A Northern Hemisphere Snow Extent Climate Data Record (Invited)

A thorough reanalysis of NOAA satellite-derived maps of Northern Hemisphere continental snow cover extent (SCE) has resulted in the creation of a more accurate and consistent SCE climate data record (CDR) than was heretofore available. This new CDR shows annual snow extents of 1-3% lower than the previous EDR throughout the satellite era. Changes from previously mapped extent results from the elimination of regions originally mapped as having a patchy snow cover, yet were digitized as being fully covered. This was particularly prevalent over the Tibetan Plateau and western North America in the 1970s and early 1980s. Also, some mountainous areas were plotted as being unrealistically extensively snow covered during the first 30 years of the 40 year NOAA product, when the spatial resolution was coarser than in the past decade. This was particularly evident where narrow ranges are found, such as the Himalayas, Sierra Nevada and in portions of the Rockies. The creation of the CDR relied on comparisons between climatologies of the first 30 years of coarse-scale weekly NOAA maps and the most recent 10 years of finer daily maps produced by NOAA, currently at the multi-agency National Ice Center. The reanalysis also benefited from a two-year overlap of the independently-produced daily and weekly products and the expertise of the Global Snow Lab staff and colleagues elsewhere who were consulted throughout the project. Results of previous studies utilizing the original dataset should not be greatly affected, assuming, as we continue to recommend, that local analyses, particularly in the aforementioned areas, not be undertaken with this regional-continental scale CDR. While we have greater confidence in the new CDR, quantitative assessments of uncertainty remain a vexing issue. This will continue to be addressed as we work with colleagues to integrate this interactively-generated visible satellite derived dataset with automated products generated from visible and microwave satellite data. As first noted in the late 1980s, NH SCE from the late winter to early summer remains well below that seen in the previous two decades. The past year has found SCE to be near a record low over Eurasia, while close to the long-term average over North America.