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Albedo is a key variable in the response of glaciers to climate. In Iceland, large albedo variations of the ice caps may be caused by the deposition of volcanic ash (tephra). Sparse in situ measurements are insufficient to characterize the spatial variation of albedo over the ice caps due to their large size. Here we evaluated the latest MCD43 MODIS albedo product (collection 6) to monitor albedo changes over the Icelandic ice caps using albedo measurements from ten automatic weather stations on Vatnajökull and Langjökull. Furthermore, we examined the influence of the albedo variability within MODIS pixels by comparing the results with a collection of Landsat scenes. The results indicate a good ability of the MODIS product to characterize the seasonal and interannual albedo changes with correlation coefficients ranging from 0.47 to 0.90 (median 0.84) and small biases ranging from −0.07 to 0.09. The root-mean square errors (RMSE) ranging from 0.08 to 0.21, are larger than that from previous studies, but we did not discard the retrievals flagged as bad quality to maximize the amount of observations given the frequent cloud obstruction in Iceland. We found a positive but non-significant relationship between the RMSE and the subpixel variability as indicated by the standard deviation of the Landsat albedo within a MODIS pixel (R = 0.48). The summer albedo maps and time series computed from the MODIS product show that the albedo decreased significantly after the 2010 Eyjafjallajökull and 2011 Grímsvötn eruptions on all the main ice caps except the northernmost Drangajökull. A strong reduction of the summer albedo by up to 0.6 is observed over large regions of the accumulation areas. These data can be assimilated in an energy and mass balance model to better understand the relative influence of the volcanic and climate forcing to the ongoing mass losses of Icelandic ice caps.We greatly thank Crystal Schaaf and Qingsong Sun from University of Massachusetts Boston for providing us MCD43A V006 data before the official release. Data from in situ mass balance surveys and on glacier automatic weather stations come from joint projects of the National Power Company and the Glaciology group of the Institute of Earth Science, University of Iceland. The staff members at the Institute of Earth Sciences, University of Iceland were funded by the the Icelandic Research Fund (project SAMAR 140920-053) and the National Power Company of Iceland. Simon Gascoin and Etienne Berthier are supported by the CNES through the TOSCA program. We thank the three anonymous reviewers and the editor of this special issue Frank Paul for their constructive comments during the peer-review process.Peer Reviewe

Evaluation of MODIS Albedo Product over Ice Caps in Iceland and Impact of Volcanic Eruptions on Their Albedo