Comparison of the spatial and temporal ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D22, PAGES 27,755-27,769,NOVEMBER 27, 1999

Comparison of the spatial and temporal distribution of fluxes of sensibleheat, latent heat and COz from grid flights in BOREAS

1994 and 1996

SegunO. Ogunjemiyoand PeterH. Schuepp Departmentof Natural ResourceSciences,McGill University, Ste-Anne-de-Bellevue, Quebec,and Centrefor ClimateandGlobalChangeResearch,McGill University,Montreal,Canada Ian J. MacPherson

Institutefor Aerospace Research,NationalResearch Councilof Canada,Ottawa

Ray L. Desjardins ResearchBranch,AgricultureandAgri-FoodCanada,Ottawa

Abstract. Analysisof airborneeddy correlationflux measurements of heat (H), moisture (LE) and CO2(C) over two 16 km x 16 km heterogeneous grid sitesin BOREAS 1994 (IFC-2) and 1996 are comparedin order to examinepersistenceand variability in the distributionsof surfacecharacteristics and fluxesbetweenthe two years. The data used were obtainedin grid patternsflown at 30 rn abovegroundlevel, undergenerallyclear sky andthermallyunstableconditions.Maps of fluxesand surfacecharacteristics were constructed by block averagingover 2 km windowsalongthe flight lines, analyzedfor similarities,and usedto quantifyspatialvariability of the fluxes. Sensitivityanalysis suggested minor effectsof boundarylayer variabilityandwindowsizeon the main featuresof the source/sink distributions. Incidentradiationwasmorehighlycorrelated with grid-averagedvaluesof C thanwith H andLE. The dominantrole of surface inhomogeneity,as opposedto local variationsin solarenergyinput, on spatialvariation of flux distributionswas confirmed,and mesoscalemotionwas foundnegligible, probablybecauseof the smallsizesof homogeneous subareaswith sufficientsurface contrastto inducethermallygeneratedmotion.CO2flux andgreenness indexwere highly correlated,but correlationwas site- and time-specific.The previouslyobservedlow correlationbetweensensibleheatflux and surfaceminusair temperaturedifference (T•Ta),primarilyover old blackspruce,wasconfirmed.The highBowenratio over the forestcontributedto the growthand developmentof the observeddeepboundarylayers over the sites,but no clear correlationemergedbetweenboundarylayer depthand observed near-surface

1.

fluxes.

the surfacecover.This doesnot invalidate thepreviously demonstrated potentialof grid flightsto maptemporaland

Introduction

Modelsof biosphere-atmosphere interactionshave been spatialvariationsin surfacefluxesand to relatethe observed developed primarilyforhomogeneous surfaces. Extrapolation flux estimates to surfacecharacteristics [Desjardins et al., of suchmodelsto heterogeneous surfaces requiresvalidation 1992; Schuepp et al., 1992; Mitic et al., 1995], in principle, by datathatintegratemeasurements overareastypicalof the butmakesitsapplication to theboreallandscape muchmore land surfaceand over an adequateperiod of time. Our of a challenge. analysisof the data obtainedby the Twin Otter aircraft

The analysis presented in thispaperusesTwin Otterdata [Ogunjemiyo et al., 1997] duringthe Boreal Ecosystem- fromBOREAS1994second intensive fieldcampaign (IFC-2) Atmosphere Study(BOREAS)1994suggests thattheprospect and the 1996 IFC to comparethe spatialdistributions of of usingboundarylayer modelsto estimatesurfacefluxes fluxesof sensible heat(H), latentheat(LE), andCO2 (C) at dependson the scalarunder consideration and the natureof thetwogridsitesbetween thesetwoobservation periods. This involvesidentification of areasat thesiteswheresignificant Copyright1999by theAmericanGeophysical Union. changes in fluxpatterns occurred andtheirlinkstopatterns of surface characteristics. Ourpaperalsoaddresses thestatistical Papernumber1999JD900052 stability of fluxmapsderivedfromoursampling andanalysis

0148-0227/99/1999JD900052509.00

procedure. 27,755

27,756

OGUNJEMIYO ET AL. ßAIRBORNE FLUX MAPPING IN 1994 AND 1996

years,andall flightsat eachsitewereflownwithin10days, exceptthe1996flightsin theSSA,whichwerespread over 20 days.Theairborne datawerecollected by theCanadian

2. Data Acquisitionand Processing 2.1.

Sites

The two grid sites,eachwith a dimensionof 16 km x 16 km, are located in the northern (NSA) and southern(SSA) studyareasof BOREAS, respectively[Ogunjemiyoet al., 1997]. The sites are heterogeneous,with different size patchesof surfacecoverandplant speciesat differentstages of growth.The land classificationimageryand the percentagesof the surfacecover typesare shownby Ogunjemiyoet al.[ 1997]. The NSA grid site is boundedby latitude55.80øN and 55.94øNand longitude98.40øW and 98.65øW, and the SSA grid by latitude53.78øN and 53.92øN and longitude 104.56øWand104.80øW.The northernhalf of theNSA grid, which includesthe old jack pine (OJP) andold black spruce (OBS) towersites,is coveredby wet anddry conifersandthe southernhalf by a former burn area in various stagesof regeneration,with onecentrallylocatedunburned"pocket". The SSA grid, which includesthe OJP, youngjack pine (YJP), and fen sites,is dominatedby wet conifermixed with fen, belowthe NW-SE diagonal,with drier conifer, logged, and regeneratingareasaboveit.

TwinOtter(TO) atmospheric research aircraft[MacPherson,

1990,1996] in a gridpattern,flownat approximately 30 m abovegroundlevel(agl),at a meanair speedof 60 m s4. Eachgrid flightconsisted of nineparallelstraightlines, spaced 2 kmapart,witheachlinesampled twicein opposite directions, in a sequence thatassured thatall gridlineswere sampled atthesamemeantime.Flighttrajectories (east-west ornorth-south) werechosen forclosest approach tocrosswind conditions.

Data were digitizedat 16 Hz. Observeddatausedin this

study include airtemperature (Ta),surface temperature (Ts) , incidentsolarradiation,reflectedred (R) andnear-infrared

(NIR)radiation, andmixingratiosof CO2andH20.Parametersderivedfrommeasured variablesincludenetradiation, Rn, greenness index, GI (from the Simpleratio SR = NIR/R), difference betweensurfaceand air temperature (AT= Ts-Ta)andpotential temperature (0). Moreinformation abouttheinstrumentation is givenby MacPherson [1996]. Thegeneral weather conditions duringtheflightsaresummarizedin Tables1 and2. All flightswere flownaround

solarnoon.Except forJulianday211in 1996,whenthesky wasovercast, theflightperiods werecharacterized bymainly The 1994 (IFC-2) data set consistedof eightgrid flights, sunny conditions. ThewindattheSSAwasgenerally stronger

2.2. Samplingand Weather Conditions

four at eachsite, flown duringJuly andAugust,andthe 1996 setconsisted of nine grid flights, four in the SSA and five in the NSA, alsoflown duringJuly and August.As seenfrom Table 1, the SSA flights precededthe NSA flights in both

in 1994thanin I996, withaveragevaluesof 4.4 and3.5 m

s'1 respectively. Thereverseis thecasein theNSAwhere averagewindspeedwas3.5 m s-• in 1994and4.1 m s4 in i996. The averageair temperatures in 1994 and in 1996

Table 1. Twin Otter Grid Flight Summary Flight

Time, Site

General Weather Conditions

1704-1859

SSA

clear, smoke from fire, north winds

N-S

1700-1854

SSA

mostlyclear, few cumulus,SW winds

26

E-W

1653-1844

SSA

clear, north winds

207

30

N-S

1705-1856

SSA

mostlyclear, few cumulus,SW-W winds

July 28

209

33

N-S

1617-1810

NSA

clear, NE winds

1994

Aug. 01

213

35

N-S

1547-1752

NSA

some cirrus, south winds

1994

Aug. 04

216

37

N-S

1546-1749

NSA

clear with some smoke, NW winds

1994

Aug. 08

220

39

N-S

1526-1748

NSA

few stratuscumulus, west winds

1996

July 09

191

58

E-W

1705-1906 SSA

1996

July 20

202

68

E-W

1605-1810

SSA

mostlyclear, few cumulus,SE winds

1996

July 27

209

71

N-S

1722-1908

SSA

overcast skies, SE winds

1996

July 29

211

72

E-W

1631-1818

SSA

clear,smallcumulusacrossgrid, SSE winds

1996

July 31

213

76

E-W

1456 1645

NSA

generallyclear, southwinds

1996

Aug. 02

215

79

N-S

1447-1638

NSA

clear, SW winds

1996

Aug. 03

216

80

N-S

1615-1801

NSA

clear with some smoke, east winds

1996

Aug. 05

218

81

E-W

1615-1810

NSA

few stratuscumulus, south winds

1996 Au•. 08

221

83

E-W

1450-1640 NSA

Year

Date

Day

Flight

Direction

1994

July 20

201

21

E-W

1994

July 21

202

22

1994

July 24

205

! 994

July 26

1994

GMT

generallyclear and sunny,SSW winds

clear, north winds

OGUNJEMIYO

ET AL. : AIRBORNE

FLUX MAPPING

IN 1994 AND 1996

27,757

Table 2a. Northernstudyareagrid averageparameters.

1994

1996

Julian

Wind,

Ta

Ts*

R,,

-L,

Flight

Day

m s-1

øC

øC

Gi*

W m'2

m

33

209

3.4

21.7

23.9

2.73

549.3

64.5

35

213

1.6

22.1

22.5

2.74

465.2

22.4

37

216

4.0

18.5

19.5

2.75

496.5

125.6

39

220

3.1

16.1

16.7

2.74

482.8

68.7

76

213

4.0

23.8

26.5

2.49

474.7

129.7

79

215

5.0

24.2

25.2

2.49

539.2

325.6

80

216

3.4

25.5

29.8

2.49

585.2

72.8

81

218

4.1

22.6

26.1

2.39

486.5

116.4

83

221

4.1

9.6

15.3

2.19

327.4

134.3

* Shiftsin instrument sensitivitymighthavecausedminorchangesin absolute values between,but not in relativedistributions within, the two years.

were 19.7øCand 21.1øC,respectively,at the NSA siteand 22.5øC and 18.7øC, respectively,at the SSA. Thermal stratification,as definedby z/L, where L is the Obukhov length,wasunstable(negativez/L) in all cases.Valuesof L are given in Tables2a and 2b.

were estimatedalongeachof the flight lines and averaged over all the flight lines to obtainthe grid averagefluxes. The mappingprocedureadoptedfor creatingtheflux maps wasthe sameas thatusedby Ogunjerniyo et al. [1997]. Flux estimateswere averaged over 2 km segments,and the estimatesfor the two repeatedpassesaveragedinto 2 km 2.3. Data Processing windows,with 1 km overlapbetweenadjoiningwindows. Vertical turbulent fluxes of sensibleheat, latent heat, and This generates a matrix of 135 datapointsper grid, with 4CO2attheflightlevelwerecomputed fromthetime-averagedkm averagesamplingper data point. Using cubic spline covariance(w'a') betweenfluctuations of verticalwind (w') interpolation,the griddeddata were smoothedand usedas andthe scalarof interest(a'). Potentialtemperature (0) was inputto GIS-basedIDRISI softwareto producethe flux maps usedto compute H, andtheuseof mixingratiosfor H20 (q) of the grid sites. The same procedureswere adoptedto (surfacetemperature and CO2 (c) obviatedthe need for densitycorrection producemapsof surfacecharacteristics indexGI). [MacPherson,1990].Turbulentfluctuations werecomputed excessover air temperature,AT, and greenness as the difference between instantaneous and mean scalar Localvariationsin surfaceemissivitywere notaccounted for.

quantities, with meansdetermined by applyingdetrending Given the fact that the PRT-5 sensor has internal calibration techniques developed by Ogunjemiyo et al. [1997]. Fluxes assuminggreen vegetationcanopieswith emissivitiesap-

Table 2b. SouthernStudyArea Grid AverageParameters Year

1994

1996

Julian

Wind,

Ta

Ts*

R,,

-L,

Flight

Day

m s-1

øC

øC

Gi*

Wm-2

m

21

201

5.7

21.5

19.3

2.71

525.9

414.2

22

202

5.1

26.5

24.7

2.70

622.9

204.5

26

205

4.9

19.2

20.1

2.84

611.4

146.3

30

207

1.8

22.9

22.7

2.77

553.8

128.4

58

191

5.0

19.6

25.8

2.3

691.5

151.5

68

202

2.9

18.4

22.9

2.28

601.1

41.6

71

209

4.2

17.0

19.6

2.20

291.2

412.2

72

211

1.9

19.9

23.4

2.26

573.9

14.6

27,758

OGUNJEMIYO

ET AL. ßAIRBORNE FLUX MAPPING IN 1994 AND 1996

2OO

ß

a

15o

E

100 50

0 185

•

I

,

190

I

•

195

I

,

200

I

,

205

I

I

210

t

215

I

220

225

Julian Day 30O

25O

•' 200

..• 150

100

,

50

19o

185

19•'

200

205

I

I

210

215

,

I

22O

225

Julian day -0.1

-0.2

• -0.3 E E

0 -0.4

-0.5

-0.6 185

,

I 190

•

I 195

•

I 200

,

I 205

•

I 210

,

I 215

,

I 220

, 225

Julian day

Figure1. Plotsof thegrid-averaged fluxes of (a)sensible heat,(b)latent heatand(c)CO2.

proaching unity(>0.98), the associated potential errorin Thesignificance of anygivenfactor,suchasradiation, in emissivity of 1 to2% isnotlikelytobesignificant. Also,no thecomplex relationships thataffectflux magnitudes, can atmospheric corrections weremadeonthesurface tempera-onlybededuced indirectly bycomparing thevariability inits turemeasurements, sincePerryandMoran[ 1994]foundsuch estimates tothatinthefluxestimates. Weusedthisapproach

corrections to be generallylessthan0.5ø for aircraftdata onthebasis ofthenormalized variances inthefluxmaps, Vr, acquired fortheir(higher) flightlevelsat 100m agl. andin theradiation maps,Vr, thatis,

OGUNJEMIYO

ET AL. : AIRBORNE FLUX MAPPING IN 1994 AND 1996

1./•i=•v _•]2

27,759

Table 3.Boundary Layer Parameters

VfNM j='-j E•[F(i d) i=l

zi

(1)

Azi,

W.

t.,

Flight rn rn rn s:• s

Vr -NMj=• ••[R(id)-•]2

o.,

q.,

K

g kg4

21 7461801.26360.06 0.08 22 1180 170 1.7 688

/=1

777

30

150

O.O5

1.6

497

0.10

0.05

1045 710

1.7

633

0.08

0.05

33

928

1.6

577

0.08

0.04

transformed into asetofvalues Z•jbysubtracting themean

35

800 820 1.4

579

0.07

0.03

andnormalizingthestandard deviationof thedifferences. The similaritybetweenthetransformed mapsisthencomputed on

37

1116

674

0.07

0.03

thebasisof similarity in thesignof Zijpairsas

39

1227 1034 1.8

657

0.08

0.03

where F(i,j) and R(i,j) are the grid data matrix in the two maps,withM rowsandN columns,andtheoverbarindicates averageof all the map data. To determinesimilaritybetween1994and 1996mapsof a givenscalaror flux, the datapointson the mapsare first

26

O.O8

644

937

1.6

58 121465

1.9 576

0.09

0.04

68

1.8

0.08

0.04

number ofZijpairs with negative, positive, ormixed signs. 71 714 252 1.0 750

O.O4

O.O5

Values ofCsvaryfromzero(nosimilarity) to 1 (absolute

72

785 614 1.5

546

0.08

0.05

Cs= rnm+n +n +p

(2)

1191

973

666

whereCs is the similaritycoefficient;m, n, andp are the similarity).

Data from serialupperair soundings [Barr and Betts,

76

1227

851

1.6

790

0.06

0.04

1994, 1997]releasedon eachof the flight dayswereusedfor boundarylayeranalysis.Because the releasetime wasabout 5-10 min beforeor after eachflight, with a 2 hour spacing,

79

678

688

1.2

591

0.06

0.05

twosuccessive soundings, coinciding approximately withstart

80

977

725

1.5

616

0.08

0.04

theboundary layer conditions during each flight. Estimated 81

1660

684 1.8 907

0.06

0.04

parameters include themixedlayerdepthzi, takenasthe

872

248

0.07

0.03

andendtimeof thegrid flightcanbe considered to document

depthover which the potentialtemperatureis essentially constant,averageboundarylayer moistureand changesin boundarylayer heightAz, moisturemixing ratio Aq, and potentialtemperature A0, whichoccurredduringthe flight period.We alsoestimatedthe convective velocityscalew,, convection timescalet,, temperature scale0,, andhumidity scaleq, in themixedlayeraccordingto Stull[1988]as

z, w,=[g_Z•(w/O/)]m,t,=•, 0,-(wO%,q,0

W,

W,

W,

(3)

83

1.4

607

NSA site.The observedtemporalvariabilityin the fluxeswas mainly due to variation in incidentsolar radiation and to a probablylesserdegreeto changesin flux footprintsand boundarylayer structure. The net radiationg nwas generallyhigherat the SSA than at the NSA, with lessthan6 % differencebetweenthe years at eachsite. It is interestingto observethatthehighestvalues of C are associatedwith lowest solar insolation,resulting from haze or cloud cover, and the lowest values of C associ-

ated with highestinsolationfrom clear skies. This is tentatively associatedwith physiologicalresponseto high vapor pressuredeficitsof coniferoustrees. Insofar as correlations 3.1. Grid-Averaged Fluxes couldbe established betweenRnandfluxesin suchsmalldata Figure 1 illustratesthe temporalvariationsof the grid sets, they were higher for C than for H and LE, and the with results averaged fluxesat thesitesfor the1994and1996observation observedtrendsbetweenC andg nare consistent periods.Each value represents samplingover a 288 km from otherstudies[e.g., Hollinger et al., 1994; Baldocchiet uptakeof CO2enhanced distance,which is expectedto providearea-averaged flux al., 1997],whichshowedecosystem estimateswith a relative error of < 5% [Lenschowand undercloudysky when the incomingsunlightis isotropic. Examinationof theboundarylayerprofilesof mixingratio Stankov, 1986; Mahrt, 1998]. Temporal variability was (O) at thestartof thegrid flightsshowed greaterattheSSAthanat theNSA; it depended onthenature (q) andtemperature of the scalarandreflectssite-dependent weathereffects.The thatthetimingof theflightscoincidedwith the growingphase composite, grid-averaged valueof H was the samein both of the boundarylayer when the residuallayer decaysas a yearsat theSSAbut24% lowerin 1996thanin 1994at the resultof turbulentmixingtriggeredby surfacewarming.The q with NSA. AveragedLE wasthe samein bothyearsat the NSA shapeof theprofiles,nearlyuniformO, anddecreasing signatures of but 19% lower in 1996 at the SSA. AveragedC was 18% heightwithinthe mixedlayer are characteristic higherin 1996thanin 1994at the SSAbut4 % lowerat the convectivemixed layer turbulence.The averagedvaluesof

3.

Results and Discussion

27,760

OGUNJEMIYO

ET AL. : AIRBORNE Ts -Ta

a

FLUX MAPPING

IN 1994 AND 1996

c

1994

GI

1994

'• :']:::::*.':•::•i::•ii:::;½.•:.,-?.•ii•i::•i';",.':" ;;•.•```•.``.•!:.*•:i:•:!{•;!•i.?;•:!:•i:•::!•:;.`:•!•iiii•i:.i:•iii•i•ii•. .'.,.•:' •:,+.•:i•::i:•.•::;•,,-.•:::½*.:,i:;•;:•., ..... -.;:.•.:*.iii.::! ,.":;::**::•'ii•i•iS**i':':*'*'"'.**'"" ........ .:.:::.:,,:., iii,:!*: ....

•-0.94

-0.03 •

•"• "?' *:':0.88 - 1.78

0.04 -0.87

'•.......... 1.79 - 2.88

-'---• 2.89 - 3.59 •

3.60 - 4.50 Ts -Ta

• 2.00 - 2.20 : -'-'........2.41-2.60

"• ........... 2.61 - 2.80

"::'':.'-'"':'.';• 2.81 - 3.00

'

•

2.21 - 2.40 •3.01 - 3.20

1996

GI

1996

'*:*•i'""•'•*"•"' '••••di "•'•'"'•'" ' ::: ''"*;-'•!i':'"" '•:•..g '"'"•'"' '••*•... '-••;:•!i'":'*'x'"'" .................... '•:''" ' ' "":.•7• "P;;:::•-i•*i;* .....

?•""'""'*"•-.•.'•.•.*a.•xS;."•.i*'".-aa•]•: -':'" '"'-s.':•.'-'....'.•Z;;,.:..-.,-.•ssiia•ii ...... '*•"ii:*'*'•;*c":: ....... :":•;'"' •

1.60 -2.40

•

•

3.07 - 3.70

-•-'•3.71 '",-'-"-'----*.. - 4.00

•

•

•'-'• ]':'":"-""-'":"""•*'""*:'•:":" 2.47 - 2.58

...........•--:-a 4.01 - 4.96

2.41- 3.06

4.97 - 5.64

•2.00

- 2.11

2.24 - 2.35

•

'"'• '•'*'"'

2.12 - 2.23

2.36 2.46

2.59 -2.70

Figure2. (a-d)Mapsof surface minus airtemperature Ts-Ta(øC)andgreenness index(GI)atthenorthern studyarea(NSA) grid in 1994and 1996.

w,, t,, 0,, andq, (Table3) aretypicalfor a deepmixedlayer with vigoroussurfaceheating[Stull, 1988; Bettsand Ball, 1994].The averagegrowthof theboundarylayerduringthe flight was greaterin eachyear at the NSA than at the SSA, just asthe averagevaluesof the mixedlayerheightz• were higherat the NSA (1056 m in 1994and 1082m in 1996)than at the SSA (902 m in 1994 and 976 m in 1996). The structure andgrowthof theboundarylayer is knownto be relatedto H andLE; a changein oneof the variablesmay be explained partly by a changein the other. However, conclusiveinferencescouldnot be drawnfrom the linear regression of z• againstH and LE in our data set. A differencein atmospheric stabilitybetweenflight days couldalso causevariationin the grid averagefluxes. As a majorcomponent of the flux footprintfunction,the stability parameter-z/L definesthe zone within which surfacecover significantlycontributesto the measuredfluxes. Unlike in 1994,the averagedvalueof L wasthe samefor bothsitesin 1996, with a wider rangeof valuesat the SSA than at the NSA (Table2). This, as well as the fact that the footprint analysisof Kaharabataet al. [1997] showedthatthe various components of thetwogridsurfaces weresampledadequately

evenat a more constrained footprintat L = -100 m, makes

stability related variations inthefootprint anunlikelycontributor to the flux variability. 3.2. Maps of Surface Characteristics

The mapsof excessof surfacetemperature over air temperature (AT)andgreenness (GI) areshown in Figures 2 and 3. Sincea calibration problemmighthaveaffected absolute values of AT andGI andsincetheemphasis is on relative spatial variability, different scaling wasusedforthese maps.Thespatial distribution of AT reflects thepatternof absorption andpartitioning of energybetween thedifferent components of the surfacecover, and the distributionof GI

documents theirphysiological status. The NSA AT maps(Figures2a and 2b) exhibitthe contrastin surfacetemperature betweenthenorthernandthe southernhalves of the site. The areas with low AT were

dominated bytallandclosed-canopy conifers, whiletheareas withhighAT wereprimarilycovered by shrubs, shortand opencanopytrees,withan oftendry underlying surfacethat

is typicalof a regenerating burnarea.Thecorrespondence

OGUNJEMIYOET AL. ßAIRBORNEFLUX MAPPING IN 1994AND 1996 a

....... •.........

Ts -Ta 1994

:..'.,

....:..• •.:::•,•?/..•:;•:•. ......

c

GI

27,761

1994

.:.................... ............. ......... ................ ............ i

::":•¾"'"•••:•• '••'"" ..2.4•::::i::i•?" "•' '•:•:.-'"-:i•' ' ?'::'%::':?':" ::•;•i:;:::".•.::• .... :-•:i ??:?"•'"'"'"'"'"'•'••••••• •.•,.,•.....:•.. :.: ..

•::•4

•-• ,*.

,• :•

ß, . .....

•

-2.06 - -0.61

Ts -Ta ......

1996

...,,• • .•:•,'.::•. -.•• •'•-:•:•i•:•"•::

•. :::•. '•..

..

,..... .....

....•.•. -,,,, .•?;;;•:•:.•-% .:;.;'

1.99- 2.26

•

••

2.56 - 2.63

!'"-•:•;J 2.64 - 3.10

•3.71 - 5.15

b

• . •;:•:;.:, • ........... .... •:•,

•

•'e•, ,•;..';•o:• 0.83 - 2.26 •

..:'::'•= •' '

......... •'•:-:.::•

I:• .........:'.':':-;'• 3.11 - 3.38

•

d

:

2..27 - 2.55 •3.39 - 3.66

GI 1996

...• ß :... '.Z;' ': ........ .•.•": ""'L•; •.:.:' ........... ß .............. '"'"':"'•'":'"'"':"' :'::•':::' ........... ...-•..:'" :;•:•:':':""

;:::•;...:: ..... ,'•...'-'

•

•+• '• '•...::;•.:..:..:..:•½:..:.:.. .......

.....

ß :;.i ;:.'. :...

:...

1.99- 3.34

•

3.35- 4.69

4.70 - 6.04

•;•.••.•

6.05 - 7.39

7.40 - 8.74

•

•

1.99-2.20 •

2..21- 2.41

•

2.42 - 2.62

2.63 - 2.83

•>•-•]

::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 2.84 - 3.04 • •3.05 - 3.25

• 8.75 - 10.1

Figure3. (a-d)Mapsof T,-TaandGI atthesouthern study areaSSAgridin 1994and1996.

area between1994and1996patterns is very strong,withcoeffi- ViningandBlad,1992],withlowAT overtheforested bythefactthatoverpartially covered canopies the cientof similarityC, of 0.94 andcorrelation coefficient r of explained measured bytheverticalradiometer isa combination 0.84, in spiteof somelocaldifferences suchastherelatively radiance fromthetop of thecanopyandtheunderlying high values of AT over black spruce (around of emissions 55.9øN/98.45øW)and over old jack pine (top left-hand surface[Sunand Mahrt, 1995], whichwas coolerat the corner)in 1996. To what degreesuchdifferencesmay be forested area than at the disturbed areas. There is some for theexpected inverserelationship between AT associated withflightorientation (predominantly N-S in 1994 evidence thesharpdistinction between theforested and and E-W in 1996), leadingto potentialdifferencesin the andGI, though areasis absent fromtheGI maps. sampling of anisotropic surfacepatches, is notyet clear.A theregenerating of GI patterns between thetwo dependence of the spatialextentof mappedtemperature Thereishigherpersistence attheSSAsite( Figures 3cand3d;C,andr, 0.94and anomalies on theorientationof flightlinesis suggested by the years thanattheNSAsite(Figures 2cand2d); comparison of temperature mapsfromN-SandE-W linesin 0.90,respectively) C,andr, 0.84and0.64,respectively), partlyassociated with the 1994data[Ogunjemiyo et al., 1997]. in thenorthern partof theburnarea. At the SSA (Figures3a and 3b) the NW-SE diagonal therapidregeneration separates thesiteintoa warmareaabovetheline anda cool areabelowit. The high AT aroundthe middleof the siteto 3.3. Flux Maps the NE edge corresponds to the loggedand regenerating 3.3.1. Spatialdistributionof fluxes.The NSA andSSA areas,whilelow AT valuesareassociated withpredominantly mapsof C, H andLE are shownin Figures4a-4f and5a-5f, wet conifersinterspersedwith fen. The similarity and respectively.In the NSA grid, both 1996 and 1994maps low correlationcoefficients between1994and1996mapsare0.92 showhighvaluesof H overtheforestandcomparatively formerburnareas.Estimated Cs and 0.90, respectively.The viewing angleproblemswith valuesovertheregenerating respectto observedAT havebeenaddressed before[e.g., andr betweenthe 1994andthe 1996mapsof H are 0.84 and

27,762

OGUNJEMIYO ET AL. ßAIRBORNE FLUX MAPPING IN 1994AND 1996 a

c

e

1994

l
0.45

H

• ......

'•'•• 108 - 123 r'"'"'" •••'•:• 124 - 139 ' ........... ,---':'-• 140 - 155 , , •>155

LE •2Ol

174 - 200

226

'•...... 227 - 253 r'-'""-l> 253

f

1996

Figure 5. (a-f) Maps of C, H, and LE at the SSA in 1994 and 1996.

betweenthe ratiosandV r showsan exponentwo factorsby comparing thecoefficiem of variationof the fit relationship incoming radiation Vr against thecoefficients of variation of tial decaywith increasingVr. The influenceof radiationon greaterin 1996thanin the fluxesV/(for LE), Vh (for H), and Vc (for C). The thevariabilityof thefluxesappeared coefficients for the fluxes showed noticeable differences

1994, which is also corroboratedby statisticaltests (see

between gridflightsandlittleor nodifference in theaverage below). From the valuesof theseratios(mostof whichare was more valuesbetweenthe two years. At the NSA site, average _>5)it is apparentthat surfaceinhomogeneity thanradiation in inducing variabilityin thesurface values of V/,Vh,andVcare0.31,0.27,and0.30for 1994and significant 0.30, 0.27, and 0.32 for 1996. In the same order, the fluxes. 3.3.3. Direct mesoscalefluxes. Vertical tramport of coefficiemsare 0.26, 0.27, 0.36 for 1994and0.37, 0.28, and 0.36 for 1996at the SSA site.The reducedvalueof V/at the scalarsat scalesgreaterthan turbulenteddiesmay be an component of thetotalsurfaceflux. Tramportby SSAin 1994is mostlikely associated with extensive precipi- important motionis generallysmallat low observation levels tation acrossthe site at the start of IFC-2, which would have mesoscale reduceddifferencein evaporation dueto variationsin surface but canbe importantat the aircraftlevelof 30 m [Sunand types(seediscussion of Bowenratio).Thismeansthatexcept Mahrt, 1994].Furthermore,thepartitionbetweencoolforests areasat thegrid sitesandtheassociated for LE at theSSAsite,thebiophysical differencesat thesites andwarmdisturbed variations in surface radiationtemperature (10ø-15øC)andair betweenthe two years were insufficiemto significantly

(2.0ø-3.0øC) aresimilarto thoseresponsible for modifythesourceandsinkconfigurations of thescalars.By temperature motionsreportedby $egalet al. [1989],Mahrt contrast,incomingradiationwassomewhat moreuniformly themesoscale distributedin 1996 than in 1994, with averagevaluesof Vr et al. [1994], andDoran et al., [1995]. We examined theinfluenceof surface-induced mesoscale 0.05 in 1994 and 0.10 in 1996 at the NSA and 0.04 in 1994 thegridby estimating mesoscale fluxesfrom and 0.12 in 1996 at the SSA. The valuesof the ratiosV//V r, fluxesacross averages andcomposited overrepeated runsof a Vh/Vr, andVc/Vragainst Vr areplottedin Figure6. Thebest segment

27,764

OGUNJEMIYOET AL. ßAIRBORNEFLUX MAPPINGIN 1994AND 1996

SSA 94 H SSA 96 H NSA 94 H NSA 96 H

---

20 ",I

¸

I

•

15

lO

¸ 0.05

0.1

0.15

0.2

0.25

0.3

Vr

35

SSA 94CSSA 96CNSA 94CNS•6 C

30

25

20

> 15 10

0.05

0.1

0.15

0.2

0.25

0.3

Vr

SSA 94LE SSA 96LE NSA 94LE NSA•6 LE

20 '• 15

10

0.05

0.1

0.15

0.2

0.25

0.3

Vr

Figure6. Plotsof theratioof V/Vr against Vr.

that flightline. Theseline estimates,symbolizedhereas Fm•,are thermalproperties,especiallyat the NSA grid, suggest is not largeenough influencedby transienteventssuchaschangesin wind speed the spatialscaleof surfaceheterogeneity andcloudcoverandshowedsignificantvariationacrossflight to enhance mesoscale motion. 3.3.4 Stabilityof flux map estimates. The flux maps lines. Highestvaluesof Fm•were associated with moisture transfer and could be _• 20% of the turbulent fluxes. The presented aboveare from griddeddatacomposited overfour mesoscale flux averaged over the grid Fmg,is primarily or more flightsundersimilar weatherconditions,with each relatedto surfaceheterogeneity,althoughit may still include grid line sampledtwice per flight. We have examinedthe someinfluenceof nonstationarity. As expected,theobserved reliabilityof themaps,in thefaceof boundarylayervariabilvalues of Fmg weremuchsmaller thanthose of Fm• ' andforall ity, by analyzingthemagnitudeof variationin flux estimates thegridflights andfluxtypes,Fmg didnotexceed 10%of the obtainedfrom six or morerepeatedpassesof theTwin Otter turbulentflux andwasfar lesswhencomposited overan IFC. under uniform radiation conditions, over the OBS site at the The low observedvalues of the mesoscalefluxes at the site, NSA. Flux averagesover adjoining,nonoverlapping 2km producedfive data pointsfor eachof the 10 km evenin thepresenceof a well- developedgradientin surface segments

OGUNJEMIYO

ET AL. : AIRBORNE FLUX MAPPING IN 1994 AND 1996

C

LE

•

178

156

• 4.2 x

underdevelopment) mayhelpto elucidate theparameters that definesuchrelationships remainsto be seen. Acknowledgments. The financialandtechnical supportfor

thesestudies fromtheCanadian NaturalScience andEngineeringCouncil,Agricultureand Agri-FoodCanada,the

NationalResearchCounciland from the Atmospheric Service of Canada isgratefully acknowledged. 105Jdeg-1(forwater,quartz,andorganic matter),thiswould Environment account for approximately 20 W m-2,suggesting thatsuch

References effectsmightpartiallyaccountfor theobserveddiscrepancies. However, consideringthatthereis little evidencethat any of Avissar, R., andR. A. Pielke,Theimpartofplantstomatal control onmesoscale atmosphericcirculation. Agtqc. For. Meteorol. , 54, thefactorsmentionedaboveweremoresignificantin oneyear 353-372, 1991. thanthe other,we assumethaterrorsin the energybalance Baldocchi, D. D., C. A. Vogel.,andB. Hill, Seasonal variation will have little effect on our analysisof relative spatial of carbondioxideexchange rates aboveandbelowa boreal distributionsbetweenthe two years. jackpineforest,Agric.For. Meteorol.,83, 147-170,1997.

Barr,A.G., andA. K. Betts,Preliminary summary of BOREAS 4.

Conclusion

upper-air soundings. Tech Rep., 12, Atmos.Environ.Serv, Saskatoon,1994.

Spatialandtemporalvariationsin the surfacefluxesof heat Barr,A.G., andA. K. Betts,Radiosonde boundary layerbudgets abovea borealforest, J. Geophys. Res.,102, 29,205-29,213, (H), moisture(LE), andCO2(C) were examinedby compar1997. ing the 1994 and 1996 airbornedata obtainedat the two 16 Betts,A. K., andJ. H. Ball, Budgetanalysis of FIFE sondes, J. km x 16 km BOREAS grid sites.A sensitivitytest showed Geophys.Res., 99, 3655-3666, 1994. thatthedominantfeaturesof thecomposite flux mapsusedin Desjardins, R. L., P. H, Schuepp, J. I. MacPherson, andD. J. Buckley,Spatialandtemporalvariations of thefluxesof carbon our studyare relativelyinsensitive to boundarylayervariabildioxide andsensible heatovertheFIFEsite,J. Geophys. Res., ity. A comparisonof the coefficientof variationof the fluxes 97, 18,467-18,475, 1992. with those of available energy showedthat variability in Desjardins R.L., et al., Scalingupfluxmeasurements of theboreal surfacefluxesat the sitewas associated more with heterogeforestusingaircraft-tower combinations, J. Geophys. Res,102, 29,125-29,135, 1997. neity of the surfacecover than with variationsin available R. L. radiantenergy.Analysisof the mapsindicatedsomenotice- Dobosy,R. J., T. L. Crawford,J. I. MacPherson, Desjardins, R.D. Kelly, S. P. Oncley,andD. H. Lenschow, able changesin the biophysicalpropertiesof the surface Intercomparison amongfourflux aircraftat BOREASin 1994, covers over the 2 year period, but the changeswere not J. Geophys.Res, 102, 29,101-29,111, 1997. enoughto significantly modifythesourceandsinkconfigura- Doran,J. C., W.J.Shaw,andJ.M. Hube,Boundary layercharactionsof the scalarsthatwere examined.The mapsof surface teristics overareasof inhomogeneous surface fluxes,J. Appl. Meteorol., 34, 559-571, 1995. characteristics showedgreatersimilarityin spatialpatterns Hollinger, D.Y., F. M. Kelliher,J. N. Byers,J. E. Hunt,T. M. between1994 and 1996thanthe flux maps.SpatialdistribuMcSeveny, andP. L. Weir,Carbon dioxide exchange between an tions of C showed the highestvariation between the two undisturbed old-growth temperate forestandthe atmosphere, years, especiallyin the NSA, where a southwardshift in Ecology,75 134-150, 1994. greenness matcheda similarshiftin CO2 absorption. Kaharabata S. K., P. H. Schuepp, S. O. Ogunjemiyo, S. Shen,M. Y. Leclerc, R. L. Desjardins, andJ. I. MacPherson, Footprint Correlationbetween greennessindex, GI and C was considerations in BOREAS, J. Geophys. Res.,102, 29, 113significantin bothyears(r valuestypicallybetween0.6 and 29,125, 1997. 0.8) andhigherat the SSA thanNSA. The significantvalues Kelly,R. D., E. A. Smith,andJ. I. MacPherson, A comparison of of H in spiteof a low surfacetemperatureexcessover the surfacesensibleand latent heat fluxes from aircraft and surface forest, and the resultingdecouplingbetweenAT and H (r measurements in FIFE 1987, J. Geophys. Res., 97, 18,44518,453, 1992. values< 0.2, negativeat the NSA andpositiveat the SSA), turbulence withinand was alsoobservedin bothyears.The high Bowenratio over Lee, X. andT. A. Black, Atmospheric abovea douglas-fir stand,II, Eddyfluxesof sensible heatand the forestattributedto physiologicalcontrolof the stomatal watervapour. BoundaryLayer Meteorol., 64, 369-389,1993. openingof plants should favor the developmentof deep Lenschow, D. H., andB. B. Stankov,Lengthscales in theconvectiveatmospheric boundary layer,J. Atmos.Sci.,43, 1198-1209, boundarylayers over the forests. However, we could not 1986. establish,onthebasisof ourobservations, a clearrelationship MacPherson,J.I., Wind and flux calculationson the NAE Twin betweenboundarylayer heightsand flux characteristics. Otter,Rep.LTR-FR-109,Natl. Res.Counc.,Ottawa,1990. Our studyunderlinesthe importanceof the role of the MacPherson, J.I., NRCTwinOtterOperations inBOREAS, Rep. surfacemosaicin forest-atmosphere exchange.It ishopedthat LTR-FR-129,Natl. Res. Counc.,Ottawa, 1996. andR. L. Desjardins, Observations the observations reportedhere will providecluesand refer- Mahrt,L., J. I. MacPherson, of fluxesoverheterogeneous surfaces, Boundary Layer encepointsfor the developmentand validationof surfaceMeteorol., 67, 345-367, 1994. vegetation-atmosphere modelsoverthe BOREAS landscape, Mahrt,L. Flux Sampling errorfromaircraftandtowers.J. Atmos. includingthosebasedon remotesensingobservations. They Oceanic.Technol.15, 416-429, 1998. should convey to the potential user our estimatesof the Mitic, C. M., P. H. Schuepp,R. L. Desjardins, and J. I. MacPherson,Spatialdistribution and c oo c cu r r e n c e o f confidencelimits applicableto the givenmapswhenusedas surface-atmosphere energyandgasexchange processes overthe "test patterns"for high-resolutionmodeling, and caution againstoversimplification in the assumption of relationships codegrid site,Atmos.Environ.,29(21), 3169-3180,1995. Ogunjemiyo, O. S, P. H. Schuepp, J. I. MacPherson, andR.L. between radiometric surface features and energy or gas Desjardins.Analysis of flux mapsversus.surface characterisexchange,whichappearto be site-andtime-specific.To what ticsfromTwinOttergridflightsin BOREAS1994,J. Geophys. Res., 102, 29,135-29,147, 1997. degreetheuseof morerefinedsurfacedescriptions (currently

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R.L Desjardins, Research Branch, Agriculture andAgri-Food

Canada,Ottawa,Ontario,Canada

I. J. MacPherson, Institute for Aerospace Research, National

Research Councilof Canada,Ottawa,Ontario,Canada

S. O. Ogunjemiyo andP. H. Schuepp, Department of Natural Resource Sciences, McGillUniversity (Macdonald Campus), 21,111

Lakeshore Road,Ste-Anne-De-Bellevue, Quebec, H9X 3V9Canada (segun@nrs. mcgill. ca; pschuepp@nrs. mcgill. ca) (Received August 2, 1998;revised December 7, 1998; accepted January21, 1999.)