Variability of total, black and white sky spectral albedo at the ASR central facility

We explore the variability of spectral albedo, including the breakdown of the white and black sky components, using data collected at the Atmospheric System Research (ASR) Central Facility located in north central Oklahoma. (Note that the former ARM program is now known as ASR). Black sky albedo is the reflectance of the surface from direct beam illumination only, and is a function of solar zenith angle. White sky albedo lacks a direct component and is independent of solar zenith angle. The instrumentation used for this analysis are lamp calibrated Multi-Filter Rotating Shadowband Radiometers (MFRSR). These instruments measure at six narrowband channels: 415, 500, 615, 673, 870, and 940 nm. The ASR program operates two MFRSRs in the usual up-pointing position measuring incoming fluxes. The program also operates two MFRSR heads mounted on towers in a down-pointing position measuring reflected fluxes. The two towers, while close in proximity, are installed in disparate locations. One tower is located in a natural grass/vegetation field. The instrument on this tower is mounted ~10m above the surface. The other tower is located in an active agricultural field. The data we present cover all of 2009. During this period the field produces two crops, and is plowed twice. The tower in this field is 60m in height, and the MFRSR is located at the 25m level.

To separate total albedo into its components we first identify day pairs that consist of one overcast day and one clear day. Using the surface albedo measured under overcast conditions we determine the white sky albedo. We then look at the clear day and extract the black sky component with the following equation:

Albedo_BS = [Total↑ - (Diffuse↓ * Albedo_WS)] / (Direct↓ * Cosθ)

A primary assumption for this analysis is that diffuse albedo is comparable under both cloudy and clear sky conditions. In this work we show the variability of total spectral albedo for the two different surfaces over all of 2009. We then break down selected days to show how the white and black sky albedo varies with different surface states and solar zenith angles.