Relating MODIS-derived surface albedo to soils and rock types over

Northern Africa and the Arabian peninsula

Elena A. Tsvetsinskaya,1 C. B. Schaaf,1 F. Gao,1 A. H. Strahler,1 R. E. Dickinson,2

X. Zeng,3 and W. Lucht4

Received 14 September 2001; revised 30 November 2001; accepted 20 December 2001; published 15 May 2002.

[1] We use the MODerate resolution Imaging Spectroradiometer

(MODIS) aboard the Terra spacecraft to derive surface albedo for

the arid areas of Northern Africa and the Arabian peninsula.

Albedo in seven MODIS spectral bands for land and three broad

bands (for shortwave, near infrared, and visible portions of the

spectrum) is produced. Surface albedo is derived from MODIS

observations during a sixteen-day period and is analyzed at 1 km

spatial resolution. MODIS data show considerable spatial

variability of surface albedo in the study region that is related

to soil and geological characteristics of the surface. For example,

solar shortwave white-sky albedo varies by a factor of about 2.5

from the darkest volcanic terrains to the brightest sand sheets.

Vegetation contribution to surface reflectance is essentially

negligible since we only considered pixels with under ten

percent fractional canopy cover. Few, if any, coupled land-

atmosphere global or regional models capture this observed

spatial variability in surface reflectance or albedo. Here we

suggest a scheme that relates soil groups (based on the United

Nations Food and Agriculture Organization, FAO, soil

classification) and rock types (based on the United States

Geological Survey, USGS, geological maps) to MODIS derived

surface albedo statistics. This approach is a first step towards the

incorporation of the observed spatial variability in surface

reflective properties into climate models. INDEX TERMS:

3322 Meteorology and Atmospheric Dynamics: Land/

atmosphere interactions; 3337 Meteorology and Atmospheric

Dynamics: Numerical modeling and data assimilation; 3359

Meteorology and Atmospheric Dynamics: Radiative processes;

3360 Meteorology and Atmospheric Dynamics: Remote sensing

1. Introduction

[2] Surface albedo represents the fraction of incoming solarradiation reflected back to the atmosphere and space. As such, it isa basic geophysical property of land surfaces and is needed for anystudy involving the linking of terrestrial surfaces to the absorptionof solar energy. In particular, surface albedo is a required compo-nent for models of global and regional climate [e.g., Dickinson,1992; Knorr et al., 2001], or for numerical weather prediction [e.g.Viterbo and Betts, 1999]. By determining in part the absorption ofsolar energy, it helps determine the sensible, latent, and soil heatfluxes, and consequently the thermal and moisture stratification ofthe atmospheric boundary layer.

[3] We focus our study on the largest desert belt in the worldthat encompasses Northern Africa and the Arabian peninsula.Much of the region is as reflective as almost any surface on earth(with the exception of fresh snow and ice). Such highly reflectivesurfaces coupled to the overlying atmosphere can be major energysinks relative to more highly absorbing neighboring regions. Suchgradients in forcing lead to regional circulations that can suppressprecipitation and can be strongly dependent on the degree ofsurface darkening by overlying vegetation [Charney, 1975; Claus-sen and Gayler, 1997].[4] Climate models commonly specify separate albedos for

vegetation and soils and determine total surface albedo as a mixtureof the two. Many such models have based their soil albedos onWilson and Henderson-Sellers [1985] who used an earlier version ofFAO [1994] to classify world soils at 1 degree resolution as light,medium, and dark. Global climate models [e.g., Bonan, 1996] havetypically averaged these over the larger model grid-squares andstretched them to a scale of 1 (light) to 8 (dark). High albedo valuestypical for the study region have prompted the creation of anadditional land/soil class of bright or hot desert in such classificationschemes, different from the other desert regions of the world[Bonan, 1996; Henderson-Sellers, 1990]. Obviously, with increas-ingly sparse vegetation, the soil albedo becomes increasinglydominant in determining the total surface albedo. Hence, determin-ing arid region ‘‘end-members’’ is especially important. Not onlymean values but spatial variability in albedo is important for theclimate of Northern Africa [Lofgren, 1995]. Although lower reso-lution global climate models typically have meshes several hundredkm on a side, the current widespread application of regional climatemodels, and development of higher resolution versions of the globalclimate models either for land or for the atmosphere as well requiredata sets at 50 km or finer resolution.[5] Considerable spatial variability in surface albedo over

Northern Africa and the Arabian peninsula has been observed bythe MODerate resolution Imaging Spectroradiometer (MODIS), aswell as by other space borne spectroradiometers [Strugnell et al.,2001; Pinty et al., 2000]. Recently attempts have been made tointroduce this spatial variability to land surface models [Wei et al.,2001; Knorr et al., 2001]. In this paper, we relate the MODISobserved spatial variability in surface albedo to specific soil andgeological features in the study region.

2. Materials and Methods

[6] The MODIS albedo algorithm [Lucht et al., 2000; Schaafet al., 2002] couples multidate, cloud-free, atmospherically-cor-rected surface reflectance with a semiempirical, kernel-drivenBidirectional Reflectance Distribution Function (BRDF) model todetermine the anisotropy of global land surfaces at 1 km spatialresolution every 16 days. The resulting BRDFs are used to producedirectional hemispherical albedo (hereafter referred to as black-skyalbedo) and bihemispherical albedo (hereafter referred to as white-sky albedo) for seven MODIS spectral bands for land and threebroadbands (visible, near infrared, and total shortwave). Note thatblack-sky albedo represents the direct beam contribution whilewhite-sky albedo represents the entire diffuse portion and these two

GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 9, 10.1029/2001GL014096, 2002

1Department of Geography and Center for Remote Sensing, BostonUniversity, Boston, Massachusetts, USA.

2Georgia Institute of Technology, Atlanta, Georgia, USA.3Institute of Atmospheric Physics, The University of Arizona, Tucson,

Arizona, USA.4Potsdam Institute for Climate Impact Research, Potsdam, Germany.

Copyright 2002 by the American Geophysical Union.0094-8276/02/2001GL014096$05.00

67 - 1

extremes can be combined as a function of the diffuse skylight fora representation of an actual albedo such as measured by fieldinstruments [Lucht et al., 2000]. The MODIS Albedo/BRDFproduct has been operational since April 2000, and has beenreleased to the public and assigned a provisional status sinceOctober 2000. Prelaunch error statistics were estimated to be under10% [Lucht, 1998] and initial postlaunch field evaluations underclear sky and stable surface conditions support these estimates[Liang et al., 2002]. The results presented here focus on the firstreprocessing of a 16-day period starting on October 31, 2000 (Dayof Year 305). This period falls into the most desert dust-freewindow of the year in Northern Africa and the Arabian peninsula,which is from November through January [Herman et al., 1997].The other 16-day periods examined were those of November 16,December 2, and December 18, 2000.[7] Soil information for the study region comes from the United

Nations Food andAgriculture Organization, FAO, digitized soil mapof the world [FAO, 1994], which is a global soils data set compiled atthe scale of 1:5,000,000. We used soil groups rather than individualsoil types in the final analysis to allow for a more useful general-ization and identification of the clearest trends in the data. Wilsonand Henderson-Sellers [1985] used an earlier version of the samesoils data set to derive global maps of soil color and texture that arecurrently used in many land surface models [Bonan, 1996; Hender-son-Sellers, 1990]. To complement the soils information, we used anauxiliary data set of the United States Geological Survey, USGS,digitized geological maps of Africa and the Arabian peninsula[USGS, 1997, 1998], which were originally compiled at the scalesof 1:12,500,000 and 1:4,500,000, respectively. Geological units ofthe maps are based on the age of rock and in some cases the type ofrock or surface sediment (e.g., volcanics, intrusives, eolian, fluvial).[8] The three data sets (MODIS albedo, FAO soils, and USGS

geological maps) were all reprojected to Lambert Azimuthal EqualArea projection, with the center of projection at 20N 20E, and re-gridded if necessary to the spatial resolution of 1 km. A mask wasimposed, based on Friedl et al. [2002], such that only pixelsclassified as barren (i.e., with under 10 percent fractional canopycover) were used for the analysis. This barren mask is consistentwith Zeng et al. [2000].

3. Results and Discussion

[9] Spectral white-sky surface albedo for the study region isshown in Figure 1. The image is an RGB (Red-Green-Blue) true-color composite of MODIS red (band 1, 620-670 nm), green (band4, 545–565 nm), and blue (band 3, 459–479 nm) bands, with

linear stretches applied to each band. Lighter color (yellow)corresponds to the highest albedo values, i.e. the most reflectivesurfaces. Albedo is shown for the entire North Africa and theArabian peninsula, prior to application of the barren land covermask. Pixels shown in white in Figure 1 indicate areas of no orpoor quality MODIS data and were excluded from the analysis.Note that black-sky albedo at local solar noon (not shown here) isessentially identical to the white-sky albedo (Figure 1) in terms ofspatial pattern. Black-sky albedo is generally somewhat lower thanwhite-sky albedo, but the difference between the two is uniformlywell under 5%. Furthermore note that although not shown here,when both white-sky and black-sky albedo were aggregated to thecoarser spatial resolutions typical for regional and global climatemodels (i.e., 50 km and 250 km, respectively), there was stillsubstantial spatial variability in albedo over the study region. Forexample, white-sky albedo values in the total shortwave broadbandat 250 km resolution ranged from 0.184 to 0.477.[10] Figure 2, in contrast to Figure 1, shows only the pixels

classified as barren (i.e., with fractional canopy cover under 10percent). In order to minimize the vegetation signal, only thosepixels were used in our analysis. Considerable spatial variability ofalbedo in the study region can be clearly seen (Figure 1) and isassociated with geological and soil variability (Figure 2). The mostreflective areas in Figure 1 generally coincide with areas of sanddune fields and sand sheets (see Figure 3a for albedo histogram andTable 1 for albedo statistics). The next most reflective soil types areregosols and arenosols, both soils with no diagnostic pedogenichorizons (or none other than an ochric A horizon), and salinizedsoils (salt and solonetz). After examining histograms of salt andsolonetz, which were essentially identical, we combined them into asingle category (Figure 3b, Table 1). Yermosols, the most abundantsoil group in the study region, are close to salt and solonetz in meanalbedo, but have a somewhat wider spread around the mean (notshown here). This wider spread and greater standard deviation(Table 1) are related to the various degrees of soil salinizationthroughout the study region and the particular combinations of thestony/sandy/clayey fractions in any given area. Desert pavement ofvarying thickness typically forms on the surface of yermosols.[11] Several soil groups have somewhat lower albedo, and a

generally higher coefficient of variation, compared to the afore-mentioned soil groups. Among them, lithosols (Figure 3c) androck both have clearly bimodal albedo histograms. After over-laying albedo and soils data, for lithosols and rock, with theUSGS geological maps [USGS, 1997, 1998], we could clearly seethat the lower mode in both cases corresponds to quaternary andolder volcanics and intrusives and the higher mode correspondsto other types of exposed bedrock. We split these modes into

Figure 1. White-sky surface albedo at 1 km spatial resolution. The image is a true-color RGB composite of MODIS band 1 (red, with alinear stretch applied from 0 to 0.5), band 4 (green, with a linear stretch applied from 0 to 0.4), and band 3 (blue, with a linear stretchapplied from 0 to 0.4). Lambert Azimuthal Equal Area projection.

67 - 2 TSVETSINSKAYA ET AL.: RELATING MODIS-DERIVED SURFACE ALBEDO TO SOILS AND ROCK TYPES

separate categories in Table 1. At the same time, we combinedlithosols and rock, since both soil groups identify areas ofexposed bedrock (with no overlaying material in case of rockand minimal material, under 10 cm, in case of lithosols), and themeans of the two modes were essentially identical between thetwo groups. Xerosols, fluvisols (soils associated with recentalluvial deposits), and solonchaks (highly saline soils) have thered band albedo between .306 and .327 (with higher standarddeviation, between .070 and .083). Their histograms suggest oneclear mode with a considerable spread around it, which couldeither be interpreted as a very weakly pronounced second modeor a consequence of artifacts of misregistration of the maps (i.e.,MODIS albedo, FAO soils, and USGS geological maps), mis-labelling of the barren pixels, errors in the soils and geologicaldatabases, and natural inherent heterogeneity associated withspatial differences in soil water content for the particular periodof analysis. Such issues are common for all surfaces but may bemore pronounced in these soils due to their smaller sample size.[12] The darkest areas in Figure 1 correspond to vertisols,

cambisols, rendzinas, and luvisols. They are mostly located inand around the Atlas mountains, in the northwestern portion of thestudy region. Our analysis is limited to supporting modelingstudies on spatial scales of 50 km or greater so soil groups under2,500 pixels are not considered. The only region affected by thisexclusion was the high altitude deserts of the Atlas mountains,where soils tend to have smaller sample size and are generallycharacterized with lower albedo values.[13] The analysis presented in Table 1 is limited to November

2000. As mentioned earlier, we did examine other temporal slicesin the relatively desert dust-free period from November 2000through January 2001 [Herman et al., 1997] and found that albedo

is very stable over the entire period. MODIS-derived white-skysurface albedo presented here is somewhat lower (although thedifference is generally small) compared to the recent Meteosat-derived broadband surface albedo of Knorr et al. [2001], who givevalues above 0.35 over most of the Saharan Africa, with one areaabove 0.500. Our values are generally somewhat higher comparedto the recent AVHRR-derived broadband albedo of Strugnell et al.[2001] and Wei et al. [2001], who suggest albedo under 0.4 overthe Saharan region. While these differences are probably due tothe specifics of the narrow spectral band to broadband conversionsemployed by different authors, lack of atmospheric correction andpoor georeferencing characteristics of the AVHRR sensor, choiceof temporal windows selected for the analysis, and the particularsof spatial aggregation of the data, the rigor of the MODIS geo-location and calibration, and its 1 km spatial resolution lendcredibility to our results.[14] Many applications of measured albedos require their link-

age to underlying surface properties. Climate models, for exampleexamine consequences of changing vegetation covers and possiblyon longer time scales, changing soil surfaces. This paper demon-strates, at least for Northern Africa and the Arabian peninsula, thatthe MODIS data for arid surfaces can be efficiently summarized interms of readily available global distributions of soil groups andgeological land forms. Although this paper has largely shown thisrepresentation for spectral and broadband total white-sky (i.ediffuse) albedo, the conclusions hold for the only slightly differentblack-sky values (i.e. direct beam; exact values are available fromthe authors). This representation spans a wide range of surfacealbedos from the extremely bright sand dune surfaces with albedoof approximately 0.5 to the darker mode of lithosol and rocksurfaces with albedo of about 0.2.

Figure 2. FAO digitized soils map [FAO, 1994], resampled on a 1 km grid. Lambert Azimuthal Equal Area projection. Pixels shown inwhite are either non-barren (i.e., fractional canopy cover from Friedl et al. [2002] is over 10%) or soil groups of under 2,500 pixels (i.e.,gleysols, phaeozems, and kastanozems).

Figure 3. Histograms of white-sky albedo for MODIS band 1 (red, 620-670 nm), for selected FAO soil groups: (a) dunes/shifting sands,(b) salt and solonetz, (c) lithosols.

TSVETSINSKAYA ET AL.: RELATING MODIS-DERIVED SURFACE ALBEDO TO SOILS AND ROCK TYPES 67 - 3

[15] Acknowledgments. This work was supported by grants fromNASA’s EOS NRA-99-OES-04 and MODIS projects. We especially thankthe anonymous reviewer for constructive and helpful comments. We thankAlessandro Baccini and Mutlu Ozdogan of Boston University for their greathelp with image registration.

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�����������Elena A. Tsvetsinskaya, Department of Geography and Center for

Remote Sensing, Boston University, 675 Commonwealth Avenue, Boston,MA, USA. (elenat@bu.edu)

Table 1. Mean and standard deviation (in parentheses) for MODIS white-sky albedo, by FAO soil group.

AlbedoCategory

FAO Soil Group(Pixel Number)

620–670 nm 300–700 nm 700–5,000 nm 300–5,000 nm

Very High Dunes / Shifting Sands(1,748,498)

0.459 (0.053) 0.295 (0.044) 0.575 (0.053) 0.426 (0.043)

High Regosols (901,044) 0.409 (0.078) 0.272 (0.056) 0.518 (0.092) 0.384 (0.068)Arenosols (426,355) 0.374 (0.073) 0.255 (0.052) 0.479 (0.083) 0.357 (0.064)Salt (30,832) andSolonetz (1,476)

0.371 (0.057) 0.250 (0.044) 0.459 (0.063) 0.346 (0.048)

Yermosols (5,258,148) 0.367 (0.073) 0.247 (0.048) 0.470 (0.089) 0.348 (0.065)Moderate Lithosols and Rock:

Othera (2,568,322b) 0.320a (0.096) 0.217a (0.058) 0.406a (0.118) 0.302a (0.086)Xerosols (301,269) 0.327 (0.070) 0.213 (0.041) 0.423 (0.088) 0.312 (0.064)Fluvisols (157,104) 0.321 (0.083) 0.216 (0.050) 0.412 (0.100) 0.306 (0.074)Solonchaks (168,391) 0.306 (0.080) 0.225 (0.057) 0.370 (0.097) 0.284 (0.072)

Low Vertisols (18,505) 0.231 (0.043) 0.163 (0.028) 0.293 (0.057) 0.220 (0.040)Cambisols (15,368) 0.226 (0.077) 0.159 (0.051) 0.314 (0.094) 0.227 (0.071)Rendzinas (4,637) 0.211 (0.079) 0.152 (0.060) 0.272 (0.097) 0.204 (0.072)Luvisols (9,327) 0.198 (0.044) 0.133 (0.029) 0.266 (0.056) 0.196 (0.041)Lithosols and Rock:Volcanics andIntrusives (178,347b) 0.187 (0.061) 0.137 (0.039) 0.223 (0.077) 0.174 (0.056)

Number of pixels in each group is shown in parentheses following the group name. MODIS band 1 (red, 620–670 nm), the visible broadband (300–700 nm), the near infrared broadband (700–5,000 nm), and the total shortwave broadband (300–5,000 nm) are shown. Note that only pixels classifiedby Friedl et al. [2002] as barren (i.e., under 10% fractional canopy cover) were considered. Note also that the black-sky albedo at local solar noon isgenerally similar to the white-sky albedo presented here (the mean of the black-sky albedo tends to be lower than the white-sky albedo values presentedhere, but the difference between the means is always well within 5%).

a In the USGS [1997, 1998] geological maps, Precambian rocks were undivided and consisted of sedimentary, metamorphic, and igneous types, all inone category. Hence, Precambrian volcanics and intrusives (i.e., mostly the areas in the rift zone along the Red Sea), which have low albedo (similar tothat of other volcanics and intrusives) were included in the ‘‘Other’’ category, substantially decreasing the mean albedo for the category and raising thestandard deviation.

bNote that we combined Lithosols (soil group of 2,464,161 pixels) with Rock (soil group 284,617 pixels). We then split the combined groups intotwo modes: Volcanics and Intrusives (consisting of 178,347 pixels) and Other (consisting of 2,568,322 pixels).

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