HYDROLOGICAL PROCESSESHydrol. Process. 21, 1534–1547 (2007)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6715Accuracy assessment of the MODIS snow products†Dorothy K. Hall1* and George A. Riggs21Cryospheric Sciences Branch, Code 614Ð1, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA2Science Systems and Applications, Inc., Lanham, Maryland 20706, USAAbstract:A suite of Moderate-Resolution Imaging Spectroradiometer (MODIS) snow products at various spatial and temporal resolutionsfrom the Terra satellite has been available since February 2000. Standard products include daily and 8-day composite 500 mresolution swath and tile products (which include fractional snow cover (FSC) and snow albedo), and 0Ð05°resolution productson a climate-modelling grid (CMG) (which also include FSC). These snow products (from Collection 4 (C4) reprocessing) aremature and most have been validated to varying degrees and are available to order through the National Snow and Ice DataCenter. The overall absolute accuracy of the well-studied 500 m resolution swath (MOD10L2) and daily tile (MOD10A1)products is ¾93%, but varies by land-cover type and snow condition. The most frequent errors are due to snow/clouddiscrimination problems, however, improvements in the MODIS cloud mask, an input product, have occurred in ‘Collection 5’reprocessing. Detection of very thin snow (<1 cm thick) can also be problematic. Validation of MOD10L2 and MOD10A1applies to all higher-level products because all the higher-level products are all created from these products. The compositedproducts may have larger errors due, in part, to errors propagated from daily products. Recently, new products have beendeveloped. A fractional snow cover algorithm for the 500 m resolution products was developed, and is part of the C5 dailyswath and tile products; a monthly CMG snow product at 0Ð05°resolution and a daily 0Ð25°resolution CMG snow productare also now available. Similar, but not identical products are also produced from the MODIS on the Aqua satellite, launchedin May 2002, but the accuracy of those products has not yet been assessed in detail. Published in 2007 by John Wiley &Sons, Ltd.KEY WORDS MODIS; snow covered area; snow products; Terra; AquaReceived 9 June 2006; Accepted 10 January 2007INTRODUCTIONSnow may cover up to about 50 million km2of the North-ern Hemisphere land surface (http://climate.rutgers.edu/snowcover/) and thus has a major impact on the Earth’senergy balance because of its high albedo and low ther-mal conductivity. Albedo, the ratio of reflected to incidentsolar energy, governs how much solar energy is absorbedby land and ocean surfaces, and can change dramati-cally, for example from 0Ð2to0Ð8 or greater, when snowfirst accumulates. Snow cover also has a major influenceon atmospheric circulation by modifying overlying airmasses.Because so much of the water supply used by humanscomes from snow cover, especially in mountainous areasthroughout the world, snow water equivalent (SWE) isa critical snowpack parameter. However SWE cannotyet be measured remotely with the accuracy requiredby hydrologic models. To obtain accurate SWE esti-mates from space, other sources of information, includ-ing station data and snow-covered area, should beused, together, to increase their usability in land-surfacemodels.* Correspondence to: Dorothy K. Hall, Cryospheric Sciences Branch,Code 614Ð1, NASA/Goddard Space Flight Center, Greenbelt, Maryland20771, USA. E-mail: [email protected]†This article is a US Government work and is in the public domain inthe USA.Moderate-Resolution Imaging Spectroradiometer(MODIS) data, available since 2000, have proven use-ful for a large variety of land, ocean and atmosphericapplications, and a multitude of MODIS standard prod-ucts is now available. The MODIS standard snow-coverproducts–providing snow extent and albedo–are useful,or potentially useful, as input to models. The accuracy ofthese snow products must be known in order to optimizetheir use.Two types of validation are addressed in this paper–absolute and relative. To derive absolute validation,the MODIS maps are compared with ground measure-ments or measurements of snow cover from Landsatdata, which are considered to be the ‘truth’ for thiswork. Relative validation refers to comparisons withother snow maps, most of which have unknown accu-racy. Thus for the studies of relative validation, it is notgenerally known which snow map has a higher accu-racy.In this paper, we provide the most up-to-date infor-mation on the accuracy of each MODIS-derived stan-dard snow cover product in collection 4 (C4 or Version4 (V004)). C4 refers to the fourth reprocessing of thedata-product suite (or the second complete reprocess-ing). The products have varying degrees of maturity, andbecause of the different spatial and temporal resolutions,the accuracy is different for different products. Thoughthe focus is on the C4 products derived from the MODISPublished in 2007 by John Wiley & Sons, Ltd.ASSESSMENT OF THE MODIS PRODUCTS 1535on the Terra satellite, denoted by MOD10, Aqua MODISproducts, designated MYD10, are also discussed.BACKGROUNDMODIS is an imaging spectroradiometer that employsa cross-track scan mirror, collecting optics, and a setof detector elements to provide imagery of the Earth’ssurface and clouds in 36 discrete, narrow spectral bandsfrom approximately 0Ð4to14Ð0µm(Barneset al., 1998)http://modis.gsfc.nasa.gov/. The spatial resolution of theMODIS instrument varies with spectral band, and rangesfrom250mto1kmatnadir.Operational data collection from the Terra MODISbegan on 24 February 2000, orbiting in a 10 : 30A.M.localtime descending node and from the Aqua MODIS on 24June 2002, in a 1 : 30P.M. local time ascending node.Together, the two overpass times (Terra in the morningand Aqua in the afternoon) allow the possibility ofdiurnal observations of snow, and the possibility to obtainmore clear views of the surface, as clouds can changein position and extent within a period of 3 h. MODISinstruments on each spacecraft are nearly identical exceptwith respect to band 6 (1Ð628–1Ð672µm). Approximately70% of the band-6 detectors are non-functional on theAqua MODIS. Since MODIS band 6 is crucial for snowdetection, the loss of band 6 detectors forced a changein the Aqua snow mapping algorithms to use band 7(2Ð105–2Ð155µm) where band 6 is used in the TerraMODIS