Broadband albedo of Arctic sea ice from MERIS optical data

• Main Authors: Pohl, Christine, Istomina, Larysa, Tietsche, Steffen, Jäkel, Evelyn, Stapf, Johannes, Spreen, Gunnar, Heygster, Georg
• Format: Text
• Language: English
• Published: 2019
• Subjects: Arctic; albedo; Sea ice
• Online Access: https://doi.org/10.5194/tc-2019-62; https://www.the-cryosphere-discuss.net/tc-2019-62/

Summer in the Arctic is the season when the sea ice covered ocean experiences rapid changes in its sea ice concentration, the surface albedo, and the melt pond fraction. These processes drastically affect the energy balance of the region and it is a challenge for climate models to represent those correctly. In this paper, the broadband albedo (300–3000 nm) of Arctic sea ice is derived from Medium Resolution Imaging Spectrometer (MERIS) optical swath data by transforming the spectral albedo as an output from the Melt Pond Detector (MPD) algorithm by a newly developed spectral-to-broadband conversion (STBC). The new STBC replaces the previously applied spectral averaging method to provide a more accurate broadband albedo product which approaches the accuracy of 0.02–0.05 required in climate simulations and allows a direct comparison to broadband albedo values from climate models. The STBC is derived empirically from spectral and broadband albedo measurements over landfast ice. It is validated on a variety of simultaneous spectral and broadband field 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 albedo converted by the STBC is 0.02. Other conversion techniques, the spectral averaging method and the linear combination of albedo values from four MERIS channels, achieve higher RMSDs of 0.09 and 0.05. The improved MERIS derived broadband albedo values 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 albedo is compared to broadband albedo from ERA5 reanalysis to examine the albedo parameterization used in ERA5. Both albedo products 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 albedo values 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 albedo data set from MERIS against other published data sets is the additional data set of melt pond fraction available from the same sensor. Melt ponds are the main reason for the sea ice albedo change in summer but currently are not represented in climate models. Additional information on melt evolution together with the accurate albedo product can aid the challenging representation of sea ice optical properties in summer in climate models.