Evaluation of VIIRS and MODIS snow cover fraction in High-Mountain Asia using Landsat 8 OLI

We present thefirst application of the Snow Covered Area and Grain size model (SCAG) tothe Visible Infrared imaging Radiometer Suite (VIIRS) and assess these retrievals withfiner-resolution fractional snow cover maps from Landsat 8 Operational Land Imager (OLI).Because Landsat 8 OLI avoids saturation issues common to Landsat 1–7 in the visiblewavelengths, we re-assess the accuracy of the SCAG fractional snow cover maps fromModerate Resolution Imaging Spectroradiometer (MODIS) that were previously evaluatedusing data from earlier Landsat sensors. Use of the fractional snow cover maps fromLandsat 8 OLI shows a negative bias of−0.5% for MODSCAG and−1.3% for VIIRSCAG,whereas previous MODSCAG evaluations found a bias of−7.6% in the Himalaya. Wefindsimilar root mean squared error (RMSE) values of 0.133 and 0.125 for MODIS and VIIRS,respectively. The Recall statistic (probability of detection) for cells with more than 15%snow cover in this challenging steep topography was found to be 0.90 for both MODSCAGand VIIRSCAG, significantly higher than previous evaluations based on Landsat 5Thematic Mapper (TM) and 7 Enhanced Thematic Mapper Plus (ETM+). In addition,daily retrievals from MODIS and VIIRS are consistent across gradients of elevation, slope,and aspect. Different native resolutions of the gridded products at 1 km and 500 m forVIIRS and MODIS, respectively, result in snow cover maps showing a slightly differentdistribution of values with VIIRS having more mixed pixels and MODIS having 7% morepure snow pixels. Despite the resolution differences, the snow maps from both sensorsproduce similar total snow-covered areas and snow-line elevations in this region, withR2values of 0.98 and 0.88, respectively. Wefind that the SCAG algorithm performsconsistently across various spatial resolutions and that fractional snow cover mapsfrom the VIIRS instruments aboard Suomi NPP, JPPS–1, and JPPS–2 can be asuitable replacement as MODIS sensors reach their ends of life.