Kilometer-ScaleGlobal Albedo from MODI Wolfgang Wanner (l), Alan H. Strahler (l), Baojin Zhang (1), and Philip Lewis (2) (1) Center for Remote Sensing, Boston University, 725 Commonwealth Avenue, Boston, MA 02215, USA, Tel. +1-617-353-2088,Fax +1-617-353-3200,e-mail
[email protected],
[email protected],
[email protected] (2) Remote Sensing Unit, University College London, 26 Bedford Way, London, WClH OAF’, UK, Tel. 44-171-387-7050-5557, Fax 44-171-380-7565, e-mail
[email protected] Abstract - Directional-hemisphericaland bihemispherical spec- sampling available is deemed insufficient. Both albedo and tral and broadband l-km land albedos will be availablefrom the BRDF will be delivered with a full range of quality controls MODIS BRDF/albedo product routinely starting in 1998. providing information about the extent of angular sampling that went into the retrieval and expected error margins. INTRODUCTION
BLACK-SKY AND WHITYE-SKY ALBEDO In climate and weather modeling, three key parameters related to the land surface that need to be taken into consideration are the aerodynamic roughness length, surface resistance, and albedo. Of these, we will provide bimonthly global albedo to the user community at a spatialresolution of one kilometer derived from Earth Observing System @OS) MODIS and MISR sensor data in form of the MODIS BRDF/Albedo Product [11. ”HE MODIS BRDF/ALBEDO PRODUCT
Land surface albedo provides a first-order feedback of radiation into the atmosphere. It characterizes the average or in.. tegral reflective properties of a land surface under prevailing illumination conditions as the integral ratios of upwelling ancl downwelling radiances, where illuminationis assumed not only from the direction of the sun but the hemispherical distribution of diffuse skylight due to atmospheric aerosols is fully taken into account. It is in this form that land surfacealbedo enters into theories of radiation transport in the atmosphere, such as are used in climate models and in atmospheric correction. However, due to the dependence of the downwelling irradiance and hence of the albedo on atmospheric state, this expression of albedo is of use only in applications that require knowledge of albedo at thle exact time of satellite overpass. If albedo is to be used in climate and biosphere models, it needs to be decoupled from the atmospheric state found during the satellite observations and expressed as an intrinsic surface property that may later be coupled to a different atmospheric state. The MODIS BRDF/albedo product takes this requirement into account by providing two measures of albedo that are intrinsic surface properties. Each describes one of two Iimi1.ing cases: the case of absence of diffuse skylight, i.e., dimtional illumination from the sun alone, and the case of perfectly diffuse illumination. The former, termed “black-sky albedo?’ (abs),is a function of solar zenith angle. The latter, termed “white-sky albedo” ( a w s )is, a constant. These albedos xe derived as the directional-hemispherical and the bihemispherical integrals of the surface bidirectionalreflectance distribution function (BRDF):
In 1998,the cross-trackscanning MODIS sensor and the alongtrack imaging MISR sensor will be launched on board NASA’s EOS-AM-1 platform. Data from both of these sensors will be combined to rountinely produce global databases of bidirectional reflectance and albedo. Albedos and BRDF will be given in seven MODIS spectral bands centered at 460,555,659,865, 1240, 1640, and 2130nm, of which the first four have corresponding MISR spectral bands. Additionally, albedos will be produced in two broadband ranges, divided by 700nm, and for the total spectral range. The product will provide both black-sky (directional-hemispherical) and white-sky (bihemispherical) albedo, defined below. Black-sky albedo will be given parametrized for its solarzenith angle dependence, allowing recovery of values for any solar zenith angle using a small look-up table that will be delivered with the product. B?ack-sky albedo will also be given explicitly for the median sun-mgi :of the observations, which is a function of geographic latitude and the time of year. Additionally, white-sky albedo will be provided. The temporal resolution of the product will be 16 days (the MISR two-look repeat time). The spatial resolution will be one kilometer, allowing reliable deduction of subresolution variability of albedo for use in coarse-resolution grids (for example in p(Q, , Q, , 4; dX) cos 0, sin 0, do, C@ GCMs) in regions of small-scalelandscape heterogeneity. x The MODIS BRDF/albedo product will provide the BRDF 0 0 of each pixel by giving the parameters of one of six semiemn/2 pirical kemel-driven BRDF models [1,2], where each model is cr,,(d~) = 2 ab,(B,;dX)sin8,cos8,dB,, suited for a structurally different type of land cover and is chosen after consideration of the RMSEs found for each model in 0 inverting the ObServed multiangle reflectances. The land cover where Qs and 0, are solar and view zenith angles, respectively, class of the pixel is used for choosing a model if the angular 4 the relative azimuth between viewing and solar direction, tlX This work was funded under the NASA MODIS project, NAS5-31369. is the wave band, and p is a.BRDF.
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0-7803-3068-4/96$5.000 1996 IEEE
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ALGORITHM THEORETICAL BASIS
Albedos will be derived through integrationof the BRDFs obtained from inverting atmospherically corrected multiangular reflectances against BRDF models. The combined MODIS and MISR observation geometry makes such inversions feasible. The models used are semiempirical kernel-driven BRDF models employingkernel functions derived from physical considerationsof theories of volume and surface scatteringof light. They are the RossThin-LiSparse,the RossThick-LiSparse, the RossThin-LiDense, the RossThick, and the LiDense models, described by Wanner et al. [2]. Additional provisions are made for snow and water reflectances. These models have been shown to provide adequate fits to field-observed BRDF data [3].
NOISESENSITIVITY ANALYSIS The sensitivity of the albedos derived to random noise is dependent on the angular distribution of the reflectance samples obtained. This distribution is specific to the orbital and instrumental characteristics of the MODIS and MISR sensors, and varies with latitude and time of year. Using the xsatview software developed by Bamsley and Moms, we have simulated the viewing geometries encountered in each 16-day period by MODIS and MISR, and have investigated the sensitivityof various parameters to random noise. Due to the mathematical form of the BRDF models used, this analysis could be carried out analytically using a theory of deriving so-called weights of determination by Gauss (see [4], and [SI at this conference). This analysis is independent of the form of the BRDF observed. Fig. 1 shows on the top left the dependence of expected black-sky albedo error on latitude for four different days of the year (left), where black-sky albedo has been derived at the mean sun angle during the observations and assuming the RossThick-LiSparse model as an example. A random noise RMSE in fitting of 10 percent was assumed in producing these plots, but note that the linear nature of kernel-driven models implies that the error scales linearly with the RMSE. As can be seen, the error in albedo is smaller than the observation error, signifying a stable retrieval of albedo at the prevailing sun zenith angle. There is a slight dependence on sampling latitude. On the top right, Fig. 1 shows, for different latitudes, the error expected when predictingblack-sky albedo at different solar zenith angles, where the observations were made at the respective solar zenith angle givingby sampling and at different latitudes. The plot shown is for sampling on the first day of the year. All curves show best retrievals in the zenith angle range where the sun is actually to be found in the sky during the observations. Extrapolationof black-sky albedo to sun zeniths of more than about 60 degrees is problematic. On the lower left, Fig. 1 shows the white-sky albedo error, again using the example of the RossThick-LiSparse model and assuming a 10 percent noise RMSE. Deriving white-sky albedo requires extrapolationof the BRDF to angles where no observations were made, hence the error is larger than for the black-sky albedo at mean sun angle. But still it is less than the noise level; it depends strongly on the latitude. Observe the shift of error as the zenith of the sun shifts with season.
A numerical 3D discrete ordinate radiative transfer code provided by Myneni [61 was used to generate different types of forward BRDFs. These were BRDFs of 6 land cover types (biome types), such as grassland/cereal crops, brushland, or forest. The MODIS BRDF/albedo models were then inverted for 16-day MODISFlISR sampling as a function of latitudeand time of year. The purpose of this study was to investigate success and failure of model interpolationand extrapolation under realistic sampling conditions. Fig. 1 shows, in the middle panels, results for black-sky albedo using the example of a grassland/cereal crop BRDF and sampling on day of the year 192. The red band results are shown on the left, the NIR results on. the right. Black-sky albedo was retrieved at the mean sun zenith angle of the observations and at 10 degrees solar zenith, requiring extrapolationof the observed BRDF. In the red, the absolute error is less than about 0.002 in most cases, with albedo values of typically 0.036. This gives an error of about 5 percent. The extrapolated albedos are better if the zenith extrapolated to is close to the actual mean zenith of observations. In the NIR, albedo at the mean sun zenith is mostly accurate to about 0.02, with albedos of about 0.42. This again is an accuray of about 5 percent. The accuracy of albedos extrapolated to 10 degrees solar zenith is about 10 percent. The lower right panel shows that the accuracy of white-sky albedo retrieval varies with latitude. At favorable latitudes, it may be retrieved to within a few percent, or 0.01 in the red and 0.05 in the NIR, given true values of 0.037 and 0.44, respectively. At less favorablelatitudesthe errors are up to 0.02 in the red and 0.07 in the NIR, or 45 and 16 percent. There is, however, a clear bias to overestimatingthe white-sky albedo. Improvements of this aspect of the models used are necessary and will possibly allow improved retrievals in the future. One should also keep in mind that these results completely depend on the assumption that the numerical forward model produces BRDFs that resemble those found in nature at all zenith angles. Accuracies of the actual product may therefore differ. REFERENCES [ l ] Strahler, A. H. et al., “MODIS BRDF/Albedo product: Algorithm Technical Basis Document”, version 3.2 and update, NASA EOS MODIS Doc., 65 pp., 1995. [2 ] Wanner, W., X. Li, and A. H. Suahler, “On the derivation of kernels for kernel-drivenmodels of bidir. reflectance,” J. Geophys, Res., 100,pp. 21,077-21,090,1995. [3 1 Hu, B., W. Wanner, X.Li, and A. H. Strahler, “Validation of kernel-driven semiempirical BRDF models for application to MODIS/MISR data,” this conference. 141 Whittaker, E., G. Robinson, “The Calculus of Observations,’’ Blachie and Son, Glasgow, 397 pp., 1960. 15 ] Wanner, W., P. Lewis, and J.-L. Roujean, “The influence of directional sampling on bidirectional reflectance and albedo retrieval using kernel-driven models”, this conf. 16 ] Myneni, R. B., Asrar, G., and E G. Hall, “A 3D radiative transfer method for optical remote sensing of vegetated land surfaces,”Rem. Sens. Env., 41, pp. 105-121,1992.
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