The surface albedo of the Vatnajokull ice cap, Iceland: A comparison between satellite-derived and ground-based measurements

The temporal and spatial variations in the surface albedo of the Vatnajokul l ice cap, Iceland, are investigated. A time series of the surface albedo i s composed for the summer of 1996 using satellite radiance measurements fro m the Advanced Very High Resolution Radiometer (AVHRR). This time series is compared with ground measurements carried out during a glacio-meteorologic al experiment during the same summer on the ice cap. The AVHRR is able to r eproduce the development in time of the surface albedo fairly well. The lar ge systematic differences found for some of the stations on the ice are att ributed to sub-pixel-scale variations in the albedo. An attempt is made to confirm this hypothesis using satellite radiance measurements carried out b y the Thematic Mapper (TM) and measurements made with a portable albedomete r. The TM has a pixel size of 30 x 30 m whereas the pixel size of the AVHRR is 1 x 1 km. Although the TM measurements show greater variability in the albedo than do the AVHRR measurements, the large systematic difference rema ins. Measurements with the portable albedometer show a large spread in the albedo at sites with large systematic differences. This implies that the sc ale of the albedo variations is smaller than the scale of the AVHRR and TM pixels.