variance_625.htmlandfollowthemenufor
simulation (LES) of a weakly stable NBL (Obukhov profilesoffluxesandmeans].
length L . 100m) by Kosovic and Curry (2000), h SinceTKEprofilesareavailablefromWRFruns,we
Smax
and h coincided with h only under steady-state use themto determinesubjective h(h )for BouLac,
Tvmax subj
conditions,afteraboutoneinertialperiodofsimulation. MYJ,andQNSEandusetheprofilesandresultingh
subj
Similarly,observationsshowthath doesnotneces- tojudgethemodelhmetricsinTable1.ForYSU,weuse
Tvmax
sarilycoincidewithh orhinmoderatelytoverystable theeddyexchangecoefficientK,notingitsrelationship
Smax
conditions(e.g.,Figs.6and7ofMahrtandVickers2006). to TKE (Shin et al. 2013). The LES-generated TKE
Indeed,asillustratedinBantaetal.(2007)andSunetal. profiles of Kosovic and Curry (2000) and Basu and
(2004) and elsewhere, the NBL often has a complex
structurethatvarieswithtime.
Bulk Richardson numbers (e.g., Vogelezang and
Holtslag 1996) and more complex formulations (e.g.,
Vickers and Mahrt 2004; Steeneveld et al. 2007) have
also been used, with varying degrees of success. A sig-
nificant shortcoming of such approaches is that radio-
sondedataneedtobesmoothedforreliableestimates.
Also, magnitudes of criteria using vertical gradients,
including Richardson numbers, tend to vary with the
verticalspacingused.
Whenturbulencedataareavailable,theheightatwhich
asecond-momentvariabledecreasestoaspecificfraction
ofitssurfaceornear-surfacemaximumprovidesauseful
estimateofh.Examplesofsuchparametersarebuoyancy
flux (Caughey et al. 1979), vertical velocity variance
(VickersandMahrt2004),verticalfluxofthecomponent
of the horizontal momentum along the surface wind di-
rection(KosovicandCurry2000),andtheturbulenceki-
netic energy (TKE; Lenschow et al. 1988). Fortunately,
suchparameters tend tobe internally consistent, at least
for weakly stable NBLs [e.g., see LES of Kosovic and
FIG.1.Steady-stateLESTKEprofilesfortheweaklystable(L.
Curry(2000),BasuandPorte-Agel(2006),andsimulations
100m)ArcticSBL.BlackreferstoKosovicandCurry(2000),red
summarizedinBeareetal.(2006);comparetohttp://gabls. referstoBasuandPorte-Agel(2006),andSGSisthesubgridscale.
AUGUST2014 LEMONE ET AL. 2711
FIG.2.CASES-97observationalarray.Numbersindicatesurfacefluxsites.Attheverticesof
the triangle lie 915-MHz RWP/MS sites BEA (elevation 478m), OXF (360m), and WHI
(430m),withcollocatedradiosondereleases.Solidlinesindicateflighttracks.Terraincontour
intervalis20m.
Porte-Agel(2006)inFig.1illustratewhatwecanexpect parallel to the surface wind, w is the vertical wind, the
for weakly stable conditions. The two profiles in the overbars indicate an average, and the primes indicate
figure have weakly concave-up, almost linear shapes, a deviation from that average. In the figure, their h is
with a maximum at the lowest grid level (10m). The roughlytheheightatwhichTKE(h)50.045TKE(10m).
authorsfoundhbydividingby0.95theheightatwhich However,werecognizethattheTKEprofilecandepart
u0w050:05(u0w0) , where the wind component u is significantlyfromthisideal.Forexample,Pichuginaand
p p sfc p
FIG.3.IllustrationofhowhSmax,hTmax,hRiloc,h1wsonde,andh2wsondeareestimatedfromradiosondedata.
2712 MONTHLY WEATHER REVIEW VOLUME142
locations.Italics bT(havg)2m,el21T(,Kkm),zsdg0930 2111(6.6) 191(37) 1c81(32) 140––5D1MT1aAtaMeByaLyE3.Netradiatio222n0-65h2(560Wa..046vUgm66RT276C2n..)e58tatsurfacefluxs222it0-55eh983s3..a7910v–66Ug8.RT54Cn..20et
e
sit 20–21May 259.965.4 245.2614.7
and).Figure2shows 1Sparseveg(V)grass(G),sites3(V)14(V/G)8(G) 02000930 189/3.6189/6.10.140.36220.1930.4031185189/2.3197/2.90.0600.12220.0420.01683.88.3cc85/2.669/20.150.10220.0160.0661412 Btvlehaanernciatefianrhcs(ietg2h0sp1eirg0ron)fiuifilpaecr.saensftionrcmceeindtihmteoruemdceafiinnnebthetehiesriigrrnahifidicinaalnttevretmluorscbioutyf-
sl Here, we estimate h, h , and h for observed
as Tvmax Smax
Whitewater(gr 5,6,and7wheat) 0930 190/6.60.4820.0575128198/2.50.08720.01543.272/2.20.1320.01312 NstuhosBenedLsamesmso,thedraeaetldx–aeaorrrbcewsisediernevfdeaoptrpiocreoonnrfiorlceueosrgmpsh,opanfaondrrdiisnoagmnnmaisnloyitsdsooiesdleaupvrsrasio,lnufipgaleetrrseaf,odatrinhomde-
ndWis Sites( 0200 185/3.60.1620.03728.2176/1.80.05420.00651.887/2.50.1120.0129.1 Ibannehaslaeyvcsiitosio,ranonfd3t,shtweabefoilduiterysPrcerBigbLimesecthsheaermemedose.dsOeclrbisbseeetrduvpiantiasoenncdst,ioPdnBat2La.
a
Beaumont(opengrassland),OisOxford(sometrees),indicatedatathatarefromtheradiosonde. 1Sites12(grass-land,nearBEA)Parameter(avgoffoura0200093030-minavg) 21)194/6.6193/9.1Direction/speed(ms21u(ms)0.300.46*222001wT()(Kms)0.0510.039cL(m)4318321Direction/speed(ms)210/4.4215/6.521u(ms)0.080.30*222001wT()(Kms)0.0110.044cL(m)3.44521Direction/speed(ms)82/4.575/5.221u(ms)0.180.28*222001wT()(Kms)0.0210.0278cL(m)2157 00wTeterandisapproximatelyequalto().ysfc asutoasstfds(chhucuuo1on.sbihueet1egrnAmdsuTfeir0dgedarrnPmpm0erhemcedavaBstseeoteotlier,waoLfidsiflrodjofsooeeees0uaobnsrlelvrr9Nnxneessckelo3eedodehaBnfl0irtscbseuonchotvLetamjhUtreeaedinsseoofictfeeTe.dgrainubtnscoovWhiCttsdtvoemnut9ieiiesnooxleat0rsruhcpnecfv-losTttomuealuieaf5aiomatstmrmrfi.ihisCbnnmnoorpeptmlaAlniheaslhprnyosraeoSofa.toew1raveEwtofiIitferizrnreSrntvmhei-mhor-ggawosn9emteiloehwne7dssndcaesbafdttwaeotsyweaiarhnlo,ra)sneyeed,TntafssdltorrunlKotha4uhilodmttdd,PErohscaeawB2wwperarpxy4lyieLeniatr-wacpedoh.mrcanerrifieonptevsidiRpiolmunddaeoemlnagelrisinpucpcidgaostaataarfeihkedoooorrttsdeeeeeesr--ft
s m
T2.Conditionsfordaysexamined(timeinUTC).BiABLE 28148-mwinddirection/speed(/ms)minisodar(sonde) Date02000930 4–5MayB195/11203/12O200/7.8182/9.3197/14W()— 216/5.510–11MayB225/11.5O206/4.5212/6.6W186/7.2213/8.6 20–21MayB83/7.569/7.4O84/5.281/7.6113/8.7(119/8.8)W87/5.3()96/5.3 00awTTNotethat()usestemperaturefromsonicanemoccbIncludesstations1–8,unlessotherwiseindicated.cDoesnotincludesite8. t2dcadMamasefi(vRwuBo0rvbdiasldWoee_CrS1amotluEfreenap4i/oaswlPsAdpit,rafcsomaoairwfpbfe)1nurP(ovr/j,pelipoR?direamBnrsadcCWrmMsisoWdreoLlafowaAitnossjthSshnbePareow(haiSpeacltd)Rrnnet1rEeetl,eaatsossd0rw.pciSrt/gefitCocaea:dta-eaof/loa,9mon/oetersnogse7waofdsfiudenmdorr,vmgnlhasun(taot(2u9iid(emFmhzMnic0oW7nrliaihegaoi,d/gbihlSnaMcbsAHo.seaRtqsiaotld.tsrrcae2arIutenpWoeeaiig)y)braido:dd,t.oD.)/fsilaPaen/ar.liinadfWitmnoo)rartAynnaad.solymwtp.deetnfSa.sgmWaonlt(duNo(y.fuaOrTlheMrovo1hascMafoth/rxaieiattoStAltlnepifdldo.cSemto)oud:hfBe3rw/nro8ciab/tde0cdLwmaaahaudimwaalntrtReEriwttee(.oeLeiaeenO/arWnwrsd‘,Bdadin‘e.owagXb.bjuePfesnrahtuaelor/paeFoeh,ndctonrsru.snl)aoeBdee)amda.,mdtuddtvdeowpoflecioatttaanfraiarduhhhoiotuntlycrl’haeexeyr---t/’.,
AUGUST2014 LEMONE ET AL. 2713
FIG.4.ComparisonofPBLschemeh(solidlines)forMYJ,QNSE,andYSUtosubjectivevaluesbasedonTKEorKhprofiles(dashed
lines).NohvalueisdefinedforBouLacinstableconditions.Squaresarethefullgridlevels(zf)forMYJ,QNSE,andYSUandhalfgrid
levels(zh)forBouLac.
Whitewater data totally absent on 29 April, of poor isappealingbecauseitdirectlyrelatestoourTKE-based
quality on 5 May, good for 10–11 and 20–21 May, and NBLdefinition.IncontrasttotheonePBLdepthchosen
Oxford winds were of marginal quality. Radiosonde by Johannson and Bergstrom, we used two depths,
dataweremissingafter0500UTCatWhitewateron11 h and h , to identify the lower and upper
1wsonde 2wsonde
May(seeFig.7). limits of the height interval through which w falls
sonde
from its ‘‘turbulent’’ to ‘‘nonturbulent’’ value. A low
b. DeterminationofNBLdepthfromobservations
bias for h of upto 30m results fromthe factthat
wsonde
Figure 3 illustrates how the heights of the NBL fea- theballoonrespondstotheturbulence,whilethesonde,
tures listed in Table 1 are identified subjectively from which collects the data, is attached to the balloon by
observations. Temperature T profiles were smooth a 30-m string. The angle of the string to the vertical is
enoughtomakeh (’h )easytodetermine,with unknown,sowedonotcorrectforthisbias.
Tmax Tvmax
an uncertainty of the order of the radiosonde data It was not possible to estimate h from w if the
sonde
spacing, 30–50m, while scatter in the wind profile can NBLwasmuchlessthan;100mdeep.Ittakestimefor
increase uncertainty for h to ;50m. Heights at thesondetounreelfromtheballoonandfortheballoon–
Smax
which the increase of wind with height suddenly slows sondesystemtoacceleratetoatypicalspeedof;5ms21,
down without reaching a maximum [a rough corre- and there were typically only 3–4 points below 100m
spondence to Pichugina and Banta (2010)’s minimum at the 10-s data rate. Given the general association of
curvature in the speed profile] were also documented, deeper NBLs with larger u (e.g., Caughey et al. 1979;
*
but this situation was rare (see Fig. 7). Evaluation of Steeneveldetal.2007),welimitedouranalysistowindier
h fromtheblendedRWP1MSdatawassometimes nights.Evenso,theballoonriseratemethoddidnotal-
Smax
complicated by a jump in speed in the transition from wayswork,becauseballooninflationwasnotoptimumor
RWPtoMSmeasurementsatabout150m,likelyare- verticalairmotionsmodifiedballoonriserate.
sult of ground clutter contamination of the RWP data.
c. NBLclassification
Verticalspacingis60mfortheRWPdataatWhitewater
and Beaumont and 5m for the MS data. Scatter Thedatausedforthisstudyweregatheredinrolling
and coarse data spacing in the radiosonde data made terrain(Fig.2)withvaryinglandcover(Table2);both
Richardsonnumbersunreliableonmostdays. causewindandturbulencetovaryhorizontally.Indeed,
Inthefinaltechnique,hisbasedontherapiddecrease according to Acevedo and Fitzjarrald (2001), Fiebrich
in sonde vertical velocity w as the balloon travels and Crawford (2001), Van de Wiel et al. (2002), and
sonde
from turbulent to nonturbulent air (Johansson and others, the turbulent near-surface flow sometimes de-
Bergstrom2005),whichresultsfromanincreaseindrag tachesfromthesurface,especiallyinlower-lyingareas.
ontheballoononceitenterslaminarflow(MacCready Withthisinmind,wecharacterizetheNBLinaregional
1965;Galliceetal.2011;Wangetal.2009).Thismethod sense.
2714 MONTHLY WEATHER REVIEW VOLUME142
TABLE4.CharacteristicsofPBLschemesforstableconditions.NistheBrunt2Väisäläfrequency.TKEunitsarem2s22.
Defaulth Background Primary
Scheme Basicphysics criterion TKE(m2s22) References
BouLac Level1.5scheme NoneforNBL 0.0001 Bougeaultand
Solveforeusingsimplifiedformof(1); LaCarrere
(1989)
KH5KM50:4Lmixe0:5; Therryand
LaCarrere(1983)
L basedonhowfarparcelcan
mix
travelverticallywithinitialvertical
velocity5(2e)0.5
MYJ Level2.5scheme h ,lowest 0.1000 Janjic(2001)
MYJ
Solvesfore,u0;(latterequation levelatwhich Mellorand
simplified)/2.5; e,0.101m2s22 Yamada(1982)
KH,M5fH,M"Lmixe0:5,(cid:2) (cid:3) (cid:2) (cid:3) (cid:2) (cid:3)# (notinterpolated) Mellorand
f 5f e,L , ›U 2, ›V 2, ›Qy ; Yamada(1974)
H,M H,M mix ›z ›z ›z
PreliminaryLmix5Lmix,P
5L kz/(kz1L ),
mix,0 mix,0
ð ð
hnbl / hnbl
L 5a (zqdz) (qdz),q5(2e)0:5,
mix,0
0 0
L adjustedtosatisfyanequationforL /q
mix mix
QNSE In(1)verticaldiffusionasinMYJ, h ,lowest 0.0050 Sukoriansky
QNSE
levelatwhich etal.(2006)
ShearandbuoyancytermsuseK 1vertical e50.0050m2s22 Deteringand
H,M
gradientsasinMYJ, (notinterpolated) Etling(1985)
«5(0:55)3e1:5/L asinMYJ, Sukorianskyand
mix
0:75kze0:5L Galperin(2008)
L 5 mix,0 ,
mix kzNL 10:75e0:5(kz1L )
mix,0 mix,0
KH,M50:55aH,MLmixe0:5,aH,M(Riloc)
basedonQNSEspectralmodel
YSU KH,M5wskz(12z/hYSU)2, Ri(hYSU)50.25 N/A Hongetal.(2006)
ws’u*forversion3.2, (interpolated), Nohetal.(2003)
ws5u*/(115z/L)forversion3.4.1; from(6)with
L 5L kz/(kz1L ),L 530m z 5zh, Louis(1979)
mix mix,0 mix,0 mix,0 1 1
U 5V 50 Bettsetal.(1996)
1 1
AccordingtoLeMoneetal.(2003),NBLsregionally varieslittlespatially,indicatingthatthesynopticflow
vary from being continuously turbulent and fully cou- on 4–5 May is continuously coupled to the surface at
pled to the surface, with air trajectories following the 0930UTC.Ontheotherhand,intermediateT and
2m,el
terrainalongthesynopticwinddirection,tohavingonly more variationinwinddirection on the nightsof 10–
weak turbulence driven by drainage winds. In the for- 11 May and 20–21 May suggest some influence by
mercase,the2-mTchangeswiththeelevation,T ; drainage flow, with possible occasional decoupling
2m,el
29.8Kkm21, following the adiabatic lapse rate, while and associated cooling, especially for the low-lying
T . 140Kkm21 for the latter case (their Fig. 5), stations.
2m,el
with a magnitude that increases with the vertical T Asimilarpictureemergesfromthelocalclassification
gradient; low-lying locations where radiative cooling is scheme of Van de Wiel et al. (2003), who use net
not offset by downslope winds or turbulent mixing radiation R and u2/h, where the friction velocity
net *
alsoincreaseT .FromTable2,T 5211Kkm21 u 5[(u0w0)2 1(y0w0)2 ]1/4 and u0 and y0 are the hori-
2m,el 2m,el * sfc sfc
(basedonaleastsquaresstraightlineforT asafunc- zontal wind components, to identify three regimes:
2m
tion of elevation for sites 1–8), and the wind direction ‘‘continuousturbulent,’’‘‘intermittent,’’and‘‘radiative.’’
AUGUST2014 LEMONE ET AL. 2715
FIG.5.ForMYJatBeaumonton5May1997,theevaluationofhcriteriabasedonTKEprofiles(shifted2m2s22
each2h);‘‘1M’’inupperleftofthefirstpanelindicatesthatcriterion1failedtoidentifyh.Sunsetwasaround
0130UTC(1930CST).
To identify the regimes for CASES-97, we plotted on May falls in the continuous turbulence regime for all
theirFig.8ourbestestimatesofhalongwiththreesetsof threeaveragesat0930UTCandamixofintermittentand
fluxaverages(forsites1–8,thetwohighestgrasslandsites continuousturbulenceat0200UTC.Theother2daysfall
1and2,andthetwofloodplainsites3and6).Assuming mostly in the intermittent regime, consistent with the
that the regimes in the figure apply (reasonable since evaluationbasedonT .Thefourthnight,28–29April
2m,el
CASES-97wasinthesamelocation),wefoundthat4–5 (not shown), falls in the radiative regime for all three
FIG.6.ForBeaumont,evaluationofeightpotentialhcriteriabasedoncomparisonstoseriesofTKEprofileslike
thoseinFig.5fornightsof4–5,10–11,and20–21May.LabelsrefertodatesinUTCandMsignifiesMay.
2716 MONTHLY WEATHER REVIEW VOLUME142
FIG.7.ObservedNBLfeatures:hTmax(redcircles),hSmax(bluecirclesforradiosondes,greensquaresforRWP1MS),h1wsonde(tan
upsidedowntriangles),andh (tantriangles).Heightsofminimumwindprofilecurvature(ifitdiffersfromh sometimeduring
2wsonde Smax
thenight):radiosonde(smallturquoisedots)andRWP1MS(smalllightgreensquares).ForWhitewater5May,smallerdotsandthe
dashedbluelineindicateheightsofsecondarymaximainTyandS.
averages and thus was considered too stable to be in- jR jinourTable3isclosetothatduringCASES-99,7.5–
net
cludedhere. 8ms21isagoodthresholdspeedat48mforCASES-97as
Following Van de Wiel et al. (2012) and Sun et al. well as CASES-99. Based on this criterion, turbulence
(2012),classifyingthedaysaccordingtowhetherthewind below 48m can be sustained on 4–5 May, at Beaumont
speedatagivenheightiscapableofsustainingturbulence andWhitewaterat0930UTC10–11May,forBeaumont
beneathalsorevealsasimilarpicture.BasedonCabauw on20–21May,andforOxfordat0930UTC21May.
data,VandeWieletal.foundthatcontinuousturbulence
is maintained when the wind at 40m exceeds ;5ms21,
withthethresholdincreasingwithjR j.Usingdatafrom 2. Modelsetupandanalysis
net
the55-mCASES-99tower,Sunetal.foundthatthreshold
a. WRFruns
speeds increase with height, with values of ;7ms21 at
40mand;8ms21at50m.TheCASES-99thresholdsare ThemodelresultsanalyzedarefromWRFversion3.2
largerthanforCabauwatleastpartiallybecauseoflarger runsdescribedinLeMoneetal.(2013)for4–5May,10–11
jR j[cf.Fig.4ofVandeWieletal.(2012)forCabauwto May, and 20–21 May, with an additional set performed
net
Table2ofVandeWieletal.(2003)forCASES-99].Since using WRF version 3.4 with YSU version 3.4.1. Each
AUGUST2014 LEMONE ET AL. 2717
ThephysicalparameterizationsincludetheNoahland
surfacemodel(ChenandDudhia2001a,b;Eketal.2003),
theRapidRadiativeTransferModel(RRTM)long-wave
parameterizationscheme(Mlaweretal.1997),theDudhia
(1989) shortwave radiation scheme, and the Lin et al.
(1983) bulk microphysics scheme. Three PBL schemes
(describedinmoredetailinthenextsection)werelinked
totheirdefaultsurfacelayeroptions(option1forYSU,
option2forMYJ,andoption4forQNSE);BouLacuses
thesameoptionasMYJ.Surfacecharacteristicsarebased
ontheModerateResolutionImagingSpectroradiometer
(MODIS)VEGPARMTableversion3.1.1,withmodified
surface roughness values z (see Table 3; LeMone et al.
o
2013). Land use types over the CASES-97 array are
mainly crop- and grassland, with the latter increasing
eastward.Allthreegrasslandsitesusedformodelobser-
vationcomparisons(Beaumont,site1,andsite2)corre-
spondtograsslandgridcellsinWRF.
b. PBLschemesinstableconditions
The four PBL schemes, BouLac, MYJ, QNSE, and
YSU,areoutlinedinTable4,alongwithreferences.The
first three, henceforth called TKE schemes, solve vari-
ousformsoftheequationforTKE(representedhereby
e),givenby
›e ›(w0e0) 1 ›(w0p0) ›U
52 2 2u0w0
›t ›z r ›z ›z
›V g
2y0w0 ›z1T w0Ty02«, (1)
y
FIG.8.TimeseriesofradiosondedepthsofNBLfeaturesSmax where the wind components (u, y, w) are in a right-
and T (symbols), minimum h , and maximum h handedcoordinatesystemwithupositiveeast,andeach
max 1wsonde 2wsonde
(lines)aftercompositingusingvaluesat0930UTCasdescribedin component isthe sum of the resolved (upper case) and
text.Forthefewtimesthatminimumcurvatureinthewindprofile
unresolved/parameterized (primed) component. For Ty
didnotcorrespondtoSmax(seeFig.7)thecorrespondingheight andairdensityrtheresolvedportionisindicatedwithan
wasusedratherthantheheightofS .Symbolsrepresentloca-
max
tion: Beaumont (squares), Whitewater (upside down triangles), overbar.Thefirsttwotermsontheright-handsidearethe
andOxford(circles). verticaldivergenceofverticalenergytransportandver-
ticalpressuretransportbyw0,1thethirdandfourthterms
are shear production, the fifth term is buoyancy pro-
simulation was run for 24h, starting at 1200 UTC
duction, andthe sixth termisdissipation. Note that the
(0600 LST), using fourtwo-wayinteracting nestedgrids
threeTKEschemesallowneitherTKEtransportbythe
with spacing of 27, 9, 3, and 1km, respectively. The
resolved flow nor horizontal TKE transport by turbu-
2128 3 2547km2 outer domain (Fig. 4; LeMone et al.
lence.
2013) extends across most of the continental United
ForallfourPBLschemes(includingYSU,forwhich
States,andtheinner1273107km2gridiscenteredonthe
the nonlocal terms are zero for stable conditions), the
CASESarray.Theverticalgridhas44sigmalevels,with
tendencyforaquantityCduetosubgridfluxesisfound
the lowest half model level just below 5m, spacing in-
usinganeddydiffusivityK from
creasing with height (e.g., Fig. 4), and the top level at c
about 16km. Initial and boundary conditions for WRF
are from the 3-h North American Regional Reanalysis
1Bothverticaldivergencetermsaremuchsmallerthantheother
(NARR; http://rda.ucar.edu/datasets/ds608.0/) data on
termsin(1),withthepressuretransportcontributionclosetozero
a32-kmgrid. intheKosovicandCurry(2000)LESofaweaklystableNBL.
2718 MONTHLY WEATHER REVIEW VOLUME142
K 5F L e0:5 (3)
M,H M,H mix
In(3),Kistheeddydiffusivity,andthesubscriptsMand
Hrefertomomentumandheat,respectively.Themaster
length scale L and the function F varies with the
mix M,H
scheme(Table4)asdoesthePrandtlnumberPr,givenby
theratioK /K .NotethatPr51forBouLac.Also,L
M H mix
forBouLacisthemaximumverticaldistancetraveledby
africtionlessairparcelwithinitialverticalvelocity(2e)1/2,
against unfavorable thermal stratification. This simplifi-
cation, which ignores the decrease with stability of the
fractional contribution of w02 to e, is expected to over-
estimateL (TherryandLaCarrere1983).
mix
For YSU, K for a stable boundary layer is calcu-
H,M
latedfromtheYSUNBLdepthh via
YSU
(cid:2) (cid:3)
z 2
K 5K 5kw z 12 , (4)
H M s h
YSU
where the von Kármán constant k 5 0.4 and w is es-
s
sentiallyequaltothefrictionvelocityu inYSUversion
*
3.2andw 5u (115z/L)21 inYSUversion3.4.1.
s *
c. EvaluationofmodelNBLdepthandselectionof
0
the5%TKE criterion
max
The model output h values, h , h , and h ,
MYJ QNSE YSU
basedonthecriterialistedinTable4andshownassolid
lines in Fig. 4, diverge much more than implied by the
actualprofilesofTKEor,forYSU,K(h indicatedby
subj
dashedlines).Also,TKEfallswithheighttoadifferent
‘‘background’’valueforeachscheme.Thus,wedefined
acandidateNBLdepthhfTKE0 astheheightsatisfying
max
hfTKE0 5f(TKEmax2TKEb)1TKEb, (5)
max
whereTKE isthebackgroundvalueandf50.05(;the
b
valueinFig.1)or0.10.ForBouLac,TKE wasalways
max
at half grid level zh (;5m). For MYJ and QNSE,
2
TKE tended to occur at the surface (full grid level
max
zf )butnotalways;soTKE wasdeterminedforthe
1 max
lowest kilometer. We also considered the same TKE
FIG.9.(left)FourpatternsinhSmaxandhTvmaxevolution,along thresholdsasinLeMoneetal.(2013),namely,0.101and
with h estimates h5%TKE0max and hMYJ (light blue line, top frame 0.2m2s22.
only).TopthreepanelsshowaveragesofBeaumont,Oxford,and
Whitewater heights; the bottom panel is for one location/date. Thecandidatehcriteriabasedonthemeanprofilesand
(right)Approximategridheightzh. RichardsonnumbersarelistedinTable1.Notethatthe
altitudeatwhichtheverticalgradientofvirtualpotential
temperatureQy,z510Kkm21iswithin;10mofhTvmax.
(cid:2) (cid:3) Thisisnotsurprising,ascanbeshownbysubtractingthe
›C52› (w0c0)5 › K ›C . (2) adiabaticlapserate9:8Kkm213(z2z1)fromQy(z1)to
›t ›z ›z c›z obtainaprofilewiththesameshapeasTy.Thus,weuse
TheeddyviscosityKMfortheTKEschemesisgivenby horTvQmya,xz5in1th0eKtekxmt2fo1rinthtehesafikgeuroefsbfroervtihtye,swakheileofuascincgurhaQcyy,z.