Development of a high spectral resolution surface albedo product for the ARM Southern Great Plains central facility

Abstract

Abstract. We present a method for identifying dominant surface type and estimating high spectral resolution surface albedo at the Atmospheric Radiation Measurement (ARM) facility at the Southern Great Plains (SGP) site in Oklahoma for use in radiative transfer calculations. Given a set of 6-channel narrowband visible and near-infrared irradiance measurements from upward and downward looking multi-filter radiometers (MFRs), four different surface types (snow-covered, green vegetation, partial vegetation, non-vegetated) can be identified. A normalized difference vegetation index (NDVI) is used to distinguish between vegetated and non-vegetated surfaces, and a scaled NDVI index is used to estimate the percentage of green vegetation in partially vegetated surfaces. Based on libraries of spectral albedo measurements, a piecewise continuous function is developed to estimate the high spectral resolution surface albedo for each surface type given the MFR albedo values as input. For partially vegetated surfaces, the albedo is estimated as a linear combination of the green vegetation and non-vegetated surface albedo values. The estimated albedo values are evaluated through comparison to high spectral resolution albedo measurements taken during several Intensive Observational Periods (IOPs) and through comparison of the integrated spectral albedo values to observed broadband albedo measurements. The estimated spectral albedo values agree well with observations for the visible wavelengths constrained by the MFR measurements, but have larger biases and variability at longer wavelengths. Additional MFR channels at 1100 nm and/or 1600 nm would help constrain the high resolution spectral albedo in the near infrared region.

Areal-Averaged Spectral Surface Albedo in an Atlantic Coastal Area: Estimation from Ground-Based Transmission

“…These three types of surface have distinct visual appearances: "brown" (soil), "green" (vegetation) and "navy and white" ocean associated with the open ocean, whitecaps at high wind speed and breaking waves near shore. Similar differences occur with regard to the type-dependent surface albedo [15][16][17]. The observed surface heterogeneity at the coastal AMF site is much higher than those typical at the continental sites with available tower-based measurements [8,16].…”

“…Similar differences occur with regard to the type-dependent surface albedo [15][16][17]. The observed surface heterogeneity at the coastal AMF site is much higher than those typical at the continental sites with available tower-based measurements [8,16]. Thus, the observed surface heterogeneity is too complicated for an accurate assessment of the areal-averaged albedo from tower-based measurements.…”

“…As a result, the measured transmission at 0.415 µm wavelength represents conditions with a "black" surface where the relative contribution of the surface reflectance to the downward radiation is small. In contrast, the surface albedo for the same surface can be large (up to 0.5) at 0.87 µm wavelength [10,16]. Thus, the measured transmission at 0.87 µm wavelength represents conditions with a relatively "bright" surface where the relative contribution of the surface reflected radiation to the measured downwelling radiation is large.…”

“…This classification of surface types together with the associated albedos [21] are commonly applied to estimate spectrally-resolved areal-averaged albedo (A a λ ) using a weighted average approach [16,22]…”

“…The associated values of the spectrally resolved surface albedo are available for different surface types from previous studies [15][16][17]23,24]. We use parameterizations of ocean [15], vegetation [23] and soil [16] surface albedo to approximate the albedo values for three major surface types observed near the AMF site (Table 1).…”