Combining NLCD and MODIS to create a land cover-albedo database for the continental United States
Introduction
Land surface albedo, the ratio of upwelling to downwelling radiative flux, is the proportion of the sun's radiant energy that is reflected by the earth's surface. It affects the partitioning of the sun's energy into sensible and latent heat and ground conduction, and is considered an essential climate variable (Schaaf, Cihlar, Belward, Dutton, & Verstraete, 2008). Although radiative and non-radiative factors can affect near surface air temperature (Zhao & Jackson, 2014), increases in albedo tend to lead to a negative radiative forcing (increased surface reflection) and decreases in albedo tend to lead to the opposite effect (Barnes and Roy, 2010, Barnes et al., 2013).
Remote measurement is needed to understand spatial heterogeneity and temporal variability of albedo at broad spatial scales (Gao et al., 2005, Schaaf et al., 2008). The promise of remotely sensed albedo has encouraged research on the performance of such measurements (see overviews by Román et al., 2009, Wang et al., 2014). Román et al. (2009) found agreement between albedo measured at flux towers and satellite-derived albedo values resolved at ~ 500 m × 500 m from the Moderate Resolution Imaging Spectroradiometer (MODIS). Similarly, Wang et al. (2014) found that flux tower and MODIS albedo values agreed when the measurements were acquired over areas of spatially homogeneous land cover.
There have been several studies comparing albedo derived from climate models, which model or prescribe albedo based on land cover (Gao et al., 2014), to albedo derived from MODIS. Wang et al. (2004) found that July albedo derived from a climate model was significantly greater than July albedo derived from MODIS for the eastern United States, whereas the reverse occurred for the southeastern United States in February. Tian et al. (2004) found that albedo derived from a climate model was greater than its MODIS counterpart throughout the continental United States for July and February. Matsui et al. (2007) reported that March, July, and November albedos derived from a climate model tended to be greater than their MODIS counterpart throughout much of the United States, although July black-sky near-infrared climate-model albedo was less than its MODIS counterpart. Lawrence and Chase (2007) reported that winter albedos derived from a climate model were significantly higher in the southeastern United States and significantly lower in the north-central United States than their MODIS counterparts, and that differences in albedo resulted in significant differences in modeled precipitation.
There is a strong correlation between land cover and albedo, such that land cover class specific albedos tend to be prescribed or modeled in most climate models (Bonan, 2002, Davidson and Wang, 2004, Davidson and Wang, 2005, Oleson et al., 2013). A typical rank order of albedo in climate models is snow, bare ground (e.g., desert pavement), cropland and grassland, deciduous forest, coniferous forest (Barnes and Roy, 2010, Betts, 2000, Bonan, 2002, Coakley, 2003, Davidson and Wang, 2004, Davidson and Wang, 2005). Notwithstanding the general trend in land cover class specific albedo, there can also be considerable variation in albedo related to canopy architecture (e.g., open versus closed forest stands), intra- and inter-annual variation in the timing, persistence, and depth of snow, soil color and wetness, and vegetation composition (Bonan, 1997, Davidson and Wang, 2004, Jackson et al., 2008, Myhre et al., 2013, Ghimire et al., 2014, Loranty et al., 2014). Davidson and Wang (2004), for example, reported essentially equivalent snow-free and snow-covered albedos for broadleaf deciduous and evergreen forested stands. Similarly, Barnes and Roy (2008) reported regionally averaged snow-free albedo values for forest and mechanically disturbed (e.g., clear-cut) that were strongly overlapping.
The advent of spatially and temporally explicit satellite derived land cover and albedo datasets creates the opportunity to advance parameterization of land cover albedo. In this paper, we describe the creation of a land cover-albedo dataset derived from 14 years of snow-covered and snow-free MODIS albedo data (Schaaf et al. 2002) for 14 of 16 land cover classes in the National Land Cover Database (NLCD 2011) (Jin et al., 2013, Homer et al., 2015). The two data products are linked by using the 30 m × 30 m NLCD to identify land cover homogeneity at the nominal spatial resolution of MODIS (500 m × 500 m). The main reasons for utilizing these two datasets are: 1) land cover homogeneity at the MODIS scale can be defined precisely using the higher resolution NLCD land cover data; 2) NLCD 2011 land cover is a time series product that includes data for 2001, 2006, and 2011, making it possible to examine land cover-related albedo change, and; 3) the MODIS albedo data have been produced since 2000, making it possible to examine the temporal behavior of albedo over a relatively long time series. We demonstrate the utility of the database by conducting a multivariate analysis of variance of albedo for each of the NLCD land cover classes.
Section snippets
Data
The database was developed from three data products: albedo from the MODIS satellite, land cover from the National Land Cover Database (NLCD) developed from the Landsat satellite, and topographic information (elevation, slope, and aspect) from the National Elevation Dataset (NED). The albedo data used in this study are from the MODIS Collection 5 BRDF/Albedo 500 m product (MCD43A3) (Schaaf et al., 2002) downloaded from the U.S. Geological Survey (USGS) (http://e4ftl01.cr.usgs.gov/MOTA) for the
Results
The number of per pixel snow-covered and snow-free high quality albedo observations ranged between 1 and 605 for the continental United States (Fig. 2a). The maximum possible value, 638, was not realized for any pixel, and only ~ 3% of all pixels have ≥ 90% of the maximum possible number of observations for the study period. The number of observations declined northward and eastward from the desert southwest. During winter months, there were few snow-covered and snow-free high quality
Discussion
The NLCD-MODIS land cover-albedo database is freely available on the MRLC website (http://www.mrlc.gov). The data, organized as described in the methods, are grouped into 16 sections (Table 5). Fourteen sections are devoted to the MODIS albedo data, one section for each year. For each year, there are six layers of albedo for each of the 46 observation dates and an additional binary layer identifying a pixel as snow-covered. Another section includes the number of high quality observations. This
Acknowledgments
The United States Environmental Protection Agency, through its Office of Research and Development, partly funded and managed the research described here. The article has been reviewed by the USEPA's Office of Research and Development and approved for publication. Approval does not signify that the contents reflect the views of the USEPA. C. Barnes' participation was underwritten by contract G13PC00028 between ASRC Federal InuTeq LLC and USGS. The authors are grateful for comments on earlier
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