Accuracy of NOAA AVHRR-based surface reflectance over a winter-time borealsurface - comparison with aircraft measurements and land-cover information

Polar-orbiting NOAA14 AVHRR (Advanced Very-I-EghResolution Radiometer) base d estimates of surface reflectance, with a spatial resolution of about 1.1 km at nadir, were derived for a single almost cloud-free image of snow-cove red, partly-forested boreal terrain in northern Finland. As a part of the W INTEX campaign, comparative measurements of reflectance were made along thr ee 30 km flight legs with downward and upward-facing broadband pyranometers installed in a C-130 aircraft. After co-location of the satellite sensor p ixels and the airborne samples, the two estimates of broadband (0.3-3 mum) surface reflectance matched to within 2%, with a standard error of 4.4% (ab solute units). The reflectance variation obtained by the aircraft along the flight legs was well matched by the satellite-based method. A method was developed for generating a parametrized regional distribution of surface reflectance by assigning random reflectance values to the main l and-cover categories. These random values, for the three land-use classes: 'open', 'transitional' and 'forest', were drawn from the approximated norma l distribution of reflectances based on the measurements from an aircraft f lying at a height of 80m above the surface. The reflectance distribution fo r the 100 km x 100 km region thus obtained using the land-cover data set wa s quite similar to the corresponding satellite-based distribution for the w inter day-time snow-covered terrain analysed here. During winter, however, frequent changes in reflectance distributions are to be expected due to the variation of snow interception on trees and snow ageing. It is thus sugges ted that the parametrized distributions of surface reflectance should be co mplemented with real-time satellite-based estimates to account for the dail y variation of surface reflectance, particularly when the snow cover is cha nging rapidly.