The Areal Averaged Albedo VAP yields areal averaged surface spectral albedo estimates from MFRSR measurements collected under fully overcast conditions via a simple one-line equation (Barnard et al., 2008), which links cloud optical depth, normalized cloud transmittance, asymmetry parameter, and areal averaged surface albedo under fully overcast conditions. The corresponding algorithm has been initially evaluated for multi-year MFRSR data collected at ARM SGP site (Kassianov et al. 2014).
Traditionally, combined sets of tower-based downward- and upward-looking instruments are used for measuring the surface albedo. The tower measurements observe the albedo over a very small area and can be considered local measurements. However, the measured local albedo may not be representative of a larger area averaged surface albedo that encompasses an inhomogeneous surface with many different local albedos. Because most radiative transfer calculations require an area averaged albedo, the use of a local albedo may be responsible for large uncertainties in these calculations. Moreover, the tower-based measurements of surface albedo are limited to SGP and NSA sites, and the TCAP AMF deployment.
In contrast to these well-established sites, the ARM Mobile Facilities (AMFs) have been deployed for many locations around the world. The AMFs have a suite of instruments for measuring solar radiation at surface, including the Multi-Filter Rotating Shadowband Radiometer (MFRSR). The MFRSR measurements have been used extensively for retrieving optical and microphysical properties of clouds, such as cloud optical depth. However, downward-looking spectral measurements are unavailable at most AMF deployments, and even when available these only provide the capacity to measure local albedos.