MOD10A1 V005 data granules include a snow albedo data array. Albedo refers to the percentage of incident solar radiation reflected back by an object. The albedo measured by MODIS is specifically termed a directional-hemispheric reflectance as the incident irradiance is considered a directional collimated beam, and the reflected radiance is integrated over the upward hemisphere.

Snow albedo is an important parameter for understanding global energy-balance as well as local- and basin-scale snow melt runoff. Four conversions are needed in order to derive broadband albedo from satellite measurements:

• Satellite sensor calibration
• Correction for atmospheric effects
• Correction for surface effects including slope, solar aspect, and reflectance anisotropy
• Conversion to a spectrally integrated albedo (Klein and Stroeve 2002).

For more information regarding the theory for snow mapping and fractional snow cover, please see the Theory of Measurements section in the MODIS/Terra Snow Cover 5-Min L2 Swath 500m, Version 5 guide document (MOD10_L2).

The MODIS science team is responsible for algorithm development. MODAPS is responsible for product generation and transfer of products to NSIDC.

Processing Steps

Please see the Processing Steps section in the MODIS/Terra Snow Cover 5-Min L2 Swath 500m, V005 guide document for a discussion of the processing steps used to generate the MOD10_L2 product. A days worth of MOD10_L2 data is processed into a single MOD10_L2G product for each day by mapping each pixel in each MOD10_L2 swath to a grid all in the MODIS sinusoidal map projections. Thus, the MOD10_L2G product is not archived, but is used as the input to MOD10A1 processing.

In MOD10A1, a daily snow cover map is constructed by examining the multiple observations acquired for a day that are mapped to each grid cell. For V005 data, a scoring algorithm based on pixel location, distance from nadir, area of coverage in a grid cell, and solar elevation, selects an observation for the day. The object of scoring is to select the observation nearest to nadir with greatest coverage at the highest solar elevation that was mapped into the grid cell (Riggs, Hall, and Salomonson 2006). The scoring algorithm is represented by the following equation:

score = 0.5 * (solar elevation) + 0.3 * (distance from nadir) + 0.2 * (observation coverage)

Results of the snow cover algorithm are used to create a daily snow map, and are stored in the Snow Cover Daily Tile Field. The corresponding fractional snow cover observation is stored in the Fractional Snow Cover field. Gridded snow cover data are stored as coded integer values with values being the same as assigned for MOD10_L2.

Snow Albedo

The prototype MODIS snow albedo algorithm for MOD10A1 V005 granules is described in detail in (Klein and Stroeve 2002) and (Klein 2003). The observation selected by the scoring algorithm described above is used by the albedo algorithm for a cell. Albedo is calculated only for areas identified as cloud-free by the MODIS cloud mask and as snow-covered by the MODIS snow algorithm. If the pixel meets these criteria, atmospherically corrected surface reflectances are retrieved from the MODIS/Terra Surface Reflectance Daily L2G Global 500m SIN Grid product, available from the LP DAAC (Klein and Stroeve 2002). Land cover type, retrieved from the MODIS/Terra Land Cover Type Yearly L3 Global 500 m SIN Grid is also available from the LP DAAC (Klein and Stroeve 2002), and is used to correct non-forested pixels for the anisotropic reflectance of snow using a static Anisotropic Response Function (ARF).

Error Sources

As with any upper level product, the characteristics of and/or anomalies in input data may carry through to the output data product. The following products are input to the algorithms used to create the MOD10A1 product:

Note: NSIDC does not archive or distribute the following products, and does not maintain the links to these products. Thus, if a link does not work, please contact the MODAPS or the LP DAAC.

• MOD10L2G Land Level 2G Snow Cover, Version 5
• MODMGGAD - Geolocation Angles Daily L2G Global 1km Day, Version 5
• MODPTHKM - MODIS/Terra Observation Pointers Daily L2G Global 500m SIN Grid, Version 5
• MOD09GHK - Surface Reflectance Daily L2G Global 500m, Version 5
• MOD12Q1 - MODIS/Terra Land Cover Type Yearly L3 Global 1km SIN Grid, Version 5
• GTOPO30 Global 30-Arc Second Digital Elevation Data Set

The Snow Albedo Daily Tile Field Local Attribute was released as a beta product in V004 data, and it progressed to a status of provisional in V005 data. Validation and evaluation of the snow albedo is ongoing and is estimated to be within 10 percent of surface measured snow albedo based on studies in the literature. However, those estimates are based on best conditions for the algorithm. Where it is difficult to calculate snow albedo, such as in steep mountainous terrain, the error in calculated albedo is likely to be large.

Snow albedo retrieved from MODIS is sensitive to a variety of atmospheric effects such as errors in modeling the ARFs used to correct the MODIS reflectances into a hemispheric albedo, and the conversion from narrow-band albedos to a broadband albedo (0.3 - 3.0 microns). In terms of atmospheric effects, excluding clouds, uncertainties in atmospheric aerosols have the largest impact on derivation of surface albedo from visible and near infrared satellite observations and (Greuell and Oerlemans 2005). Since it is impossible to retrieve aerosols over bright surfaces such as snow and ice, climatologies of aerosols are used in the polar regions. In the MODIS atmospheric correction for snow-covered and ice-covered regions, the aerosol optical depth at 550 nm is set at 0.05. However, there are many locations and times of the year that the aersol optical depth exceeds this value. Thus, for pixels with viewing angles less than 50 degrees, variation in aerosol optical depth impacts the albedo by less than 0.01. For viewing angles greater than 50 degrees, the impact will be larger; however, it is not clear as to how large (Greuell and Oerlemans 2005).

Erroneous albedo values also occur wherever clear-sky conditions are misidentified. However, in general, the MODIS cloud mask is conservative, classifying clear sky pixels as cloudy. In the ARF conversion, the model assumes a constant snow grain size of 250 microns and ignores diffuse sky radiation assuming only direct radiation (Klein and Stroeve 2002). The ARFs under real sky conditions are slightly different depending on the ratio of direct to diffuse sky radiation, and it is a potential error source in the ARF values used in the derivation of the albedo. Another potential error source is the fact that snow grain size is variable, and different ARFs are needed depending on how fine or coarse the snow grain size is. Thus, fine-grain snow has a small backscattering peak at viewing angles of 50 degrees, whereas medium snow grain sizes do not. The narrow-to-broadband conversion also depends on atmospheric effects, and may not be suitable for atmospheric conditions different from the assumptions used in developing the narrow-to-broadband conversion model (Painter and Dozier 2004). Finally, accurate instrument calibration is essential in order to provide high-quality scientific data sets. There are uncertainties in MODIS albedo as a result of calibration problems, around two percent for the Terra instrument based on comparisons made using data in Greenland (Greuell and Oerlemans 2005), which is similar to that reported by (Justice 2002).

The snow albedo product in MOD10A1 and MYD10A1 is subject to a different land/water mask mismatch. The albedo algorithm requires as input a global Digital Elevation Model (DEM) from which slopes and aspects are computed. The GTOPO30 DEM is used for this purpose, and it has a land/water mask that does not precisely match the MODIS land/water mask. Pixels where there is a mismatch between the GTOPO30 and MODIS masks are flagged as such in the data.

Principles of Operation

The MODIS instrument provides 12-bit radiometric sensitivity in 36 spectral bands, ranging in wavelength from 0.4 µm to 14.4 µm. Two bands are imaged at a nominal resolution of 250 m at nadir, five bands at 500 m, and the remaining bands at 1000 m. A ±55 degree scanning pattern at a 705 km altitude achieves a 2330 km swath with global coverage every one to two days.

The scan mirror assembly uses a continuously rotating double-sided scan mirror to scan ±55 degrees, driven by a motor encoder built to operate 100 percent of the time throughout the six year instrument design life. The optical system consists of a two-mirror off-axis afocal telescope which directs energy to four refractive objective assemblies: one each for the visible, near-infrared, short wave-infrared, and long wave-infrared spectral regions.

Technical Specifications

Spectral Bands

For information on the 36 spectral bands provided by the MODIS instrument, see the MODIS Spectral Bands Table.

Sensor or Instrument Measurement Geometry

The MODIS scan mirror assembly uses a continuously rotating double-sided scan mirror to scan ±55 degree, with a 20.3 rpm. The viewing swath is 10 km along track at nadir, and 2330 km cross track at ±55 degree.

Manufacturer of Sensor or Instrument

MODIS instruments were built to NASA specifications by Santa Barbara Remote Sensing, a division of Raytheon Electronics Systems.

Calibration

MODIS has a series of on-board calibrators that provide radiometric, spectral, and spatial calibration of the MODIS instrument. The blackbody calibrator is the primary calibration source for thermal bands between 3.5 µm and 14.4 µm, while the Solar Diffuser (SD) provides a diffuse, solar-illuminated calibration source for visible, near-infrared, and shortwave infrared bands. The Solar Diffuser Stability Monitor (SDSM) tracks changes in the reflectance of the SD with reference to the sun so that potential instrument changes are not incorrectly attributed to changes in this calibration source. The Spectroradiometric Calibration Assembly (SRCA) provides additional spectral, radiometric, and spatial calibration.

MODIS uses the moon as an additional calibration technique and for tracking degradation of the SD, by referencing the illumination of the moon since the moon's brightness is approximately the same as that of the Earth. Finally, MODIS deep space views provide a photon input signal of zero, which is used as a point of reference for calibration.