Comparison of Modis and Misr-derived Surface Albedo with in Situ Measurements in Greenland

Accurate estimation of snow albedo is essential for monitoring the state of the cryosphere. The high albedo of snow-covered surfaces allows little energy to be absorbed by the snowpack. However, as snow ages and/or begins to melt the albedo decreases and more energy is absorbed by the snowpack. This paper examines the retrieval of snow albedo from the MISR and MODIS instruments over the Greenland ice sheet. Two different methods are used to compute the albedo. The first method uses spectral information from MISR and MODIS. The second method uses angular information from MISR to develop a statistical relationship between in situ albedo and the red channel reflectance at all nine MISR viewing angles. Good agreement with in situ measurements is found using either method, although problems with instrument calibration, snow BRDF models and narrow-to-broadband albedo relationships can cause the albedo error to still be large with the first method. With a few exceptions, the satellite-derived surface albedo is within about 6% of that measured at the stations. INTRODUCTION The surface albedo is an important climate parameter, as it influences the amount of solar radiation absorbed by the surface. Satellite remote sensing offers a means for measuring and monitoring the surface albedo of snow-covered surfaces. Several studies have attempted to estimate the albedo over snow in the polar regions using visible and near infrared (NIR) data from sensors such as the NOAA Advanced Very High Resolution Radiometer (AVHRR) [e.g. (1)-(2)]. Snow albedo is also one of the standard products to be generated in the near future from data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument flown on the Terra satellite (3). Snow albedo from instruments such as AVHRR and MODIS rely on using variations in spectral reflectance to derive the albedo. With the Multiangle Imaging SpectroRadiometer (MISR), also flown on Terra, the possibility exists to use the angular signatures in addition to spectral signatures for highresolution snow albedo retrievals. This study evaluates snow surface albedo retrievals from MISR and MODIS through comparisons with ground-based surface albedo measurements obtained in Greenland. Two different techniques will be examined for deriving the surface albedo. One using angular information from MISR and the other based on spectral information from both MISR and MODIS. Data from automatic weather stations (AWS) during 2000 and 2001, in addition to other in situ data collected during summer 2000 provide the ground-based measurements with which to compare coincident clear-sky satellite albedo retrievals. Proceedings of EARSeL-LISSIG-Workshop Observing our Cryosphere from Space, Bern, March 11 – 13, 2002 89