This study assesses the Moderate-resolution Imaging Spectroradiometer (MODIS) BRDF/albedo 8day standard product and products from the daily Direct Broadcast BRDF/albedo algorithm, and shows that ...these products agree well with ground-based albedo measurements during the more difficult periods of vegetation dormancy and snow cover. Cropland, grassland, deciduous and coniferous forests are considered. Using an integrated validation strategy, analyses of the representativeness of the surface heterogeneity under both dormant and snow-covered situations are performed to decide whether direct comparisons between ground measurements and 500-m satellite observations can be made or whether finer spatial resolution airborne or spaceborne data are required to scale the results at each location. Landsat Enhanced Thematic Mapper Plus (ETM+) data are used to generate finer scale representations of albedo at each location to fully link ground data with satellite data. In general, results indicate the root mean square errors (RMSEs) are less than 0.030 over spatially representative sites of agriculture/grassland during the dormant periods and less than 0.050 during the snow-covered periods for MCD43A albedo products. For forest, the RMSEs are less than 0.020 during the dormant period and 0.025 during the snow-covered periods. However, a daily retrieval strategy is necessary to capture ephemeral snow events or rapidly changing situations such as the spring snow melt.
•We evaluated the MCD43A Albedo during dormant and snow covered period.•Spatial representativeness analysis is necessary for the albedo evaluation.•MODIS albedo performs well during vegetation dormancy and snow cover.
Clumping index (CI), quantifying the level of foliage grouping within distinct canopy structures relative to a random distribution, is a key structural parameter of plant canopies and very useful in ...ecological and meteorological models. Previously, a Normalized Difference between Hotspot and Darkspot (NDHD) index has been proposed to accurately retrieve the global CI using POLarization and Directionality of the Earth's Reflectances (POLDER) data at ~6km resolution (Chen et al., 2005). In this study, we show for the first time a global CI map at 500m resolution derived using the Bidirectional Reflectance Distribution Function (BRDF) product from Moderate Resolution Imaging Spectroradiometer (MODIS), which currently provides the finest pseudo multi-angular data for the global land surface. In computing the NDHD we found that the hotspot calculated from the MODIS BRDF product is underestimated in comparison with POLDER measurements very near the hotspot. Without correcting the bias in MODIS data, the MODIS-derived CI could be overestimated. We have developed an approach to correct the MODIS hotspot magnitude with co-registered POLDER-3 data acquired at about the same time. Field-measured element-CIs in zenith angles of 30–60° by the Tracing Radiation and Architecture of Canopies (TRAC) instrument from 38 sites together with corresponding needle-to-shoot area ratios were used to evaluate the performance of hotspot-corrected NDHD and to tune the parameters to get accurate CI. It is found that the MODIS-derived CI has a dependence on the solar zenith angle, and this dependence in the red band is smaller than that in the near infrared (NIR) band. The highest correlation between field measurements and MODIS-derived CI is achieved when hotspot at nadir is used. The correct estimation of forest species (and therefore the crown shapes) from land cover is very important to the accuracy of MODIS-derived CI. For the red band, the coefficient of determination (R2) between MODIS-derived CI and field-measured CI is 0.76 for all sites and R2 is 0.53 for only the needle-leaved forest sites after the hotspot correction, indicating that a strong correlation between MODIS-derived NDHD and CI indeed exists. The MODIS-derived CI is regularly smaller than the field measurement. The red band in the MODIS BRDF product is found to be better than the NIR band for deriving CI for the vegetation covers investigated (mostly dense forests). Therefore a global clumping index map has been produced using the corrected MODIS hot\spot in the red band and optimized parameters. However there were 10 broad-leaved forest sites used in the validation, which are too few for a reliable validation. For the broad-leaved forest area, where the same crown shape (ellipsoid) is used in the algorithm, the global CI spatial pattern is interestingly very similar to the pattern of the land cover distribution, suggesting that the MODIS-derived map has effectively captured the structural differences among cover types.
► MODIS hotspot is corrected by POLDER-3 hotspot. ► The global clumping index map is derived at 500m resolution. ► The clumping index map is validated by 38 TRAC ground measurements.
Increasing the accuracy and spatial coverage of Leaf Area Index (LAI) values is an important part of any attempt to successfully model global atmosphere/biosphere interactions. It is further a ...fundamental parameter in land surface processes and Earth system climate models. Remote sensing methods offer an opportunity to improve on each of these requirements but are typically limited by the necessity for validation using labor intensive and sparsely collected in situ measurements.
In this paper we present the results of an intercomparative study of ground-based, airborne and spaceborne retrievals of total LAI over the conifer-dominated forests of Sierra Nevada in California. The efficacy of LVIS (Laser Vegetation Imaging Sensor) airborne waveform lidar LAI measurements (total and vertical profile) has previously been validated at the site specific level using destructive sampling. We also explore the efficacy of ground based measurements obtained from hemispherical photography, LAI-2000, and ground based lidar, acknowledging discrepancies existing between the systems and collected data. We highlight their use and role in validating the relationship between ground and airborne estimates of total LAI (LVIS LAI correlation with i) hemispherical photographs, r2=0.80, ii) LAI-2000, r2=0.85, and iii) terrestrial lidar, r2=0.76. The existence of such relationships offers immediate implications for LAI estimation where LVIS data is available, creating the potential to obtain, not only total LAI values but also corresponding vertical LAI distributions from a ground validated source previously unobtainable at this spatial scale.
The ability to validate airborne lidar LAI data collected at different spatial scales to the available ground measurements allows further upscaled validation using global lidar datasets provided by spaceborne lidar, such as the Geoscience Laser Altimeter System (GLAS). In the absence of adequate ground validation plots coincident with GLAS footprints, GLAS LAI validation is examined using geographically limited but spatially continuous LVIS data. Under favorable conditions, significantly the absence of slopes greater than ~20°, the comparison between LVIS and GLAS LAI values obtained using a recursive algorithm constrained by independently validated LAI limits exposes the capability of GLAS as an accurate standalone LAI sensor (r2=0.69, bias=−0.05 and RMSE=0.33). The correlation comparison between LVIS and GLAS LAI estimates not only significantly exceed those associated with equivalent space borne passive remote sensing datasets, such as MODIS (r2=0.20, bias=−0.16 and RMSE=0.67) but also offers significant advantages to future research including the prospective validation of regional and global LAI products and data comparison with ecosystem model inputs. The encountered effectiveness of these relationships allows the implementation of a scaling-up strategy where ground-based LAI observations are related to aircraft observations of LAI, which in turn are used to validate GLAS LAI derived from coincident data. Successful implementation of this strategy paves the way for the future recovery of vertical LAI profiles on a global scale and opens up the potential for fusion studies to incorporate widely available and spatially abundant passive optical datasets.
•We derive independent LAI data from terrestrial, airborne and spaceborne lidar.•Airborne lidar is used as a validation link between field and satellite data.•Our method can correct part of the slope effect in LAI retrieval.
A thorough evaluation of the capability of modeling vegetation lidar returns is a critical aspect of deriving vegetation structure from lidar measurements. This study assesses the performance of the ...Analytical Clumped Two-Stream (ACTS) canopy radiative transfer model to simulate large-footprint lidar waveforms. Modeled lidar waveforms were compared to airborne Laser Vegetation Imaging Sensor (LVIS) data collected in deciduous and conifer forests: Harvard Forest, MA; Bartlett Experimental Forest, NH; and Howland Experimental Forest, ME. The simulated and LVIS lidar waveforms have coefficients of determination R2>0.9 and RMSE~0.01 at both plot and stand level for most sites. The ACTS model also produces realistic multi-peak returns from vegetation for the multi-layer and multi-species canopies with R2~0.79–0.86 and RMSE~0.01 between the simulated and LVIS waveforms. This validation work lays the foundation to retrieve vegetation structure and above-ground biomass directly from lidar waveforms through model inversion with the ACTS model.
•The ACTS canopy radiative transfer model was used to simulate lidar waveforms.•The modeled waveforms were compared with airborne LVIS data in temperate forests.•Coefficients of determination R2>0.9 and RMSE~0.01 at both plot and stand levels
A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and ...reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at local solar noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.
This study assesses the MODIS standard Bidirectional Reflectance Distribution Function (BRDF)/Albedo product, and the daily Direct Broadcast BRDF/Albedo algorithm at tundra locations under large ...solar zenith angles and high anisotropic diffuse illumination and multiple scattering conditions. These products generally agree with ground-based albedo measurements during the snow cover period when the Solar Zenith Angle (SZA) is less than 70°. An integrated validation strategy, including analysis of the representativeness of the surface heterogeneity, is performed to decide whether direct comparisons between field measurements and 500-m satellite products were appropriate or if the scaling of finer spatial resolution airborne or spaceborne data was necessary. Results indicate that the Root Mean Square Errors (RMSEs) are less than 0.047 during the snow covered periods for all MCD43 albedo products at several Alaskan tundra areas. The MCD43 1-day daily albedo product is particularly well suited to capture the rapidly changing surface conditions during the spring snow melt. Results also show that a full expression of the blue sky albedo is necessary at these large SZA snow covered areas because of the effects of anisotropic diffuse illumination and multiple scattering. In tundra locations with dark residue as a result of fire, the MODIS albedo values are lower than those at the unburned site from the start of snowmelt.
► We evaluated the MCD43A Albedo at tundra under large solar zenith angles. ► Spatial representativeness analysis is needed for the albedo evaluation. ► The full expression blue sky albedo match the ground measurements very well.
A ground-based, upward-scanning, near-infrared lidar, the Echidna® validation instrument (EVI), built by CSIRO Australia, retrieves forest stand structural parameters, including mean diameter at ...breast height (DBH), stem count density (stems/area), basal area, and above-ground woody biomass with very good accuracy in six New England hardwood and conifer forest stands. Comparing forest structural parameters retrieved using EVI data with extensive ground measurements, we found excellent agreement at the site level using five EVI scans (plots) per site (
R
2
=
0.94–0.99); very good agreement at the plot level for stem count density and biomass (
R
2
=
0.90–0.85); and good agreement at the plot level for mean DBH and basal area (
R
2
=
0.48–0.66). The observed variance at site and plot levels suggest that a sample area of at least 1
ha (10
4
m
2) is required to estimate these parameters accurately at the stand level using either lidar-based or conventional methods. The algorithms and procedures used to retrieve these structural parameters are dependent on the unique ability of the Echidna® lidar to digitize the full waveform of the scattered lidar pulse as it returns to the instrument, which allows consistent separation of scattering by trunks and large branches from scattering by leaves. This successful application of ground-based lidar technology opens the door to rapid and accurate measurement of biomass and timber volume in areal sampling scenarios and as a calibration and validation tool for mapping biomass using airborne or spaceborne remotely sensed data.
► Ground-based, full-waveform, scanning Echidna lidar retrieves mean tree diameter, stem density, basal area, and above-ground biomass with R2=0.94-0.99. ► We scanned six 1-ha sites, five scans per site, in three New England forests including deciduous and coniferous stands. ► Best agreement is obtained when five scans are averaged to represent one hectare. ► Occlusion of far trunks hidden behind near trunks is corrected using geometrical probability theory. ► Biomass is estimated by a pooled allometric equation using the two leading dominant species at the site.
The recent paper by Wang and Zender Wang, X., & Zender, C. S. (2010). MODIS snow albedo bias at high solar zenith angles relative to theory and to in situ observations in Greenland. Remote Sensing of ...Environment. draws erroneous conclusions about solar zenith angle biases at high latitudes by not making appropriate use of the extensive quality flags available with the MODIS BRDF/Albedo. Coarse resolution MODIS white-sky albedo data are compared with actual blue-sky field albedometer measurements from the Greenland GC-Net. By utilizing large area averages of the MODIS data product that combine both high quality and poor quality data indiscriminately, the authors erroneously conclude that the accuracy deteriorates for solar zenith angle (SZA)
>
55° and often becomes physically unrealistic for SZA
>
65°. Once the quality flags are considered, however, the comparisons demonstrate that the MODIS product performs quite well out to the recommended limit for product use of 70° SZA. This verifies the conclusions of an earlier more rigorous evaluation performed by Stroeve et al. Stroeve, J., Box, J. E., Gao, F., Liang, S., Nolin, A., & Schaaf, C. B. (2005). Accuracy assessment of the MODIS 16-day albedo product for snow: comparisons with Greenland in situ measurements. Remote Sensing of Environment.. With over a decade of observations and products now available from the MODIS instrument, these data are increasingly being used to evaluate and tune climate and biogeochemical models. However, such use should take into account the documented quality and limitations of the satellite-derived product.
Spectral albedo may be derived from atmospherically corrected, cloud-cleared multiangular reflectance observations through the inversion of a bidirectional reflectance distribution function (BRDF) ...model and angular integration. This paper outlines an algorithm suitable for this task that makes use of kernel-based BRDF models. Intrinsic land surface albedos are derived, which may be used to derive actual albedo by taking into account the prevailing distribution of diffuse skylight. Spectral-to-broadband conversion is achieved using band-dependent weighting factors. The validation of a suitable BRDF model, the semiempirical Ross-Li (reciprocal RossThick-LiSparse) model and its performance under conditions of sparse angular sampling and noisy reflectances are discussed, showing that the retrievals obtained are generally reliable. The solar-zenith angle dependence of albedo may be parameterized by a simple polynomial that makes it unnecessary for the user to be familiar with the underlying BRDF model. The algorithm given is that used for the production of a BRDF/albedo standard data product from NASA's EOS-MODIS sensor, for which an at-launch status is provided. Finally, the algorithm is demonstrated on combined AVHRR and GOES observations acquired over New England, from which solar zenith angle-dependent albedo maps with a nominal spatial resolution of 1 km are derived in the visible band. The algorithm presented may be employed to derive albedo from space-based multiangular measurements and also serves as a guide for the use of the MODIS BRDF/albedo product.
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