Abstract
We present here a simple retrieval of the areal-averaged spectral surface albedo using only ground-based measurements of atmospheric transmission under fully overcast conditions. Our retrieval is based on a one-line equation. The feasibility of our retrieval for routine determinations of albedo is demonstrated for different landscapes with various degrees of heterogeneity using three sets of measurements: (1) spectral atmospheric transmission from the Multi-Filter Rotating Shadowband Radiometer (MFRSR) at five wavelengths (415, 500, 615, 673, and 870 nm); (2) tower-based measurements of local surface albedo at the same wavelengths; and (3) areal-averaged surface albedo at four wavelengths (470, 560, 670 and 860 nm) from collocated and coincident Moderate Resolution Imaging Spectroradiometer (MODIS) observations. These integrated datasets cover both temporally long (2008–2013) and short (April–May 2010) periods at the Atmospheric Radiation Measurement (ARM) Southern Great Plains site and the National Oceanic and Atmospheric Administration (NOAA) Table Mountain site, respectively. The calculated root mean square error (RMSE), defined here as the root mean squared difference between the MODIS-derived surface albedo and the retrieved areal-averaged albedo, is quite small (RMSE ≤ 0.015) and comparable with that obtained previously by other investigators for the shortwave broadband albedo. Good agreement between tower-based measurements of daily-averaged surface albedo for completely overcast and non-overcast conditions is also demonstrated.
Highlights
Albedo, defined as a ratio of the upwelling irradiance from a surface over the downwelling irradiance reaching that surface, plays an extremely important role in the Earth’s radiation balance [1,2].To estimate the broadband and/or spectral surface albedo, both tower-based solar radiation measurements [3,4] and remote sensing methods [5,6] have been used
The identical (0.015) root mean square error (RMSE) values obtained for the retrieved areal-averaged Multi-Filter Rotating Shadowband Radiometer (MFRSR)-based and measured point surface albedo under overcast conditions suggest that the degree of surface heterogeneity is quite small for the National Oceanic and Atmospheric Administration (NOAA) site during the period considered here (April–May 2010)
In contrast to the previous studies that relied on time-consuming numerical radiative transfer (RT) simulations, our approach uses the semi-empirical equation [17], which analytically links the optical properties of an overcast cloud layer and the surface albedo with atmospheric transmission
Summary
Albedo, defined as a ratio of the upwelling irradiance from a surface over the downwelling irradiance reaching that surface, plays an extremely important role in the Earth’s radiation balance [1,2]. In contrast to limited tower-based data, ground-based measurements of downwelling irradiances (in the absence of towers) are abundant and have much larger spatial coverage These measurements have been used extensively for retrieving optical and microphysical properties of overcast clouds, such as cloud optical depth (τ) and droplet effective radius (re). Our study aims to demonstrate the retrieval feasibility for days with fully overcast and non-overcast (defined as being either clear or partly cloudy) conditions, and landscapes with various degrees of heterogeneity To this end, our efforts address three main questions: (1) How do the retrieved spectral surface albedo values compare to those obtained from independent satellite- and tower-based measurements?
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