Areal-Averaged Spectral Surface Albedo at ARM ENA Coastal Site: Estimation from Ground-based Transmission

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Description

The surface albedo affects substantially the variability of the Earth’s radiation balance, and this variability is sensitive to multiple natural and man-made factors. The assessment of these variations may be very challenging for heterogeneous surfaces with complex landscapes consisting of more than one land cover type. Conventionally, the assessment is based on the combined use of “point” tower-based measurements of surface albedo together with high-resolution remotely sensed satellite images. Since the tower-based albedo data are available only at several well-established sites, application of this conventional approach is quite limited. In this context, the development of additional tools for obtaining the areal-averaged albedo product is highly desirable. Recently, we have developed a simple, ground-based retrieval of the areal-averaged and spectrally-resolved surface albedo (Kassianov et al., 2014) to address the abovementioned issues associated with the complexity and heterogeneity of the landscape of interest. Our approach involves atmospheric transmission measured by the Multi-Filter Rotating Shadowband Radiometer (MFRSR) at five wavelengths (415, 500, 615, 675, and 870 nm) under fully overcast conditions. For relatively homogenous surfaces at the ARM SGP and NOAA Table Mountain site, the agreement between the spectrally-resolved values of surface albedo retrieved by our MFRSR-based approach, and those measured by towers and derived from MODIS observations, is good. Here, we illustrate how our simple retrieval can be extended to the even more challenging problem of getting the areal-averaged and spectrally-resolved surface albedo in coastal areas with highly heterogeneous surface (land and adjusted water areas). In particular, we use the MFRSR data collected during the 19-month (June 2009 – December 2010) period at the ARM Eastern North Atlantic (ENA) coastal site. The surface albedo obtained by three methods (MFRSR retrieval, MODIS data and composite-based albedo) exhibit similar spectral signatures with a marked drop in the surface albedo around 660 nm wavelength. Depending on the choice made, a good agreement between the composite-based and areal-averaged MFRSR albedos can be accomplished within a given level of acceptable uncertainty. An extension of our retrieval to hyperspectral ground-based instruments with fine spectral resolution, such as SAS-He, is discussed as well.