25-years of summer-time daily melt-pond fraction on Arctic sea ice derived from TERRA-MODIS sensor measurements

We report on a novel data set of the melt-pond fraction on Arctic sea ice. Melt ponds on Arctic sea ice are an important phenomenon of the summer-melt process. They reduce the surface albedo of sea ice substantially, by that influence the net shortwave radiation balance, and with that the amount of solar radiation energy that is received by the sea ice-ocean system in the Arctic during summer. This has also implications for under-ice biogeochemical processes and ice mechanics. Melt ponds increase the uncertainty in the sea-ice concentration retrieved from satellite microwave radiometer measurements. Melt ponds have been observed by a number of satellite sensors, mostly in the optical and near-infrared wavelength range. Here we present an updated version of a spectral un-mixing approach published earlier that led to a data set of melt-pond fraction on Arctic sea ice with 8-daily sampling for months May through August from 2000 through 2011. The approach is based on reflectance measurements of channels 1, 3 and 4 of the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Earth Observation Satellite (EOS) Terra satellite. We modified the approach and derived the daily melt-pond fraction on Arctic sea ice at 500 m and at 12.5 km grid resolution for months June through August from 2000 through 2024 from MODIS v6.1 observations. In addition, we provide the net ice surface fraction – aka the fraction of sea ice without melt ponds – and the fraction of open water between the ice floes. We performed an evaluation of our data set against various independent observations. Our MODIS melt-pond fraction agrees within -3 % to +4 % with independent estimates of the melt-pond fraction from very-high resolution optical satellite imagery and from Operation Ice Bridge Digital Camera System imagery. The MODIS open-water fraction is smaller by 2 % to 6 % than that from independent estimates. The net ice-surface fraction tends to be larger by 2 % to 9 % than that from independent estimates. The 12.5 km gridded product shows a slightly worse (by 1 %) agreement. While our 12.5 km gridded MODIS product under-estimates the melt-pond fraction from very-high resolution optical satellite imagery by about 2 % in the mean (median: 3 %), the Medium Resolution Imaging Spectrometer (MERIS) product over-estimates these independent estimates by about 8 % (median: 9 %). Our MODIS melt-pond fraction data set is available from Sadikni and Kern (2025): https://doi.org/10.25592/uhhfdm.18069.