NASA Technical Reports Server (NTRS) 20010090460: Case Studies of Water Vapor and Surface Liquid Water from AVIRIS Data Measured Over Denver, CO and Death Valley, CA

10 Pages·0.6 MB·English

by  NASA Technical Reports Server (NTRS)

CASE STUDIES OF WATER VAPOR AND SURFACE LIQUID WATER FROM AVIRIS
DATA MEASURED OVER DENVER, CO AND DEATH VALLEY, CA
1.2
B.-C. Gaol, K.S. Kierein-Young ,A. F. H.Occtz i.2,
E. R. Westwater3,B. B. Stankov3, and D. Bkkenheuer4
ICSES/CIRF_, Campus Box 449, University of Colorado. Boulder, CO
2Deparlment ofGeological Sciences, University ofColorado, Boulder, CO
3NOAA/ERL/Wave PropagationLaboratory.Boulder, CO
4Cooperative Institute forResearch inthe Atmosphere (CIRA), Ft. Collins, CO
Abstract. High spatial resolution column atmospheric water vaporamounts and equivalent liquid water
thicknesses of sttffaee targets arc retrieved from spectral data collected by the Airborne Visible/Infrared Imaging
Spectrometer (AVIRIS). The retrievalsaremade using anonlinearleast squ_es curve fitting technique. Two case
studiefsromAVIRIS (lalancquireodverDeaver-PlattevairlelacC.oloradoandoverDeathValleyC.alifornia are
IxesenteTdh.ecolumnwatervaporvaluesderivefdromAVIRISdataovertlmDenver-Plattovairlelaearccompared
withthoseobtainefdromradiosondegsr,oundlevelupward-lookimnigcrowaveradiometerasn,dgeostadonary
satellimteeastu_montsT.hecolumnwatervaporimageshowsspatiavlariatiopnattcmsrelatetdothepassageofa
weather frontsystem. Thecolumn watervapor amountsderived from AVIRIS data over Death Valley decrease with
increasing surfaceelevation. The derived liquidwater image clearly shows mfface drainage patterns.
I. Introduction
Water vaportsore of the most importantatmosphericgases. The integrated water vapor content from ground
to space has importantapplications tometeorology, hydrology, climate, andradio intefferometry. Inthis paper, the
integrated water vapor content isreferred toascolumn water vaporamountoras prceipitable water vapor (PWV).
The liquid water content of vegetation isrelated to the stressconditions of vegctaaon. Remote sensing of liquid
water content of vegetation has important applications to forestry andagriculture. The soil moisture content is
important toagriculture, forestry, hydrology, andengineering geology.
Satellite remote sensing of PWV with an acem'aey of approximately 10%over theoceansusing microwave
emission measurements isaproventechnique (Alishouse, 1983).However, the variability of land surface
microwave emissivities poses diffieultles indetermining PWV over land from satellite microwave data. Column
water vapor retrievals over land from satellite IR emission measurements using, for example, the split window
technique (Cheste,rs ctaI., 1983) arepossible during clear conditions.
Recently, _ore have beenseveral reported successes inremotesensing ofcolumn atmospheric water vapor
amountsfrom akeraft measurements ofsolar radiation reflected by the landsm'faec near1lira (One andGoeaz, 1990;
Conel ctal., 1989; Prouinet al., 1990). _ remote sensing techniques arereferred toas optical techniques. Gao
and Goeaz (1990) have also reported the derivation of moisture content ofve.getafion using liquid water absorption
featuresnear 1Idm.
Column water vapor amounts canbeoblained fromthe ground withupward-looking microwave radiometers
(Hogg et al., 1983). The Wave Propagation Laboratory (WPL) of the National Oceanic andAtmospheric
Administration (NOAA) operates alimited network of dual-channel microwave radiometers (Westwater andSnider,
1989). Radiometers arenow routinelyoperatedatStapletonInternational Airport in De.aver, Colorado andat
Plattevillc, Colorado (approximately 60 km north of Denver). Inorder toverify the optical techniques, an
experiment was designed. Specifically, the Airborne Visibledlnfrared Imaging Spectrometer (AVIRIS) (Vane, 1987)
was requested tomake measurements over the Denver-Plattcville area. Column water vapor amounts were tobe
retrieved from AVIRIS data andthen compared withcolumn water vapor amountsmeasured with the two microwave
radiometers menlioncd above. The AVIRIS made the measurements around 19:40 _ ')2March 1990. At this
time, the sky was clear. However, snow started later inthe day. The AVIRIS measurements happened to be
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=
conducted inthc middle of anotherexperimental project, theWinter Icing andStorm Project _ISI_O) (Stankov et
al., 1990)conductedbyNOAA ForecastSystems[_boratory(FSL) fordetermining theudlityofunattended
microwaveradiometersindetecting,andprovidinginputto forecastsofaircrafticing.WIsPg0 collecteddatafrom
microwaveradiometers,infraredradiometers,lid_rceilome_ers,radio-acousticsoundingsystems(RASS),and
radiosondesatseverallocationsinColorado.Also,meteorologicalparameterswerecollectedfrom approximately20
surfacestationsinColorado.Meanwhile, datafromtheVisible InfraredSpinScanRadiometer(VISSR)
Atmospheric Sounder(VAS) ontheNOAA GeostationaOl perationalEnvironmentalSatellite(GOES)were
collectedandarchivedbyNOAA FSUsProgramfor RegionalObservingandForescastlngServices(PROFS).The
WISPg0 dataprovidedinformationontic meteorologicaclonditionsaroundtheAVIRIS measurementsandallowed
betterinterpretationoftheAVIRIS andthemicrowaveradiometersdata.Inthispaper,thewatervapor
measurementsw_thAVIR1S, microwaveradiometers,andVAS arecompared.
AVIRIS measurements over Death Valley, Californiawere obtained on September 29, 1989 aspartof the
Geologic Remote Sensing Field Experiment (GRSFE). At the time of the overflight, some surface areaswere wet,
based on fieldobservations. Measurements of reflectance spectraof samples collected from the field clearly showed
liquid water absorption features near 0.98, 1.2, 1.6,and2.2 l.tm.Because of the presence of wet surface areas when
the AVIRIS measurements were made, wedecided tofurther test the ability of the algOrithm(Oao and Goetz, 1990)
for simultaneous retrievals of the atmospheric water vapor amountand the surface liquid water content from the
AVIRIS data. Theresults are described inthis paper.
II. Imaging Spectrometers: the Optical Technique
Imaging spectrometers acquireimages inhundreds.of contiguous spectral bands such that foreach picture
element (,pix¢l)acomplete reflectance or emittance spectrumcan bederived from the wavelength region covered
(Goclz et al., 1985). AVIRIS isnow an operational imaging _ometer. This instrument images the Earth's
surface in'224 spectral bands, each approximately 10nmwide, covering the region 0,4-2.5 I.tm,from an ER-2
aircraftat an altitude of 20 kin. The ground instantaneousfield of view is20 x20 m2.
A technique forsimultaneous retrievals of atmosphericwatervaporamountand surface liquid watercontent
has been deserihed byOao and Goetz (1990). In thistechnique, the quantitative retrieval ismade bycurve fitting
AVIRIS spectra with calculated spectra near I pm usingatmospheric and liquid water transmissionmodels, anda
nonlinear least squares fitting technique. Figure I shows an example of SlXX:tralcurve fitting for retrieving water
vapor amount only. During the fitting process, the water vaporamountandthe spectral background, a linear
function of wavelength, are allowed tovary.Thebest estimates of the water vapor amount and the spectral
background are obtained when the sum ofthe squared differences between the observed and the calculated spectra is
minimized. Examples of fitting AVIRIS spectra over wet areas for simultaneous retrievals of atmospheric water
vapor and surface liquid water content arepresentedlater inthispaper. Our technique isapplicable forretrievals from
AVIRIS data obtained onclear days with visibilities 20km orgreater, Because atmospheric scattering isnot
modeled directly, the technique isnot applicable for retrievals from AVIRIS data measured on h_y days. Under
these circumstances, the scattering effect must be modeled.
lIl. Results
Column watervaporamounts are retrieved from AVIRIS data measuredover the Denvet-Platteville area,and
bothcolumn water vaporamountsand surface liquid water thicknesses areretrieved from AVIRIS data measured
over Death Valley using the spec0ralcurve fiuing technique.
A. Denver-Platteville, Colorado
Figure 2ashows a0.86 lUn image of the Denver-Platteville area.TheAVIRIS radiances were averaged
spatially on a 2 x2 pixel basis when the image was produced,The spatial averaging was necessary because of the
limitation of ourImage processing hardware.The Denver Stapleton International Airport isat the lower left partof
the image; Platteville isatthe upper part,Highway 85,which connects Denver andPlatteville, isseen as acurved
line. The image covers a surface area of approximately 11x68km2.
Column water vaporamounts were retrieved bycurvefitting the 0.94-I.tm watervaporband. TheAVIRIS data
were averaged spatially ona 4x4 pixel basis todecrease the computer time. The retrieval took approximately 25
hours on a Microvax II1computer. Low vertical resolution temperature, pressure, and water vapor volume mixing
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ratio profiles, measured near the airport by asix-channel microwave radiometer, we_ used in the spectral
calculationdsuringthecurvefittinpgrocess.Figure2b showsanimageprocessedfromth_re.eyed column water
vaporvalues.To produceawatervaporimagohavingthcsamesizeastheimageinFig.2a.thetea-lovewdater
vaporvalucswere zoomed spatialloyn a:2x2 pixclbasis.The narrowverticablaron therightsideofFig.2b
givesthescaleofwatervaporvaluesfrom0.53cm (black)to0.76cm (white)a,verysmallrange.The column
watervaporvaluesintheentirescenehada mean of0.640cm andastandarddeviationof0.044cm. Horizontal
linesinFig.2bareductosmallerrorsinthcAVIRIS radiom_ic calibration..L_j_secralefealurcsarealsoobvious
in Fig. 2b. From the airport to Plattevillc, the image shows adark-bright-dark-bnght pattern.
A topographic map of the AVIRIS sceneisshown in Fig, 2c. Generally, the surface elevation decreases from
the airport area (-1630 m) to PlattcviUc (-1450 m). Small variations of surface clcvation in the east-west direction
are also present. Column watervapor amount usually decreases as the surface elevationincreases, because
atmospheric water vapor concentration decreases rapidly with altitude. Therefore, the column water vapor amount
from the airport to Platteville was expected to incrca._. The dark-bright-dark-bright pattern from bottom to top in
Fig. 2b indicates that real spatial variatiotl, not related to the topographic effects, of water vapor distributions
existed when the AVIRIS data were acquired. The observed spatial variation of column water vapor amount on the
order of 0.1 cm or less indicates'the high precision with which column water vapor amounts can be determined from
AVIRIS data.
Prccipitablc water vapor fields, atagrid spacing of approximately 10km, over the Rocky Mountain foothills
extending roughly 300 km in both the east-west and the north-south directions were obtained from VAS data using
a regression technique de.scribed by Birkcnheuer (1991). Bias existed between PWV values obtained from rite VAS
data alone and the "true"PWV values. Inorder to remove l,hcbias, PWV values obtained from VAS data alone were
raised or lowered by a constant based on PWV values measured with microwave radiometers at the airport and at
Platteville. The resulting bias ctxT_ted PWV field at 18:45 UTC 22 March Opproximatcly one hour before the
AVIRIS measurements) revealed agradient structure in the Dcnvcr-Plalt_ville area, which is similar to the gmdicnt
structure in Fig. 2b.
Figure 3shows time scfles of PWV at (a) Platteville, Co)Denver, and (c) Elbcrt on 22-23 March 1990. Elbcrt
is located approximately 60 km southeast of Dcnvcr. The continuous curves show 2,rain time series from
microwave radiometers. The mcaanglcs are from the Cross-chain Loran-C Atmospheric Sounding Sysw.xn (CLASS)
radiosondes. The squares arc from the Natio_lal Weather Service (NWS) radiosondes. The dark ckcles am from the
VAS adjusted images, and the open circles from the AVIRIS measurements. The PWV values from AVIRIS and.
from the microwave radiometer measurements agreed to within 0.1 cm, but the rcladve difference is greater than
10%.
Because tl_ PWV values from the microwave radiomcter at Eibc.rt were not used in the bias corrections
discussed above, comparing PWV values from the VAS data with bias corrections to those from the Elbcrt
microwave radiometer measurements is a blind test of the VAS PW'V analysis technique. Fig. 3c shows that the
VAS PWV analysis reported aconsistently higher level of PWV over Elbcrt. A possible explanation for the
discrepancy is presented below. Thc ground instantancous ficld or view of VAS data is approximately 10kin. This
large field of view would tend to blend the lower moisture level at the higher elevation terrain, where the microwave
radiometer was located, with the higher moisture levels atadjacent lower elevation terrains, cffcctivcly raising the
analyzed amount of PWV in a region like that of Elbcrt.
Fig. 3b shows that aPWV dlffercnce of 3mm existed bctwecn the radiometcr and the radiosonde data at2300
UTC. This differenc_ was duc to the NWS balloon moving into a region of dry air, and did not rcflcc[ the build-up
of moisture that was observed during this dmc by all three microwave rudiomctcrs. That a build-up was occurring
was also supportcd by the two spccial CLASS radiosondes released at Elbcrt at0000 and 0300 UTC 23 March. This
increase in moisture was followed by awcl|-documcnted event of supercooled liquid water that lasted for three days
(Stankov ct al., 1990). The conclusion of the NWS baIloon moving into adry air region was based on the analyses
(Gao ctal., 1991) of wind profiles measured at the airport and of meteorological data from approximately 20 surface
stations Ioeatcd indifferent parts of Colorado.
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• B. Death Valley, California
Death Valley islocatc<linthesoutheastern partof California, neartheNevada border.AVIRIS damover the
areawere collected on September 29, 1989during theGeologic Remote Sensing Field Experiment (GRSFE).
Figure4a shows anAVIRI$ image (0.68 tim) of the site. The valley flooris located between the Trail Canyon
alluvial fan(upper left comer) andthe hills of Artists Drive (right side). One road inthe upperpartof the image
traverses the valley floor. Another road located on the fight portionof the image isnearly parallel tothe valley
floor. There area number ofgeologic units within the AVIRI$ scene (HuntandMabey. 1966). The valley floor
consists of playa and salt units mixed with clay. Thebright white areas aremostly puresalts. Fig. 4b shows a
topographic map of thescene. The upperleft comer ofthe image has anelevation of approximately 60 m. The
highest portionof the Artists Drive hills has anelevation ofapproximately 240 m. The valley floor is very flat and
below sea level, with elevations between approximately -85 and-73 m.The central portion of the valley floor is
slightly lower in elevation thanthe western and the eastern portions of the valley floor. Also, the southern portion
of the valley floor is slightly lower thanthe northern portion of thevalley floor.-
Atthe time of the AVIRIS overflight, some surfaceareaswe.rewet. Reflectance spectraof samples collected
from the field were made with theGeophysical andEnvironmental Research (GER) portable specmmleter. Curve 1
of Figure 5 shows areflectance spectrum era field samplecollected over awet spot within the bright white areain
the upperpartof Fig. 4a. The liquid water features near 0.98, 1.2, 1.6,and 2,2 I.u'nare clearly seen. Curve 2 of Fig.
5shows a spectrum obtained byratiolng an AVIRIS spectrum (Spool) over the wet bright white area in theupper
partof Fig. 4a against anAVIRIS spectrum (Spec2) over adrier areajust left ofthe wet white area.The same set or
liquidwater features are also obvious. Curve 3shows aspectrum obtained byratioing the gpe..c2against an AVIRIS
spectrum (Spec3) over a higher hill area.Atmospheric water vapor features near 0.94 and 1.14 pm areseen. This
indicates that the lower elevation valley floor has more water vaporthanthe hill areas.
The spectral curve fitting techniquedescribed byGao andGoctz (1990) was used toderive slmultaneously thc
atmospheric column watervaporamount andthe equivalent liquidwater thickness of the surface from tic AViRIS
data. Inthistechnique, the atmosphericwater vapo¢transmiuanccs werecalculated with anarrow-bandmodel andthe
liquidwater transmittances were calculated usingthe liquidwater absorptioncoefficients compiled byPalmer and
Williams (1974). Fig. 6a shows the curve fitting ofSpecl by including only atmospheric wa(ex vaporabsorptions
inthe fining process. The fittedspectrum has largervalues ofreflectance thantheobserved spectrum near 1.2p.m.
The overall fitting between the two spectra ispoor.Fig. 6b shows the curve fitting by including both the water
vaporand the liquid water absorptions in the fittingprocess. Significant improvement inthe overall fit isachieved.
Figures 7aand 7bshow the column watervaporimage andthe liquid water image derived from the AVIRIS
da_a.The water vapor amounts over the hillsaresma/ler thanthose over the valley floor. This isdueto the
decreasing aLmospheric watervaporconcentration withaltitude.Theslighdy s_rnallcrwater vapor amountsnearthe
upper left comer ofFig. 7athan those over thenearby valley floor can alsobe attributed tothe topographic effect.
The liquid water image inFig. 7b shows clearly the drainagepatterns. Basedon the topographic map of Fig. 4b,
liquid water will flow from the hills onthe rightside and from theareanear the upper Icft comer into the valley
floor afterrain.Also, beamusethe southern portion of thevaIlcy floor has slightly lower elevations thanthe
northern portion of the valley floor, the liquid water will further flow from top tobottom. A roadtraversing the
vallcy in theupper partof Fig. 4a isslightly elevated and has acted as a dam, blocking the natural flow of water
from the upper valley tothe lower valley andcausing more white salt tobedeposited inareasnorth of the road.The
long narrow drainage pattern(Fig. 7b) below theroadindicates thatthe pipe beneath the roadallows the water to
flow from the northern tothe southern portions of thevalley floor. The liquid water thicknesses within the drainage
pattemg vary between approximately 0.04 and0.46 cm. Areas having large amounts of white salt deposiL_tendto
have larger liquid water thicknesses. This may bca trueindication of the wetness of the salts. Field observations
showed thatthewhite saltareas am wetter thannearby areas not covered bythe white salts. On the other hand, the
solar radiation near 1Hmcan penetrate deeper intothewhite salt thanother materials. This canalso effectively
increase the liquid water thicknesses over the white salt areas.
By comparing Figures 7aand7b, itcan be seen that over thevalley floor the areas with more liquid water
seem to havemore atmospheric watervapor. This ismost evident over the two large areashaving white salt
deposits (one inthe upper part of the scene and one inthe lower partof the sere). Therefore, the spatial variation
inatmosplmfic water vapor may be related totheevaporation ofsurface liquid water. The spatial variation of
column water vapor amountsover the valley can notbe attributedtothe topography because the valley floor is
225
relativelfyiatwith elevation differences of 12mor less. Inthe future,we plantomake furtherstudies of the
correlations among water vapor, surface liquidwater, topography, andmineralogy.
IV. Summary and Conclusions
We comparedPWV soundingfsromthethre__cparatrecmolcsensinsgystemsO:pticalV,AS, andmicrowave
radiomctexTsh.emicrowave_mcnts werealsocomparedwithsoundingfsrombothNWS andCLASS
radiosondeTsh.cmi_owavcandopticamleasurementasgreedtowithinIram,andthecomparisonwsiththe
mdiosond_were also eithergood orexplainable. Gradientstructuresof water vaporareobsexvcdinbothPWV
images derived from AVIRIS and VAS data.Because of itslargefieldof view, VAS has difficulties inresolving
small scale features neat sharp discontinuities causedbyterrain.Each of thetechniquescanprovide complementary
information of PWV. Theoptical technique provides column water vapor amounts duringclear conditions ata
precision better than l mm andathigh spatial resolution. Themicrowave radiomeaea"provides nearly continuous
dataduringboth clear andcloudy conditions=butonly ataIimitcd seaof locations. Timcolumn water vapor
amountsderived frommicrowave radiometer datacanbe usedtoquantitadvdy adjust satellite PWV images provided
by the VA$ sounder (Birkeathcuer,1991).Because ofthegood horizontal resolution provided bythe optical
technique, optical soundings could provide significant insightintohorizontal transportof water vapor.Such
soundings could also be useful for GOES VAS verification.
The column water vapor amountsderived fromAVIRIS dataover Death Valley decrease with increasing
surfaceelevation. Theliquid water image shows surface drainagepatterns. Thespatial variations inatmospheric
watervapor ovcxthe valley may be relatedtothe evaporationof surface liquid water. The distributionof mineral
depositosverthevalleymayalsobcrelatetdothesurfacderainagpeatterns.
Atmospheric water vapor is avet), complex, highly mobile species and we have a long way togo tofully
understandandanalyzethivsariablc.
Acknowledgments
Theauthors arcgrateful toR.O. Green of thelet Propulsion Laboratoryforproviding the AVIRIS spectral
data. This work was partiallysupported bythe IetPropulsionLaboratory, California Instituteof Technology under
contract 958039 and the FAA underthecontract DTAF01-90-Z-02005.
Referenceg
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microwave radiometer data on thelocal scale, d. Appl. Meteor., (in press).
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460, 1990.
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vapor measurements using downward-looking optical andinfrared imaging systems andupward-looking
microwave radiometers, Submitted to J.Appl. Meteor. inApril, 1991.
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Airborne Imaging Spectrometer Data",J. Geophys. Rex., 95, 3549-3564, 1990.
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226
Hunt, C.B.,andD. R.Mal:cy, StratigraphyandstructureDcadiValley,California,U. S.GeologySurvey
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useofmicrowave radiometers,RAS$, andce-ilomclmsfor detectionofedrcrafticingconditions.HOAA Tcch.
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radiation, AlcaFrequenza. LVIII,31-38, 1989.
1.0 [ ..... _ 9_ f
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OiO I I - - I
10 ..... ' -
i°
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"100.5_ " o:b o.bs z.bo z.o5
WAVELENGTH (/_m)
Fig. 1. An example of curve fitting of spectra. Thetop plot shows the observed spectrum(solid line) and the
fitted spectrum (dashed line). TIc bottom plot shows the percentage diffcxeaccs between the observed and the
fiued spectra.
227
(a) (b) (c)
,4.-
'8
mQ.
Fig. 2. Column water vapor reiriovals from AVIRIS
data measured over the Denver-Plattcville area in
Colorado on March 22, 1990. (a) An image of the
sccn_ processed from radiance of one channd centered
at 0.86 IJm,(b) a column water vapor image over tl_
scene retrieved by curve fitting the 0.94-pm water
vapor band absorption region, and (c) a topographic
map of the scene. The elevations in the topographic
map are In units of feet (1 foot = 30.48 cm). The
distance from left to right side of the two images in
> this figure is approximately I1kin,
N
t.
<[
¢3
1.5
1.3
1.1
III.q
r_ #°7
p. |.S (a)
I.l
Fig. 3. Time series of prccipitable water 1.1
vapor at (a) Plattevill¢, Co)l)¢nver, and (c) I.I
i.I
Elbcrt on 22.23 March 1990. Continuous
curves arc from microwave radiom_ers, 6.7
I.S
rectangles from CLASS radiosondes, squares p-
from NWS radiosondes, dark circles from 0.3
VAS-adjust_ image, and open circles from
AVIRIS. 1.1
l°|
o
1> |,5 (c)
_ I_ os 0],," oI_I5 ot..J--2.].-l-=_ 0, oo 2s 2z 21 2o l_ le l"t 16 15 14 13 12x:
<_ Time UTC
23-MAR-i_)g0 22-KAR-ISg0
228
6b)
Iim,a_mm,.,4m.llalt..-a,qmq
Fig. 4. (a) An image of Death Valley. California processed from radiance of one AVIRIS channel at 0.68 l_m,
and (b) a topographic map of the scene. The elevadons in the topographic map are in units of feet (I foot _ 30.48
era). The distance from left to right side of the image in this figure is approximately 12 kin.
229
2.0 I ] __ f I_._ __ i I__ I I . I _ i .
1.5-
f -.-..V,,/..V.\--
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3
0.5
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0.4 0.6 0,8 1,0 1.2 1.4 1,6 1.8 2.O 2.2 2,4
WAVELBNGTIt01m)
Pig. 5. A re_,ctance t'pectrum(multiplied by 5)of a wet sample collected from the field andtwo ratioed spectra
from the AVIRIS data.See textfordescriptions of the indeed spectra.
. ..I • i i I
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f_ IJo
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,,, , ,,i
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WAVELENGTH(Ia.m)
Fig. 6. Curve fitdng of anAVIRIS spectrumoverabright, wetsaltarea.The topplot showsthe observed
spectrum(solidline) andthefitted spectrum(dashedline).Only thewatervaporabsorptionisincludedinthe
fittingprocess.Thebottomplotissimilartothetopplot,except thatboththewatervaporandtheliquidwater
absorptionsareincludedinthefittingprocess.
230
,fi
,¢
It,q
231