Broadband albedo of Arctic sea ice from MERIS optical data

Abstract. Summer in the Arctic is the season when the sea icecovered ocean experiences rapid changes in its sea ice concentration, the surface albedo, and the melt pondfraction. These processes drastically affect the energy balance of the region and it is a challenge for climate modelsto represent those correctly. In this paper, the broadband albedo(300–3000 nm) of Arctic sea iceis derived from Medium Resolution Imaging Spectrometer(MERIS) optical swath data by transforming the spectral albedoas an output from the Melt PondDetector (MPD) algorithm by a newly developed spectral-to- broadbandconversion (STBC). The new STBC replaces the previously applied spectral averaging method to provide a more accurate broadband albedoproduct which approaches the accuracy of 0.02–0.05 required in climate simulations and allows a direct comparison to broadband albedovalues from climate models. The STBC is derived empirically from spectral and broadband albedomeasurements over landfast ice. It is validated on a variety of simultaneous spectral and broadbandfield measurements over Arctic sea ice, is compared to existing conversion techniques and shows a better performance than the currently published algorithms. The root mean square deviation(RMSD) between measured and broadband albedoconverted by the STBC is 0.02. Other conversion techniques, the spectral averaging method and the linear combination of albedovalues from four MERIS channels, achieve higher RMSDs of 0.09 and 0.05. The improved MERIS derived broadband albedovalues are validated with airborne measurements. Results show a smaller RMSD of 0.04 for landfast ice than the RMSD of 0.07 for drifting ice. The MERIS derived broadband albedois compared to broadband albedofrom ERA5 reanalysis to examine the albedoparameterization used in ERA5. Both albedoproducts agree in the large-scale pattern. However, consistency in point-to-point comparison is rather poor, with correlations between 0.71 and 0.76 and RMSD in excess of 0.12. This suggests that the climatological sea ice albedovalues used in ERA5 are not adequate and need revising, in order to better simulate surface heat fluxes in the Arctic. The advantage of the resulting broadband albedodata set from MERIS against other published data sets is the additional data set of melt pondfraction available from the same sensor. Melt pondsare the main reason for the sea ice albedochange in summer but currently are not represented in climate models. Additional information on melt evolution together with the accurate albedoproduct can aid the challenging representation of sea iceoptical properties in summer in climate models.