Surface fluxes of sensible and latent heat over a tall-grass prairie in central Kansas, as measured by 22 surface stations during FIFE 1987, are compared with values gained indirectly by linear extrapolation of aircraft-measured flux profiles to the surface. The results of 33 such comparisons covering the period 26 June to 13 October 1987 indicate that the sensible heat flux profiles were generally more linear with less scatter in the measurements at each level than were the latent heat flux profiles, the profile extrapolations of sensible heat flux in general underestimate the surface averages by about 30 percent, with slightly better agreement during periods of small flux, and the profile extrapolations of latent heat flux in general underestimate the surface averages by about 15 percent, with overestimates during periods of small fluxes (dry conditions) and overestimates during periods of large fluxes (moist conditions). Possible origins of the differences between the two sets of measurements are discussed, as directions for further research

Kelly, Robert D.

Macpherson, J. Ian

Smith, Eric A.

English

NASA Technical Reports Server

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A Comparison of Surface Sensible and Latent Heat Fluxes
from Aircraft and Surface Measurements in FIFE 1987
Robert D. Kelly
Department of Atmospheric Science
University of Wyoming
P.O. Box 3038
Laramie, WY 82071-3038
Eric A. Smith
Department of Meteorology
Florida State University
Tallahassee, FL 32306
J. Ian MacPherson
National Research Council
Building U61, Montreal Road
Ottowa, Ontario
Canada K1A OR6
(NASA-CR-137365) A COMPARISON OF SURFACE
SENSIBLE AN0 LATENT HEAT FLUXES FROM
AIRCRAFT AND SURFACE MEASUREMENTS IN FIFE
1987 (Wyoming Univ.) 19 p CSCL 08B
G3/_3
N91-12136
Unclas
0311007
https://ntrs.nasa.gov/search.jsp?R=19910002823 2020-03-19T20:08:11+00:00Z
Abstract
Surface fluxes of sensible and latent heat over a tall-grass prairie in central Kansas, as mea-
sured by 22 surface stations during FIFE 1987, are compared with values gained indirectly by linear
extrapolation of aircraft-measured flux profiles to the surface. The results of 33 such comparisons
covering the period 26 June to 13 October 1987 indicate that l) the sensible heat flux profiles were
generally more linear with less scatter in the measurements at each level than were the latent heat
flux profiles, 2) the profile extrapolations of sensible heat flux in general underestimate the surface
averages by about 30%, with slightly better agreement during periods of small flux, and 3) the profile
extrapolations of latent heat flux in general underestimate the surface averages by about 15%, with
overestimates during periods of small fluxes (dry conditions) and overestimates during periods of
large fluxes (moist conditions). Possible origins of the differences between the two sets of measure-
ments are discussed, as directions for further research.
1. Introduction
During theFirst ISLSCPField Experiment(FIFE),whichconcentratedsurface,airborne,
andsatellitemeasurementsovera 15x15km areaof tall-grassprairie in centralKansas(Sellerset
al., 1988), an important objective was to compare fluxes of sensible and latent heats as measured
at the surface and in the boundary layer (BL). To that end, surface and airborne flux data were col-
lated during four Intensive Field Campaigns (IFCs) in 1987. The IFCs were spaced over the grow-
ing season in order to cover a complete cycle of vegetative change from "first greenness" in the
spring to senescence in the fall, and covered the periods 26 May-6 June, 25 June-11 July, 6-21 Aug.,
and 5-16 Oct.
To gather the flux measurements, 22 surface stations were distributed across the FIFE site,
with 6 using the eddy correlation method with instrumentation at 2 m agl, and 16 using the Bowen
ratio method, again with instrumentation at 2 m agl. At the same time, several twin-engine turbo-
prop aircraft were used for eddy correlation flux measurements in the BL.
The purpose of this paper is a comparison of fluxes measured at the surface, presented as
site-wide and time averages of the data collected at the surface stations, with values extrapolated
to the surface from BL flux profiles determined by the aircraft. These extrapolations were done by
simple linear regression, based on aircraft fluxes measured over repeated, constant-altitude flights
at 2-4 levels in the BL. In all, 33 profiles were analyzed for the period 26 June-13 Oct., 1987, which
encompas se s IFCs 2-4, using data collected by the Canadian National Research Council Twin Otter
(TW) and the University of Wyoming King Air (KA) research aircraft. The surface data correspond-
ing to the times of the aircraft profiles were taken from the continuous set of surface data, and repre-
sent measurements by Florida State University, the University of Nebraska, xxx, xxx, xxx.
Three of the days for which comparisons are presented in this paper have previously been
examined with different methods by Betts et aI. (1990, hereafter referred to as B90). For these cases
(20 Aug., and 8 and 13 Oct.) the aircraft data were collected in a pattern allowing full analysis of
the BL budgets for sensible and latent heats. In each case, linear extrapolation of the eddy correlation
flux profilesto thesurfacegaveunderestimatesof thesurfacefluxes,asrepresentedby site-wide
averagesof thesurfacedata.Surfacesensibleheatfluxesweregenerallyunderestimatedby about
40%,andsurfacelatentheatfluxesbyabout30%. As discussedbyB90, theerror in theextrapola-
tionsmaybeattributedtoseveralsources,including i) underestimationof thelarge-eddy,or long-
wavelength,contributionto theBL fluxes,dueto therelativelyshort(15km) flight pathsusedin
FIFE, and2) underestimationof thelong-wavelengthcontributiondueto the5-km cutoff, high-
passfilter appliedto theaircraftdatabeforecalculationof thefluxes. Specialflightswereincluded
in the 1989FIFEoperationstoexplorebothof thesepossibilities.In addition,sincethepublication
of thatpaper,thesurfacesensibleheatflux measurementsfromthe 16Bowenratiositeshavebeen
reviseddownwardby about10-15%,afterit wasfoundthatmanyof thenetradiationflux instru-
mentsusedatthosesitesin 1987hadoverestimatedthelong-wavelengthradiativefluxes.
Thus,thecomparisonspresentedherediffer fromthoseof B90in thattherevisedsurfaceflux
datawereused,andall the available aircraft profiles from 1987 have been included. Data from the
cases analyzed by B90 were treated in the same way as the other profiles, i.e., surface fluxes were
predicted by linear regression.
2. Data collection and processing
The following two sections describe the collection and processing techniques used for the
two sets of data compared in section 3. Description of the instruments themselves, both for the sur-
face sites and for the two aircraft, may be found in the following publications: Betts et al. (1990),
XXX.
2.1. Surface flux measurements
At each of the 22 surface stations, which were distributed over the 15x15 km FIFE area,
fluxes were recorded every 30 minutes, at 15 and 45 minutes past each hour, as averages of the pre-
vious xx minutes of flux measurement. (See xxxxx (19xx) for a description of the method of site
selection and a map of the exact station locations.) For the time periods used in this study, which
correspond to the times one or both of the aircraft were over the site measuring the BL flux profiles,
4
anaverageof 20 stations successfully reported surface flux data (range 15-22). To obtain surface
flux values for comparison with the aircraft extrapolations, simple arithmetic means were calculated
from all the reported fluxes at each reporting time during which an aircraft was present. These area-
averages were in turn averaged over the number of reporting times during which an aircraft was pres-
ent (2-5 times per profile). Thus, the surface values given below are labeled time-area averages.
In addition, the maximum and minimum area-averages while an aircraft was present were deter-
mined for each case.
2.2. Aircraft flux measurements
As described by B90, data used in the eddy correlation measurements by the Twin Otter were
sampled at 16 Hz, then low-pass filtered at 5 Hz, and high-pass filtered at 0.012 Hz. This preserved
data for spatial scales from about 10 m to 5 km for an aircraft speed of 50 m s-1. Correspondingly,
the King Air data were sampled at 10 Hz, then low-pass filtered at 2 Hz, and high-pass filtered at
0.017 Hz, preserving data for spatial scales from about 40 m to 5 km for an aircraft speed of 85 m
s-:. The 5-km high-pass fihers (0.012 Hz for TW and 0.017 Hz for KA) were used in FIFE in an
effort to avoid problems associated with undersampling wavelengths longer than 5 km when using
only a 15 km flight path.
As mentioned above, profiles of the eddy correlation fluxes were studied for cases in which
repeated constant-altitude flight segments had been flown at 2-4 different levels in the BL. As FIFE
progressed in 1987, the pattern in which these segments were flown was changed. This resulted in
three general patterns for obtaining profiles: 1) a single "stack" of level segments, with 2-6 passes
at each level and all at the same azimuthal orientation, 2) a stack of "L" shaped, constant altitude
passes, with 2-4 passes at each of 2-4 levels, and 3) a double stack of level segments at constant
orientation, with one stack along the north edge and the other along the south edge of the site, giving
4-6 passes at each of 2-4 levels. In addition, and also as a progression of experimental design in
1987, some of the stacks were flown in a sequence allowing removal of linear time trends in the data
(B90); these are referred to here as "time-centered" cases. In the analysis presented here all profiles
wereprocessedin thesamemanner,regardlessof whetheror not thedatahadbeencollectedin a
time-centeredpattern.
Figs. 1 and2 showexamplesof aircraftprofilesfor sensibleheatflux (FH)andlatentheat
flux (FE)for twocases,onefrom aperiodof abundantsoilmoistureandactiveplantgrowth(6July),
andthe otherfrom aperiodof dry soil andvegetativesenescence(8 Oct.). Thesurfacedataare
shownin thefiguresasopenboxes.Thedot in'eachbox is thetime-areaaverageflux, andtheleft
andright limits of eachboxaretheminimumandmaximumareaaveragesfor thatcase(seeabove).
Theregressionlinesfor boththesensibleandlatentheatfluxesarealsoshown.Thesolidboxesin
thefigure showtherangesof predictedsurfacevaluesfromF0--_toF0+(Yfor eachregression,where
F0is thepredictedsurfaceflux. Theerror,or standarddeviation,of theflux estimateatthesurface
wascalculatedas
so(F0 Tnn \ ;2
In this expression (Morris and Rolph 1981) h0 is the surface pressure altitude, h- is the average pres-
sure altitude for the n data points in the regression, Sh is the standard deviation of h, and s is the stan-
dard error of the regression.
One of the most striking features of the two cases is the reversal of profiles: Latent heat
fluxes were almost three times the sensible heat fluxes on 6 July, and about half the sensible heat
fluxes on 8 Oct., reflecting changes in soil moisture and in the level of plant activity. Also note that
the predicted surface sensible heat flux, FH0, is smaller than even the range of surface averages for
8 Oct. A similar under-prediction is seen for the 6 July surface flux of latent heat. The scatter of
data points at each level on the profiles is typical of the FIFE BL data, and reflects, at least in part,
the time changes that occurred during a period of BL sampling (about 1.5 hr on 6 July and about 2.25
hr on 8 Oct.). A more quantitative indication of the amount of scatter and the degree of linearity
6
for eachprofilesis thecorrelationcoefficient,r, of the linearregression(seeTables1and2). On
6 July r was0.71and0.27(for thesensibleandlatentheatfluxes,respectively),while on8 OcL it
was0.95and0.94.
3. Results
Results of the surface averaging and prof'de regression are summarized in Tables 1 and 2.
Entries in these tables for the BL profiles include the number of passes and flight levels used in con-
structing the prof'de, whether or not the flight pattern was time--centered, the correlation coefficient
of the linear regression, and the F0 and F0 4- t_ values for each prediction of surface fluxes. Entries
for the surface measurements include the minimum, average, and maximum for the site-wide aver-
age fluxes measured during each aircraft profile, the number of sites reporting, and the number of
reporting times for each case. The 14 Aug. values for latent heat fluxes are considered anomalous
and are not included in the analysis and conclusions presented below.
Several patterns are noted in these summaries. First, the correlation coefficients for the flux
profiles (r) are were different for the sensible and latent heat cases. For the sensible heat fluxes,
22/33 cases had r > 0.7 and 16/33 had r > 0.8. In contrast, the corresponding fractions for the latent
heat fluxes were 4/24 and 2/24, respectively. Thus the sensible heat flux profiles were generally
more linear with less scatter in the measurements at each level than were the latent heat flux profiles.
A second pattern visible in the r values concerns changes with season. In the sensible heat
data r was slightly higher for the October cases than for the June-August cases. For the latent heat
data there were not any changes in r from the beginning to the end of the analysis.
The third pattern in these data results from comparing the F0 and surface average values, i.e.,
by examining the ratio F0:(surface average) for each case. For the sensible heat fltJxes this ratio aver-
aged about 0.71, and did not change noticeably with IFC or season. Thus, the profile extrapolations
were nearly all underestimates of the surface averages by about 30%. In contrast, the same ratios
for the latent heat fluxes averaged 0.85 (mostly underestimates) in June-August, when the fluxes
rangedover230-443W m-2, butaveraged1.71(consistentoverestimates)in October,whenthe
fluxesrangedover30-70 W m-2.
Theprofileestimatesof surfacefluxesareplottedagainstthesurfaceaveragesinFigs.3and
4, showinggraphicallysomeof thepatternsnotedabove.In Fig. 3,for thesensibleheatfluxes,all
theentriesbut4aretotherightof the1:1line(dotted),indicatinganunderestimateof surfacevalues
by extrapolationof profilesto thesurface.Note,also,thattherewasapparentlylittle differencebe-
tweenthe two aircraft in thisregard. Theestimateswereslightly betterduring thesmallerfluxes
of IFC 2 thanduring thelargerfluxesof IFCs3 and4. In Fig. 4, for latentheatfluxes, theentries
aregenerallycloserto the 1:1line,with overestimatesin IFC 4 andunderestimatesin IFCs2 and
3.
4. Conclusions
As noted above, the sensible heat flux profiles were generally more linear with less scatter
in the measurements at each level than were the latent heat flux profiles. Possible explanations here
include the fact that latent heat fluxes (as derived from fluxes of water vapor) are generally more
difficult to measure from an aircraft than sensible heat fluxes and thus more scatter may be expected
in the data. This difference in the character of the profiles also raises the question of whether or not
there is a physical or biophysical explanation of why water vapor fluxes might have varied more
widely with time within a given time interval over the FIFE site than did sensible heat fluxes.
In all, the comparisons presented here point clearly to the differences between the aircraft
profile predictions and surface measurements of surface sensible heat flux, and show that these dif-
ferences were quite consistent throughout the FIFE operations of 1987. The recent downward revi-
sion of sensible heat flux values for most of the surface stations is reflected in the average 30% under-
estimate for the 33 cases given here, compared to the 40% underestimate described by B90.
However, the exact origins of the remaining differences are still unclear. As stated by B90, they may
be a result of undersampling of large-eddy contributions to the total flux and/or a result of the filter-
ing applied to the data. In addition, the simple arithmetic averages of the surface measurements may
bebiasedbydifferencesinvegetation,terrain,andsoilcharacteristicsateachsite.Toour knowledge
FIFE is one of the first extensive comparisons of aircraft estimates of these fluxes with surface mea-
surements. It may well be that the measurement techniques used on all the surface and airborne plat-
forms need to be reexamined.
Finally, although there is more scatter in the latent heat flux data forming the aircraft profiles
than in the sensible heat data, the estimates of surface latent heat fluxes using aircraft profiles were
in general underestimates of the surface averages by only about 15% for the 1987 FIFE cases 0390
found underestimates of about 30%). Imbedded within this result is the fact that small values of la-
tent heat flux were overestimated, while the larger values were generally underestimated. This
change in the nature of the prediction certainly points to a need to reexamine the platforms and the
analysis techniques used for both the surface and the airborne data.
5. Acknowledgements
R. D. Kelly was supported by NASA--GSFC under Contract NAG-5-913. He wishes to
thank E. Gasaway, G. Gordon, and K. Chartier for their efforts in the field, and S. Allen for drafting
the figures. The enthusiastic cooperation of all FIFE participants is gratefully acknowledged.
E. A. Smith...
J. I. MacPherson ...
6. References
Betts, A. K., R. L. Desjardins, J. I. MacPherson, and R. D. Kelly, 1990: Boundary-layer heat and
moisture budgets from FIFE. Bound.-Layer Meteor., 50, 109-137.
Morris, C. N., and J. E. Rolph, 1981: Introduction to Data Analysis and StatisticalInference. Pren-
tice-Hall, 389 pp.
Sellers, P. J., F. G. Hall, G. Asrar, D. E. Strebel, and R. E. Murphy, 1988: The First ISLSCP Field
Experiment (FIFE). Bull. Amer. Meteor. Soc., 69, 22-27.
Figure captions
Fig. 1. Aircraft and surface flux measurements, 6 July 1987, 1117-1253 LT.
Fig 4.
Aircraft data points
are pass-averages for sensible heat (circles) and latent heat (+) fluxes, with regression lines
extended to the surface. Solid boxes at surface show ranges of regressed flux estimates.
Open boxes near surface show mean (dot) and minimum-to-maximum range of surface
area-wide averages.
Same as Fig. 1, for 8 October 1987, 1314-1534 LT.
Scatter diagram of surface sensible heat fluxes derived from aircraft profiles (abscissa) vs.
average sensible heat fluxes measured at the surface (ordinate), including both Twin Otter
O'W) and King Air (KA) data from IFCs 2-4 in 1987. Symbols denoting aircraft and IFC
are defined in the diagram. Linear regressions are shown for the two aircraft data sets togeth-
er (TW, KA; solid line), and for each set separately (KA, dash-dot; TW, dashed). Dotted
line is 1:1 slope.
Same as Fig. 3, but including Twin Otter and surface data only.
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A comparison of surface sensible and latent heat fluxes from aircraft and surface measurements in FIFE 1987

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