A Comparison of Satellite-Derived SpectralAlbedos to Ground-Based BroadbandAlbedo Measurements Modeled to SatelliteSpatial Scale for a Semidesert Landscape

Wolfgang Lucht,* Andrew H. Hyman,* Alan H. Strahler,*Mike J. Barnsley,† Paul Hobson,† and Jan-Peter Muller‡

This study describes the use of ground-based albedome- lator data over the 1.131.1-km area of an Advancedter measurements for validating albedo measurements ac- Very High Resolution Radiometer pixel shows that thequired from space at the 1-km spatial resolution of Mod- instrument tower at the field site, in the vicinity of whicherate Resolution Imaging Spectroradiometer (MODIS). the ground albedo measurements were made, was locatedThis work forms an element of the Grassland Prototype in an area where the local spatial heterogeneity of albedoValidation Exercise initiative, a prelaunch Earth Observ- is similar to that at the coarser satellite resolution. Thising System field experiment conducted in May 1997 at explains the agreement between the ground-based andJornada, NM, USA. In lieu of MODIS data, we present the satellite-derived albedo values despite the differencealbedo comparisons using 1.1-km Advanced Very High in spatial scale. These results provide the first step in es-Resolution Radiometer and 637-km Polarisation and Di- tablishing appropriate strategies for albedo validation forrectionality of the Earth’s Radiation data. To derive MODIS. Elsevier Science Inc., 2000broadband albedo from the satellite data, we calculatespectral albedo through the multiangular inversion of asemiempirical bidirectional reflectance distribution func- INTRODUCTIONtion model and perform narrowband-to-broadband al-

Land surface albedo is a key parameter in modeling radi-bedo conversion using an observed spectrum as interpo-ative transfer in the atmosphere. Feedback mechanismslator and extrapolator of spectral albedo. Comparisons ofbetween the amount of solar energy reflected by the sur-ground-based albedo values to the satellite-derived valuesface and the climate make a knowledge of albedo at ameet the 60.05 suggested accuracy requirement forrange of spatial scales relevant to understanding theglobal climate modeling and are close to the 60.02 sug-global radiation balance and its influence on climate andgested standard. Spatial modeling based on assigning typi-vegetation dynamics (Henderson-Sellers and Wilson, 1983;cal albedos to three land cover classes obtained throughAvissar and Verstraete, 1990; Dickinson, 1995; Lofgren,classification of 12-m resolution Thematic Mapper Simu-1995).

In current general circulation models (GCMs), landsurface albedo is one important source of radiative un-*Department of Geography and Center for Remote Sensing, Bos-

ton University, Boston certainties. Climate modeling requires albedo with an†Department of Geography, University of Wales, Swansea, Sin- absolute accuracy of 60.05 according to Henderson-Sell-gleton Park, Swansea SA2 8PP, UK

ers and Wilson (1983) and of 60.02 according to Sellers‡Department of Geomatic Engineerig, University College Lon-don, Gower Street, London WC1E 6BT, UK (1993). A global albedo data set meeting either of these

Address correspondence to Wolfgang Lucht, Potsdam Institute accuracy requirements is currently not available. The tra-for Climate Impact Research, Postfach 601203, D-14412 Potsdam, ditional approach to providing coarse resolution albedoGermany. E-mail: wolfgang.lucht@pik-potsdam.de

Received 22 December 1998; revised 27 October 1999. values has been to associate typical albedos to surface

REMOTE SENS. ENVIRON. 74:85–98 (2000)Elsevier Science Inc., 2000 0034-4257/00/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(00)00125-5

86 Lucht et al.

types derived from a global land cover classification. This A linear mixture model is formed relating the broadbandalbedos to the proportions of land cover components de-approach may be limited by many factors, which include

the representativeness of the characteristic land cover rived from classified hemispherical photography acquiredat each location. The study reveals that solar zenith angleclasses, classification accuracy, and failure to account for

the effects of latitude, seasonal variability, and the bidi- and soil moisture effects require careful considerationwhen acquiring ground-based measurements to explainrectional reflectance distribution function (BRDF) of the

surface. Satellite remote sensing is the only feasible tech- the spatial variability of albedo.Even if the albedo at a tower site is well character-nique to obtain reliable and accurate estimates of surface

albedo at the global scale, including frequent temporal ized, a key question arising in view of the fact that somespatial variability in albedo may be expected in the im-observation (Li and Garand, 1994; Lewis et al., 1999). In

view of this need, albedo is a standard product to be de- mediate vicinity of any tower even in relatively homoge-neous locations is whether a single albedometer mountedlivered from data acquired by the Moderate Resolution

Imaging Spectroradiometer (MODIS) on the Earth Ob- on a tower is sufficient for characterizing the albedo ofa test site at the much coarser 1-km spatial scale of aserving System (EOS) AM-1 platform (Lucht et al., 2000)

launched on Dec. 18, 1999. Making use of seven spectral satellite data product like the MODIS albedo product.In this paper, we build on the previous study andchannels between 470-nm and 2,130-nm wavelengths,

this product will provide spectral and broadband albedos address the issue of spatially scaling ground-based pointmeasurements to the spatial scale of satellite observations.once every 16 days in the visible, near-infrared, and total

shortwave bands from an analysis of the atmospherically In lieu of using MODIS data, we provide comparisonsof broadband albedometer measurements in the field tocorrected BRDF observed for each pixel. The spatial res-

olution will be 1 km. broadband albedos derived at the 1.1-km resolution of theAdvanced Very High Resolution Radiometer (AVHRR)Development of appropriate ground-based validation

techniques is critical to assessing the uncertainties associ- and at the 637-km resolution of the Polarisation and Di-rectionality of the Earth’s Radiation (POLDER) instru-ated with such a data product derived from satellite im-

agery. The main challenge in land satellite data validation ment. Red and near-infrared band, atmospherically cor-rected AVHRR and POLDER multiangular reflectanceis to attain adequate ground sampling of observed bio-

physical variables, which exhibit spatial and temporal observations are used to invert for the parameters of asemiempirical BRDF model, from which spectral albe-variance, at the spatial scale of a satellite sensor. The

MODIS Land Validation Plan has been structured to dos and subsequently broadband albedos are derived.These are then compared to albedo estimates derivedprovide a hierarchical test site scheme (Privette et al.,

1997b). Primary validation is based on sites designated from the ground-based measurements made at the towersite and modeled to a coarser spatial scale.as Tier 3 and Tier 4, which consist of a fixed tower with

above-canopy instrumentation to continuously providecanopy-scale radiometric and meteorological variables. In BACKGROUNDaddition, measurements of leaf area index (LAI) andfraction of absorbed photosynthetically active radiation A limited number of previous comparisons between

ground-based and satellite-derived total shortwave broad-(FAPAR) along transects radiating from the tower areplanned at less frequent temporal intervals as well as ra- band albedo observations are found in the literature.

These identify some of the considerations and difficultiesdiometric characterizations of the larger area containingthe tower from low-flying light aircraft. associated with acquiring ground-based broadband al-

bedo measurements and scaling to satellite spatial reso-To develop validation strategies, the MODIS Land(MODLAND) science team has coordinated several pre- lutions.

For polar regions, DeAbreu et al. (1994) report de-launch field campaigns. Two field experiments calledPrototype Validation Exercises (PROVE) were carried tailed comparisons between coincident ground measure-

ments and AVHRR total shortwave broadband albedosout during 1997 (Grassland PROVE and Forest PROVE),representing a short and a tall vegetation canopy. Albedo using data corrected for viewing geometry and atmo-

spheric effects. Uncertainties in the spatial heterogeneitydata collected during the more extensive GrasslandPROVE campaign at a semidesert site at Jornada, NM of the surface and differences in irradiance conditions in-

troduce difficulties in measuring albedo at the satellitein late May 1997 are addressed in this study. In keepingwith the tower-based validation concept, the spatial vari- resolution. Comparisons of broadband albedo derived

from the AVHRR with surface measurements over theability of albedo in the immediate vicinity of a tower lo-cation was studied in detail and reported by Barnsley et Greenland ice sheet by Stroeve et al. (1997) exhibited

good agreement (to within 5% before snow melt). How-al. (2000). This analysis shows relationships between thevariations in proportions of different scene components ever, it was noted that the potential presence of melt

ponds in an AVHRR pixel could lead to 11% underesti-present at measured locations and associated small butconsistent variations in the observed broadband albedo. mation error if not accounted for. Broadband albedos for

Comparison of Satellite and Ground Albedos 87

a 333 pixel window centered on tower sites for fivetypes of terrain in a subartic landscape near the HudsonBay are derived by LaFleur et al. (1997) from LAND-SAT Thematic Mapper (TM) reflectances using coeffi-cients published by Brest and Goward (1987). Compari-son with coincident surface pyranometer measurementsshowed a mean difference of 60.01.

At midlatitudes, Schaaf and Strahler (1994) used redand near-infrared (NIR) spectral albedos derived fromgeometric-optical modeling of Advanced Solid StateArray Spectroradiometer (ASAS) multiangular reflec-tance measurements over a spruce forest in Howland,ME, and Brest and Goward’s (1987) spectral-to-broad-band conversion equation. The resultant modeled albedovalues exhibited close agreement with values from coinci-dent measurements from pyranometers on a meteorolog-ical tower. In the Southern Great Plains, OK, Minnis etal. (1997) showed consistent trends in surface albedo and

Figure 1. Location of Jornada Experimentaltop-of-atmosphere albedo from Geostationary Opera-Range study site (marked by JRN). Source:tional Environmental Satellite (GOES) data and identi-http://jornada.nmsu.edufied that asymmetrical diurnal variation in albedo of 10%

may be attributable to dew.Li and Garand (1994) used colocated measurements

afternoon albedo asymmetries can be a complicatingfrom the Earth Radiation Budget Experiment sensorsource of error in satellite-based albedo retrievals.(ERBE) and tower observations in regions of plain ag-

ricultural land at the Boulder Atmospheric Observatory(BAO) and for snow at Saskatoon, Canada. ERBE sur-

GROUND-BASED ALBEDO MEASUREMENTSface albedos at 35-km resolution were calculated fromAND MODELING TO DIFFERENTtop-of-atmosphere albedo, solar zenith angle, and totalSPATIAL SCALESprecipitable water using a simple parameterization and

correcting for spectral and angular effects. A bias error Study Siteof 0.93% was reported as the mean difference between Grassland PROVE (Kannenberg, 1997; Rahman et al.,the estimated and observed albedo, and a random error 1997) took place form May 20 to 30, 1997 at the Unitedof moderate magnitude (3.07%) as the root-mean-square States Department of Agriculture—Agricultural Re-error of the differences from which bias error is sub- search Service/Long-Term Ecological Research (USDA-tracted. The authors attributed the bias to the different

ARS/LTER) Jordana Experimental Range, located 37 kmeffective field of views of the pyranometers and satellitenorth of Las Cruces, NM (32.58 N, 106.88 W, see Fig. 1).radiometers and to errors in geolocation such that theThe range consists of a very flat valley of approximatelytower could occur in any part of a pixel or possibly even800 km2 bounded to the east and west by mountains.in an adjacent pixel.Land cover may be categorized into three broad types,In the low latitudes a study was carried out in Nigernamely grassland, transitional (mixed grass, soil, andas part of the Hydrological-Atmospheric Pilot Experi-shrubs), and shrubland, of which the first two were vis-ment (HAPEX)-Sahel field campaign. Ground-basedited for this study. The shrubs growing on the sandy soilspoint sample measurements of albedo made at hourly in-in the vicinity of the tower at the transition site aretervals (cf. Allen et al., 1994) exhibited good agreementhoney mesquite (Prosopis glandulosa) and soaptree yuccawith values derived from ASAS overflights and BRDF(Yucca elata), with the dominant grass being black gramamodeling except at extreme solar zenith angles (Lewis et(Bouteloua eriopoda) (Dick-Peddie, 1993). The catego-al., 1999). In this study, the deviations at high solar ze-ries reflect slow land cover change from grasses tonith angles may have been due to uncertainty in the esti-shrubs, which is probably occurring following overgraz-mation of diffuse illumination or to overestimation of theing at the beginning of the century. The climate is semi-importance of the NIR to broadband albedo at high solararid, and as a result vegetation cover throughout the val-zenith angles. Lewis and Barnsley (1994) also noted thatley is sparse with an average LAI of about 0.5. Thean isotropic diffuse approximation may introduce errorsoccurrence of just a few unpaved roads, data towers, waterin calculating albedo for high solar zenith angles. Grantwells, and fences means that man-made structures willet al. (1999) pointed out in their analysis of extensive ob-

servations of an Australian grassland site that morning/ provide a minimal contribution to remote sensing data.

88 Lucht et al.

Figure 2 presents three overview photographs of thestudy area. In Figure 2a the view is north of the toweralong an unpaved road. Ground cover is characterized bya shrub (mesquite/yucca) and soil mix with some grass.By contrast, the ground cover to the south of the towerhas a higher coverage of senescent grass with a lesser butstill significant presence of mesquite and yucca (Fig.2b).Figure 2c looking to the southwest reveals a fenced sec-tion that has similar ground cover to the area south ofthe tower, which is largely devoid of shrubs and gradu-ates to a sparse grass and soil mix toward the west. Toplace these photographs into the spatial context of thelandscape, Figure 3 presents a Daedalus AADS-1268Thematic Mapper Simulator (TMS) 7, 5, and 3 (NIR,red, and green bands) (LANDSAT TM equivalent bands4, 3, and 2) false-color composite image with the towerlocation indicated by the crosshair. The TMS image hasnot been geometrically corrected and therefore the dis-tortions are unknown, but as a guide to the dimensions,the y-dimension of the image is approximately 2 km.Three land cover classes typical for the vicinity of thetower, “shrubs and soil” (SS), “grass” (G), and “grass andsoil” (GS), are distinguishable on the image.

Albedometer Field Observations: MethodTotal shortwave broadband albedo (305–2800 nm, nu-merically equivalent to 305–5000 nm due to the verysmall value of solar irradiance at the larger wavelengths)and NIR broadband albedo (695–2800 nm) were mea-sured at the study site using two Kipp and Zonen CM14albedometers. These instruments consist of two CM11pyranometers mounted back-to-back, one looking up-ward and one downward, and each equipped with a filterdome providing a near-perfect linear spectral responsewithin the specified broadband range. The instrumentspecifications meet the requirement of “secondary stan-dard,” the most stringent accuracy class specified in ISO9060 for instruments to measure solar radiation as de-fined by the World Meteorological Organisation (WMO)(Kipp and Zonen, 1997). Both albedometers weremounted concurrently on a black tripod at a height of4.1 m above the surface and manually leveled using the

Figure 2. View of the study site from the top of the towerbuilt-in bubble level. Albedo measurements were takenat the transitional site looking (a) north, (b) south, andat 10-m sampling intervals along four short (100-m) tran-(c) southwest. The landscape to the north and west of the

sects radiating to the north, south, east, and west from tower is characterized by a mosaic of shrubs (mesquitea tower at the transitional site, and along a 150-m tran- and yucca) and soil with some grass and herbs. The

landscape to the south and east is characterized by asect at a separate grassland site. As reported in Bransleydenser coverage of grass with some scattered shrubs.et al. (2000), corrections were applied to remove solarLooking to the southwest, note the two distinct surfacezenith angle and soil moisture effects. The solar zenith coverages. The shrub and soil mosaic is replaced by a shrub-

angle dependence of albedo at the transitional site was devoid area to the south. This zone exhibits a transitionestablished from diurnal albedo observations made dur- from grass to a sparse grass and soil mixture from east to

west. The color of the soil is reddish. Photograph takening the campaign (Jim Conel, Jet Propulsion Laboratory)by Paul Hobson (University of Wales, Swansea).with an albedometer of identical make. Since only total

shortwave data were acquired, the correction of the NIRdata were performed using the same angular dependence.

Comparison of Satellite and Ground Albedos 89

normalized to the solar zenith angle 18.38, correspondingto local noon on May 26, 1997 (cf. Barnsley et al., 2000).

The 44 albedo observations at the tower site wereaveraged to produce an estimate of the albedo of theland surface at the tower location (see Table 1). The to-tal shortwave albedo is 0.221, the visible and NIR broad-band albedos are 0.124 and 0.311, respectively.

However, in view of the observed range of potentialalbedos at Jornada and a nonnegligible standard devia-tion of 0.012 of the 44 albedos averaged at the tower,one must conclude that a single albedometer measure-ment in a heterogeneous landscape, such as Jornada,provided for example by a tower instrument, could wellprovide a biased estimate of albedo at the coarser spatialresolution of satellite observations. The next task istherefore to investigate modeling of the albedo at the ki-lometer spatial scale, with an emphasis on simple, practi-cal methods.

Figure 3. False-color composite converted to gray scalederived from 4-m resolution TMS data using bands 7, 5, and Albedo Modeled to Satellite Resolution Using3 (TM equivalent 4, 3, and 2). The tower location is marked Land Cover Classification andby a crosshair. The area shown is slightly larger than an Ground ObservationsAVHRR nadir footprint, about 2 km on one side.

The land surface surrounding the tower was character-ized using Daedalus AADS-1268 TMS images acquiredby aircraft, which were supplied by Thomas SchmuggeOther potential sources of error in the measurements are(USDA/ARS Hydrology Laboratory). These data werethe remaining leveling uncertainty, which may amount tocollected on June 19, 1997 for another project immedi-58, and nonperfect cosine response at the largest anglesately following Grassland PROVE with a spatial resolu-especially for the more isotropic upwelling radiation,tion of 4 m and 12 m. Figure 3, showing a false-colorleading to an underestimation of albedo. Data were nor-composite of these data for an area that is somewhatmalised to 18.38 solar zenith angle, corresponding to lo-larger than an AVHRR nadir footprint, suggests againcal noon. The consistency of variations seen along thethat the variation in land cover in the immediate vicinitytransects in repeat measurements on different days gaveof the tower is not homogenous. Derivation of surfacegood confidence in the accuracy of the instruments. Foralbedo at the 1-km spatial scale of AVHRR, POLDER,example, total shortwave readings were repeatable to lessor MODIS from ground point measurements could bethan 0.01. Small remaining differences in the form of an

offset were attributed to soil moisture variations due to approached by considering typical albedos of differentsparse rainfalls in the late afternoons of the days previous land cover types in the vicinity of the tower.to some of the field days. Visible broadband albedos A more sophisticated possibility would be to spec-(305–695 nm) were derived from the total shortwave and trally unmix each pixel from a data source with high spa-NIR broadband observations by weighting with the asso- tial resolution such as Landsat TM with respect to theciated integrals of the solar downwelling radiation. Refer constituent scene components and to use the linear al-to the paper by Barnsley et al. (2000) for additional de- bedo mixture model for Jornada by Barnsley et al. (2000)tails regarding the acquisition and processing of these to derive per-pixel broadband albedos at fine spatialdata. scale. However, large-scale validation should rely on

methods that avoid time-consuming procedures whileAlbedo at the Tower Site preserving good accuracy. In this study, a simpler

method that is more realistically implemented on a rou-Albedo measurements made at the tower site and at thetine basis was therefore investigated. The spatial variabil-grassland site showed some spatial variability. The mini-ity of albedo within a coarse resolution pixel was takenmum and maximum broadband albedos from all of theseinto account by performing a land cover classification ofmeasurements gave an indication of the broad range ofthe area and giving typical albedos for each land covervalues potentially true for an unknown pixel observed bytype based on albedometer measurements made on thea satellite at Jornada. The lowest total shortwave albedo,ground. This represents a simple form of spatial model-0.17, was associated with a grass-dominated site and theing that may be sufficient for most validation purposes.highest, 0.25, where soil cover was predominant, giving

a range of 0.08. All albedos reported in this study are To enable a subsequent comparison with albedos de-

90 Lucht et al.

Table 1. Comparison of Mean Local Noon Broadband Albedo from the 44 Transect Sample sites Near the Tower andBroadband Albedos Derived from the AVHRR for the Pixel Containing the Tower and Those Below, to the Right,and Below Right

Below Right of Below Right Mean of44 Transect AVHRR AVHRR AVHRR AVHRR AVHRR

Mean Albedo Measurements Tower Pixel Tower Pixel Tower Pixel Tower Pixel Pixels

Total shortwave 0.221 0.224 0.231 0.227 0.224 0.226Visible 0.124 0.127 0.143 0.137 0.128 0.134NIR 0.311 0.311 0.311 0.307 0.309 0.309

rived from concurrent AVHRR observations, a 2.232.2- from the ground albedometer measurements (see Tablekm area of the 12-m resolution TMS data containing the 2). The shrub and soil class was characterised by theAVHRR pixel with the tower in the central 1.131.1-km mean of the 22 measurements along the short 100-mportion (see Fig. 4) was selected as the area of interest transects leading to the north and west of the tower.and classified using the ISODATA unsupervised classifi- Measurements from the short south and east transectscation algorithm with a 10-dimensional feature space were excluded, given the occurrence of grassy areas inconsisting of TMS bands 1 through 10. Ten clusters were these directions. The albedo selected for the grass classgenerated, which were then amalgamated into three was taken as the mean of several locations taken at theclasses, “shrubs and soil,” “grass,” and “grass and soil,” midpoint of a 1-km long transect leading east of thereflecting the relative dominance of mesquite/yucca, tower, which traverses a grassy plateau. Ground-basedgrass, and soil cover (Fig. 5). An assignment of each of measurements were not made for the grass and soil class;the 10 clusters to the three classes was made through however, examination of the TMS hyperspectral profilesvisual assessment of various false-color composite images of the three classes indicated that the grass and soil classderived from the 4 m resolution TMS data (Fig. 3), the was located approximately midway between the otherpartially overlapping portion of Airborne Visible/Infrared classes, and thus mean albedo of the two known classesImaging Spectrometer (AVIRIS) imagery acquired dur- was therefore assumed. By finally performing the aver-ing the campaign, and photographs of the study area age over the 1.131.1-km area of the AVHRR nadir foot-taken from the top of the tower (Fig. 2a–c). print containing the tower, the ground-based albedo at

To calculate broadband albedo at the 1.1-km resolu-tion, typical albedo values were assigned to the threeclasses in the classified TMS scene, which were derived Figure 5. Classified 2.232.2-km area centered on the

AVHRR pixel containing the tower. The classification is per-formed on 12-m resolution TMS data. The three classes are:black, shrubs and soil; white; grass; gray, grass and soil.Figure 4. Identification of numbering of the nine 1.131.1-km

cells within a larger 2.232.2-km area centered on theAVHRR pixel containing the tower. Cells are numbered atthe upper left.

Comparison of Satellite and Ground Albedos 91

Table 2. Typical Local Noon Albedo Values Assumed to values obtained from the ground measurements for theCharacterize the Three Land Cover Classes Used to Derive coarser spatial scale are rather similar to those obtainedBroadband Albedo at the Satellite Scale Using the Land from the transects directly at the tower site (Table 1).Cover Classification Approach

Obviously, the variability present at the tower site is ofTotal Shortwave NIR Broadband a similar nature in terms of the relative proportions of

Land Cover Class Broadband Albedo Albedo different types of land cover on the scale of 1.1 km.Shrubs and soil 0.226 0.323Grass 0.186 0.267Grass and soil 0.206 0.295 DERIVATION OF BROADBAND ALBEDOS

FROM SATELLITE DATA: METHODOLOGY

Satellite Datathat spatial resolution was established. The total short- Moderate-resolution multiangular satellite data were ac-wave, visible, and NIR albedos found are 0.216, 0.116, quired from the AVHRR and POLDER space sensorsand 0.310. for the derivation of space-based albedo estimates.

However, the exact location of the tower in the AVHRR NOAA-14 imagery with a resolution of 1.1 kmAVHRR imagery may be considered uncertain by at least at nadir were available in the form of eight cloud-freeone half pixel. Derivation of albedos using the same daytime passes in May and early June. Preprocessingmethod for other potential AVHRR footprint locations performed by Jeff Privette (Goddard Space Flight Cen-around the tower permits an evaluation of the sensitivity ter) consisted of geometric correction to a map and at-of albedo to misregistration. The relative proportions of mospheric correction using the 6S code (Second Simula-the three land cover classes were extracted from nine tion of the Satellite Signal in the Solar Spectrum;1.131.1-km areas of interest as defined in Figure 4. Area Vermote et al., 1997) and concurrent (within 5 minutes5 corresponds to the central 1.131.1-km and areas 1–4 of the overpass) ground measurements of atmosphericand 6–9 correspond to various 0.5-km displacements. Ta- properties provided by a Reagan sun photometer (Kurtble 3 presents the land cover class proportions and the Thome, University Arizona) and a Cimel sun photometerresulting broadband albedos for each of the subregions. (Jim Conel, Jet Propulsion Laboratory). Aerosol opticalTable 4 shows the mean and one standard deviation of depth was between 0.05 and 0.12 at 550 nm. Surface re-the total shortwave broadband albedo for the nine areas. flectance for atmospheric correction was estimated from

The albedos for the eight shifted cells exhibit only handheld radiometer readings. During subsequent pro-small deviations from the broadband albedo values calcu- cessing and comparison with other imagery, it was foundlated for the central cell (cell 5). The minimal and maxi- that there was a remaining two-pixel misregistration inmal values of the broadband albedos indicates that deri- the north–south direction. Data from the red channelvation of albedo estimates using the land cover (630 nm) and the NIR channel (913 nm) were used.classification method should provide estimates of total From the POLDER instrument, data with a 637-kmshortwave, visible, and NIR broadband albedo within resolution at nadir were available for the study period.60.013, 0.017, and 0.007, respectively, when locating a Level 1 normalized radiance data were supplied by Marc1.131.1-km AVHRR footprint anywhere within a Leroy (Centre National d’Etudes Spatiales). Data for 22,2.232.2-km area centered on the AVHRR pixel believed 23, and 25 May provided imagery free of cloud contami-

nation over the study area. Nonpolarized bands centeredto contain the tower site. It is interesting to see that the

Table 3. Proportional Cover of Three Land Cover Classes and Local NoonBroadband Albedo Values Calculated Using the Land CoverClassification Method for Nine 1.131.1-km Cells within a 2.232.2-km AreaCentered on the AVHRR Pixel containing the Tower at the Transition Site(Cell 5)

Proportion of Broadband albedo

Cell Grass Shrub and Soil Grass and Soil Shortwave Visible NIR

1 0.006 0.989 0.005 0.225 0.111 0.3222 0.390 0.603 0.007 0.210 0.113 0.3013 0.611 0.385 0.004 0.201 0.108 0.2894 0.001 0.990 0.009 0.226 0.122 0.3235 0.233 0.756 0.012 0.216 0.116 0.3106 0.425 0.570 0.004 0.209 0.112 0.2997 0.073 0.771 0.156 0.220 0.118 0.3148 0.244 0.659 0.097 0.214 0.115 0.3079 0.388 0.597 0.015 0.210 0.113 0.301

92 Lucht et al.

Table 4. Summary Local Noon Broadband Albedo Statistics for the ValuesDerived from Land Cover Classification for Nine 1.131.1-km CellsCentered on the Tower

Broadband Albedo Mean Standard Deviation Minimum Maximum

Total shortwave 0.215 0.008 0.201 0.226Visible 0.114 0.004 0.108 0.122NIR 0.307 0.011 0.289 0.323

at 490 nm, 565 nm, 675 nm, and 910 nm were atmo- lated to obtain black-sky albedo, which is surface albedoin the absence of diffuse skylight (cf. Lucht et al., 2000).spherically corrected using 6S and, as for the AVHRR,The integral was evaluated for a solar zenith angle ofground measurements of aerosol optical depth taken18.38, that of solar noon on May 26, 1997, since this isclose to the overpass times.the solar zenith angle to which the concurrent albedome-ter field measurements were also corrected (Barnsley etDerivation of Spectral Albedos from Satelliteal., 2000). The bihemispherical integral of the BRDF, aObservations through BRDF Model Inversionconstant termed white-sky albedo, provided the surfaceImagery acquired by the NOAA-14 AVHRR sensor fromalbedo under completely diffuse illumination.several orbits provided a data set of eight observations

Surface albedo in the given wave band, finally, is de-with varying view and illumination geometry per groundrived from the values of black-sky and white-sky albedopixel, allowing the inversion of a BRDF model and sub-by means of the interpolation (Lewis, 1995) shown insequent derivation of surface albedo from this model.Eq. (1):Similarly, during each overpass of the study site, POL-

DER acquired images that sample a swath in angulara(K,hi)5[12d(K,hi)]q(K,hi)1d(K,hi)q

5(K) (1)space through the viewing hemisphere. To adequatelycharacterize the surface BRDF and obtain reliable re- where d(K,hi) is the proportion of diffuse illumination insults from the inversion process, POLDER observations waveband K, q is the BRDF, and the bars represent di-from three passes with clear sky conditions over Jornada rectional-hemispherical and bihemispherical integration,obtained over the duration of the field campaign were respectively. This approximation assumes that the sensi-amalgamated into a single data set for the BRDF in- tivity of surface albedo to the directional distribution ofversions. sky irradiance is negligible, which Lewis and Barnsley

BRDFs were derived from the observations for each (1994) document is reasonable except for large solar ze-spectral band through BRDF model inversion using the nith angles. The proportion of direct and diffuse irradi-Algorithm for MODIS Bidirectional Reflectance Aniso- ance for each waveband was obtained using 6S for a UStropy of the Land Surface (AMBRALS). The BRDF Standard 62 atmosphere and a continental aerosolmodel used was the a semiempirical kernel model con- model. Spectral albedos were calculated for the samesisting of the RossThick and the reciprocal LiSparse ker- values of aerosol optical depth as used in atmosphericnel (Wanner et al., 1995; Lucht et al., 2000). This model correction. Since the albedos computed for aerosol opti-has been shown to allow a diverse range of possible cal depths representative of the lower and of the higherBRDF shapes and remains stable for sparse angular sam- readings were obtained at local noon, the baseline timepling (Lucht, 1998; Privette et al., 1997a). Observed for the albedometer measurements on different daysmultiangular reflectances are modeled as a linear super- were well within a difference of 0.01, and an averageposition of mathematical kernels, or basic BRDF shapes, aerosol optical depth was employed.that are defined to represent different types of scatteringreflectance as a nonlinear function of the viewing and Narrowband to Broadband Albedo Conversionillumination geometry (Roujean et al., 1992). The Ross- Spectral albedos are related to broadband albedoThick kernel is a single-scattering approximation to radia- through weighting with and normalization to the solartive transfer in a homogeneous canopy of leaves. The re- downwelling radiative flux W for a solar zenith angle ofciprocal LiSparse kernel is a geometric-optical kernel h [see, Eq. (2)]:describing the casting of shadows by ensembles of sphe-roidal protrusions of the surface. The BRDF model pa- a(hi)5#

solarW(K,hi)a(K)dK/#solar

W(K,hi)dK (2)rameters retrieved in the inversion are the respectiveweights of the two kernels in the observed BRDF and Satellite sensors acquire spectral albedo only in a smallan isotropic constant regulating overall scene reflectance. number of spectral bands. Therefore, these either need

The directional-hemispherical integral over the view- to be interpolated for subsequent convolusion with thesolar spectrum, or spectral-to-broadband conversion fac-ing angle hemisphere of the retrieved BRDF was calcu-

Comparison of Satellite and Ground Albedos 93

tors weighting each narrowband albedo value by an ap- erated by fitting a spline. The resulting spectrum was fi-nally adjusted to fit, in the sense of least squares, thepropriate factor need to be derived (Kriebel, 1978; Brest

and Goward, 1987; Liang et al., 1999). Since solar irradi- BRDF-derived spectral albedos by means of a linearshift and stretch. In this way, the spectral functional de-ance varies as a function of solar zenith angle and atmo-

spheric state, a single-value broadband albedo is not pendence of the AVIRIS reflectance observations, ad-justed to provide the magnitudes of the satellite-deriveduniquely characteristic of a surface type.

Brest and Goward (1987) applied the method of spectral BRDFs in the available wavebands, is used asan estimator of the spectral dependence of albedo overweighting narrow spectral bands to nadir LANDSAT

measurements assuming a Lambertian reflectance. Broad- the whole broadband spectral range. Despite neglectingthe spectral dependence of BRDF shape, this procedureband surface albedos were calculated using a three-part

weighted average of LANDSAT visible, NIR, and MIR should yield results that are superior to either using oneof the widely employed simple weighting schemes or us-surface reflectance values. Irons et al. (1988) derived to-

tal shortwave broadband albedo without the assumption ing just a static standard reference vegetation spectrum.The resulting spectral albedo profile was finallyof Lambertian reflectance using a set of measurements

at multiple view angles from a truck-mounted radiometer weighted by the downwelling irradiance to producebroadband albedos. The solar irradiance spectrum usedfor seven bands between 450 nm and 2350 nm. Broad-

band reflectance factors were computed by appropriate in this is from Analytical Spectral Devices, Inc. (1996)for clear sky and good visibility conditions approximatelyweighting of the spectral observations by the solar down-

welling flux before total shortwave broadband albedo was 1 hour before local noon on April 23, 1996 at the Cloudand Radiation Testbed central facility in the southerncalculated through numerical integration of the broad-

band reflectance factors over the viewing hemisphere. Great Plains. Unfortunately, no concurrent colocateddownwelling spectral irradiance distribution was mea-This method has also been shown to provide broadband

albedo estimates that are comparable to pyranometer-de- sured at Jornada. However, the spectral weighting per-formed depends only on the realtive shape of the spec-rived values for discrete waveband, bidirectional, ground-

based radiometer measurements of bare soil and grass trum, not on its absolute magnitude.plots using both numerical integration and integration ofan empirical model fit (Ranson et al., 1991). However, Satellite-Derived Albedos at Different

Spatial ScalesStarks et al. (1991) report that albedo estimates from bi-directional reflectance data using this method exhibited Broadband albedos were derived from the AVHRR dataa systematic overestimation trend when compared to py- at three different spatial scales. First, albedos were com-ranometer measurements. They suggest that of the vari- puted for the 1.131.1-km pixel containing the tower. Toous potential sources of error, the bias was largely attrib- test for the sensitivity of the values to geolocation errorsutable to only acquiring data in the solar principal plane. and because the tower is likely located in the southeast-The influence of inappropriate weighting coefficients ern part of the AVHRR pixel containing the tower, albe-could not be quantified. More promising results were re- dos within a 232 block of AVHRR pixels containing theported by Wanner et al. (1997) with regard to deriving locations to the immediate south, east, and southeast ofalbedo from MODIS spectral bands. Albedos in the the tower pixel were also investigated. Results are listedseven MODIS land bands were splined with tie points in Table 1. The total shortwave, visible, and NIR broad-to compute broadband albedo. Potential accuracies for band albedos of the tower pixel are 0.224, 0.127, andthree different land cover types to derive total, visible, 0.311, respectively. The albedos within the block deviateand infrared broadband albedo are within 4% relative. from this at most by 0.007, 0.016, and 0.004, respec-

In this study, to provide broadband albedos from the tively. Finally, albedos were derived for each pixel of asatellite-derived spectral albedos, the method of spectral 10310 block of AVHRR pixels containing the tower site.interpolation and extrapolation was used. The function The resulting albedo map is shown in Figure 6, theemployed for interpolation was not a spline, however, means are given in Table 5. The tower is located in thebut a hyperspectral AVIRIS reflectance profile that was AVHRR pixel in the third row down, fifth column fromextracted from the vicinity of the tower site. It was mea- the left. Total shortwave albedo variations between 0.193sured from the NASA ER2 aircraft in 224 contiguous and 0.307 are seen that can be related to variations inspectral bands between 400 nm and 2500 nm on May 27, the landscape seen in LANDSAT TM imagery. It is in-1997 at a spatial resolution of 20 m and atmospherically teresting to note that already, from a purely visual in-corrected using 6S and concurrent ground characteriza- spection of Figure 6, one can conclude that the tower istion of the atmosphere by Greg Asner (University of Col- located within a pixel that has an albedo approximatelyorado). The profile was smoothed using a median filter corresponding to the average albedo of the larger 10310and the remaining erroneous values in regions affected area. The mean albedo of the block is 0.222 in the totalby water absorption were manually removed. A continu- shortwave, 0.143 in the visible, and 0.310 in the NIR

broadband.ous profile in the range of 305 nm to 2800 nm was gen-

94 Lucht et al.

compared. Figures 4 and 5 summarize graphically the re-sults of this study.

Figure 4 places the observed total shortwave albedosin the context of the range of albedos associated with avariety of land cover classes found in the literature, ex-cluding snow and ice. These values, plotted in column 1of Figure 4, are derived in all but one case from ground-based albedometer measurements. Mean daily clear skysummertime albedos are extracted for six surfaces fromFigure 5 in LaFleur et al. (1997). The mean daily totalshortwave broadband albedos are: lake, 0.055; sedge tun-dra, 0.112; open spruce and tamarack forest, 0.12; birch/willow (shrub) forest, 0.161; beach ridge, 0.163, andmixed tundra, 0.173. Values are also taken from Allen etal. (1994), who report total shortwave broadband albedofor two contrasting semiarid Sahelian environments inNiamey, Niger. The agricultural fallow, consisting of aground layer of annual herbs and grasses, scatteredshrubs, and occasional trees, exhibited an annual meanalbedo of 0.22 with minimum and maximum monthlymean albedos of 0.177 and 0.242, respectively. A con-trasting open natural brushland (tiger bush), consistingof bands of dense vegetation and areas of bare crustedsoil, exhibited an annual mean albedo of 0.25 with mini-mum and maximum monthly albedos of 0.212 to 0.269,respectively. Finally, the range of surface albedos esti-mated from ERBE data for the brightest desert, the Sa-hara (Li and Garand, 1994), is plotted. All of these dif-

Figure 6. 10310, 1.131.1-km AVHRR total shortwave ferent albedo values, associated with various snow- andalbedo map for local noon. The pixel containing the tower ice-free surfaces, represent a range of total shortwaveis highlighted with a border, and the pixel that exhibits

broadband albedos that could be derived for a site whosethe most dominant gray vegetation/dormant grasscharacteristics are unknown other than that snow and icecover is the dark pixel in center right.cover is absent.

Column 2 of Figure 4 shows that range from largestPOLDER broadband albedos were derived through (soil-dominated) to lowest (grass-dominated) value of all

BRDF inversion and subsequent narrowband-to-broad- albedo measurements made at Jornada for this study. Itband albedo conversion for the pixel containing the tran- gives an indication of the potential range of albedos onesitional site tower. POLDER pixel resolution 637 km. may expect to observe from space for an unknown loca-The resulting total shortwave, visible, and NIR albedo tion at Jornada. This range, 0.17 to 0.25, is much smallerare 0.223, 0.134, and 0.303, respectively (Table 5). than the range of potential values given in column 1 and

exhibits values comparable to those observed in thebroadly similar landscapes of Niger.Comparison of Field-Based and

Column 3 of Figure 4 shows values representativeSatellite-Derived Albedosof a single-site tower measurement. The mean and one

Total shortwave, visible, and NIR broadband albedos at standard deviation (0.012) of the 44 total shortwavea range of spatial scales from the ground-based measure- broadband measurements made along four short radialments and from the satellite observations can now be 100-m transects at the tower site are plotted. This column

locates the observation expected from a tower at the tran-sitional site within the range possible at Jornada in

Table 5. Local Noon Broadband Albedos Generated for the general.Average of a 10310 AVHRR Grid and POLDERThe final ground-based measurement is given in col-

Broadband Albedo Broadband Albedo umn 4, showing the value of total shortwave albedo for10310 AVHRR POLDER the 1.131.1-km AVHRR pixel containing the tower as

Total shortwave 0.222 0.223 derived from assigning typical albedo values to the threeVisible 0.143 0.134 different landcover classes in the area and weighing byNIR 0.310 0.303 the proportion of each class. The error bars give the vari-

Comparison of Satellite and Ground Albedos 95

Figure 7. Summary graph of total shortwave broadband albedo for local noon at various spatial scales. Column 1 presentstypical total shortwave broadband albedo values for a variety of surface types. These represent the range possible fora completely unknown pixel. Column 2 presents the range recorded at Jornada with sites that were soil-dominated exhibitingthe highest albedos and sites that were gray vegetation/dormant grass-dominated the lowest. These represent thepossible range of values expected for an unknown location at Jornada. Columns 3 and 4 present albedos derived from groundmeasurements that locate the transitional tower site within the potential range at Jornada. Column 3 and 4 presentalbedos derived from ground measurements that locate the transitional tower site within the potential range at Jornada.Column 3 plots the mean of the 44 measurements on the short north, south, east, and west transects at the towersite and error bars representing one standard deviation. Column 4 presents albedo derived from 1.131.1-km classified mapsof the tower area by assigning typical albedos to the land cover classes and weighting by their proportional cover.The errors bars represent one standard deviation based on the sensitivity analysis to the location of the 1.131.1-km areawithin a 2.232.2-km area centered on the pixel containing the tower. Column 5 plots (from left to right) albedoderived through narrowband-to-broadband conversion for the 1.131.1-km AVHRR pixel containing the tower, a 232 blockincluding the AVHRR pixel containing the tower, and the mean of the 10310 AVHRR block with the error barsrepresenting one standard deviation. The final column, 6, plots total shortwave broadband albedo obtained for the towersite from POLDER at 637-km spatial resolution.

ation of albedo introduced by shifting this AVHRR foot- umn 5 of Figs. 5 and 6). The broadband albedos of theprint within a 2.232.2-km are centered on the tower lo- AVHRR pixel containing the tower show excellent corre-cation. Interestingly, the mean total shortwave albedo of spondence to the ground-based albedos. The relative de-the AVHRR pixel containing the tower agrees with that viations in the total shortwave, visible, and NIR broad-measured in the immediate vicinity of the tower to 0.05, bands from the average of the 44 tower transector about 2% relative. In other words, the tower is located measurements is just 1%, 2%, and 0%, respectively, andat such a location within the 1.131.1-km area measured the relative deviations from the 1.1-km landcover-basedby the albedo that is representative of the area as a upscaled albedos are 4%, 9%, and 0%, respectively.whole. The agreement in this visible broadband is within However, given that the tower is located toward the bot-7%, and in the NIR broadband is within 1% relative. tom of the AVHRR pixel in the upper left pixel of theThe tower transect mean is slightly higher in all cases. 232 block, it may be more appropriate to compare theFigure 5 displays these numbers. mean of a 2.232.2-km cell that has the tower approxi-

Thus, through spatial modeling it is possible to show mately at the center. The numbers obtained, plotted asquantitatively that the ground-based transect measure-the middle point of column 5 of Figures 4 and 5, arements of albedo, which sampled sites in both the shrub/rather similar to those for the tower pixel, and thereforesoil and the grass areas, provide a good approximation tothe comparison comes to a similar conclusion. While thethe spatial variability at coarser scales in this landscape.agreement between the 1.1-km ground-based upscaledIf the tower site had been located in a region of higheralbedos and the satellite-derived albedos is still withinalbedo as observed to the south of Figure 6 or possibly0.010, 0.018, and 0.001, or 5%, 16%, and 0% relative,even in the shrub and soil area to the immediate westfor the shortwave, visible, and NIR broadbands, theof the tower (see Fig. 3), the ground measurements arelandcover-derived albedos are slightly smaller than thoseunlikely to have compared as favorably to the mean ofderived from the satellite. These results suggest that al-the landscape at coarser resolution.bedo validation at the satellite spatial scale may be possi-In the next and decisive step, the values obtained

now are compared to three AVHRR-derived results (col- ble through characterization of the validation site based

96 Lucht et al.

Figure 8. Total shortwave, visible, and NIR broadband albedo at various spatial scales for local noon in comparison. Columnnumbering follows that established in Fig. 4. Columns 3 and 4 present albedos derived from ground measurements.Column 3 plots the mean of the 44 measurements on the short north, south, east, and west transects at the tower site,while column 4 presents albedo derived from 1.131.1-km classified maps of the tower area by assigning typicalalbedos to the land cover classes and weighting by their proportional cover. Column 5 plots (from left to right) albedoderived through narrowband-to-broadband conversion for the 1.131.1-km AVHRR pixel containing the tower, a 232block including the AVHRR pixel containing the tower, and the mean of the 10310 AVHRR. The final column, 6, plotsbroadband albedos obtained for the tower site from POLDER at 637-km spatial resolution.

on sufficient ground-based sampling to derive the mean and the NIR broadbands. The relative difference to the1.1-km AVHRR albedo at the tower site is 0%, 6%, andalbedo that is detected at the 1.1-km scale from space.

The third entry in column 5 shows the mean and 3%, and that to the mean of the 10310 pixel AVHRRblock is the most realistic comparison in terms of scale,one standard deviation of the block of 10310 AVHRR

pixels. The similarity of the numbers achieved demon- 0%, 7%, and 2%.strates that with respect to total shortwave and NIRbroadband albedo, the tower is situated in a location that CONCLUSIONSresembles the average properties of the wider area,which does display variability in albedo. The visible In conclusion, it seems that the agreement to within a

few percent relative found between broadband albedosbroadband albedos are somewhat different at the twospatial scales. Relative differences between the 10310 derived from ground measurements and those derived

from the AVHRR and POLDER sensors in space is ex-pixel and the tower pixel AVHRR albedo are 1%, 13%,and 0% for the total shortwave, visible, and NIR broad- plained by two different things (Figs. 7 and 8).

First and most important, the satellite retrievals ledbands. Had the tower been located in one of the less-typical pixels, the AVHRR pixel containing the tower and to albedo values that have a similar magnitude to those

observed on the ground. Given the sequence of steps in-the AVHRR albedo of the larger area would not havematched as closely. As it is, the mosaic of shrubs, soil, volving atmospheric correction using observed atmo-

spheric properties, the inversion of the reciprocal Ross-and grass around the tower produces an albedo that isapproximately midway between the minimum and maxi- Thick-LiSparse semiempirical BRDF model from sparse

angular sampling (at least in the case of the AVHRR),mum observed over the larger area.Finally, column 6 of Figures 4 and 5 plots the results evaluation of the BRDF at a solar zenith angle deter-

mined not by the time of the satellite overpass but byfrom POLDER. The POLDER sensor, with its 637-kmspatial resolution, views a considerably larger footprint local noon and spectral-to-broadband conversion using

an observed spectrum of the region, it may be seen as athan both the round measurements around the towerand the 1.1-km resolution of an AVHRR pixel. One form of validation of the method used for deriving al-

bedo from satellite observations that the agreement is astherefore may not expect to arrive at the same result atboth spatial scales. Nevertheless, comparison with the al- close as is the case. This is especially true in view of the

large range of broadband albedo values possible, both inbedo values derived from the field measurements, thespatial land cover modeling, and the AVHRR shows that the Jornada region and in general for an unknown lo-

cation.the agreement is quite close. The relative difference tothe ground measurements at the tower site is 1%, 8%, Second, the data produced demonstrate that the

tower at the transitional site at Jornada was located,and 3%, respectively, for the total shortwave, the visible,

Comparison of Satellite and Ground Albedos 97

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