Note: The snow cover variables in Version 6 of this data set differ substantially from Version 5. Fractional snow cover, binary snow-covered area, and spatial QA (
Snow_Spatial_QA) have been discontinued. See Table 2 for details.
NDSI snow cover, raw NDSI, screen results, basic QA, and snow albedo for each pixel are written to the HDF-EOS formatted data files as Scientific Data Sets (SDSs) according to the HDF Scientific Data Set Data Model. In addition, Version 6 includes two SDSs that contain pointers to the swath selected as the observation of the day. The SDSs for this data set are described in the following table:
Table 3. Scientific Data Sets and Descriptions
|Scientific Data Set
NDSI snow cover plus other results. This value is computed for MOD10_L2 and retrieved when the observation of the day is selected. Possible values are:
- 0–100: NDSI snow cover
- 200: missing data
- 201: no decision
- 211: night
- 237: inland water
- 239: ocean
- 250: cloud
- 254: detector saturated
- 255: fill
A basic estimate of the quality of the algorithm result. This value is computed for MOD10_L2 and retrieved with the corresponding observation of the day. Possible values are:
- 0: best
- 1: good
- 2: OK
- 3: poor (not currently in use)
- 211: night
- 239: ocean
- 255: unusable input or no data
||Bit flags indicating screen results and the presence of inland water. See Interpreting
NDSI_Snow_Cover_Algorithm_Flags_QA for a description. These flags are set when MOD10_L2 is generated and retrieved with the corresponding observation of the day. Bits are set to on (1) as follows:
- Bit 0: Inland water
- Bit 1: Low visible screen failed. Snow detection reversed.
- Bit 2: Low NDSI screen failed. Snow detection reversed.
- Bit 3: Combined temperature/height screen failed. On means either:
- brightness temperature ≥ 281 K, pixel height < 1300 m, flag set, snow detection reversed to not snow, OR;
- brightness temperature ≥ 281 K, pixel height ≥ 1300 m, flag set, snow detection NOT reversed.
- Bit 4: Shortwave IR (SWIR) reflectance anomalously high. On means either:
- Snow pixel with SWIR > 0.45, flag set, snow detection reversed to not snow, OR;
- Snow pixel with 25% < SWIR <= 45%, flag set to indicate unusual snow conditon, snow detection NOT reversed.
- Bit 5: spare
- Bit 6: spare
- Bit 7: solar zenith screen failed, uncertainty increased.
||Raw NDSI (i.e. prior to screening) reported in the range 0–10,000. Values are scaled by 1 x 104. This value is computed for MOD10_L2 and retrieved with the corresponding observation of the day
Snow albedo plus other results. Possible values are:
- 1–100: snow albedo
- 101: no decision
- 111: night
- 125: land
- 137: inland water
- 139: ocean
- 150: cloud
- 151: cloud detected as snow
- 250: missing
- 251: self-shadowing
- 252: land mask mismatch
- 253: BRDF failure
- 254: non-production mask
||Pointer to the orbit number of the swath that was selected as the observation of the day. The pointer references by index the list of orbit numbers written to the ORBITNUMBERARRAY metadata object in ArchiveMetadata.0.
||Pointer to the granule (swath) that was mapped into the tile. The pointer references the corresponding value in the GRANULEPOINTERARRAY metadata object written to ArchiveMetadata.0. See Using granule_pnt for more information.
Pixels determined to have some snow present are subjected to a series of screens that have been specifically developed to alleviate snow commission and omission associated with the most common error sources. In addition, snow-free pixels are screened for very low illumination conditions to prevent possible snow omission errors. Screen results, as well as and the location of inland water, are stored as bit flags in the
NDSI_Snow_Cover_Algorithm_Flags_QA SDS. The following sections describe each data screen and the conditions that result in its bit flag being set.
Low Visible Reflectance Screen
This screen is applied to prevent errors from occuring when the reflectance is too low for the algorithm to perform well, such as in very low illumination or on surface features with very low reflectance. This screen is also applied to pixels that have no snow cover present (snow-free pixels) to prevent possible snow omission. If the MODIS Band 2 reflectance is ≤ 0.10 or the Band 4 reflectance is ≤ 0.11, the pixel fails the screen and is set to no decision in the NDSI snow cover SDS. The results of this screen are tracked in bit 1 of the
Low NDSI screen
Pixels detected as having snow cover with 0.0 < NDSI < 0.10 are reversed to no snow and flagged by setting bit 2 in the
NDSI_Snow_Cover_Algorithm_Flags_QA SDS. This flag can be used to find pixels where snow cover detections were reversed to not snow.
Estimated surface temperature and surface height screen
This screen serves a dual purpose by linking estimated surface temperature with surface height. It is used to alleviate errors of commission at low elevations that appear spectrally similar to snow but are too warm. It is also used to flag snow detections at high elevations that are warmer than expected. Using the estimated MODIS Band 31 brightness temperature (Tb), if snow is detected in a pixel with height < 1300 m and Tb ≥ 281 K, the pixel is reversed to not snow and bit 3 is set in the
NDSI_Snow_Cover_Algorithm_Flags_QA SDS. If snow is detected in a pixel with height ≥ 1300 m and Tb ≥ 281 K, the pixel is flagged as unusually warm by setting bit 3 in the
High SWIR reflectance screen
This screen also serves a dual purpose by: a) preventing non-snow features that appear similar to snow from being detected as snow; b) allowing snow to be detected where snow-cover short-wave infrared reflectance (SWIR) is anomalously high. Snow typically has a SWIR reflectance of less than about 0.20; however, this value can be higher under certain conditions like a low sun angle. The SWIR reflectance screen thus utilizes two thresholds. Snow pixels with SWIR reflectance > 0.45 are reversed to not snow and bit 4 of
NDSI_Snow_Cover_Algorithm_Flags_QA SDS is set. Snow pixels with 0.25 < SWIR reflectance ≤ 0.45 are flagged as having an unusually high SWIR for snow by setting bit 4 in the
Solar zenith screen
When solar zenith angles exceed 70°, the low illumination challenges snow cover detection. As such, pixels with solar zenith angles > 70° are flagged by setting bit 7 in the
NDSI_Snow_Cover_Algorithm_Flags_QA SDS. This solar zenith mask is set across the entire swath. Note: night is defined as a solar zenith angle ≥ 85°. Night pixels are assigned a value 211.
Ice/snow covered lake ice are detected by applying the snow algorithm specifically to inland water bodies. These data are provided so that the MODIS user community can evaluate the efficacy of this technique. Inland water bodies are flagged by setting bit 0 in the
NDSI_Snow_Cover_Algorithm_Flags_QA SDS. Users can extract or mask inland water in the NDSI snow cover SDS using this flag. The algorithm relies on the basic assumption that a water body is deep and clear and therefore absorbs all of the solar radiation incident upon it. Water bodies with algal blooms, high turbidity, or other relatively high reflectance conditions may be erroneously detected as snow/ice covered.
The GRANULEPOINTERARRAY metadata object written to the ArchiveMetadata.0 structure contains a pointer for each granule that was staged for input to a tile; however, more granules are staged than are actually used. Each granule that is mapped into a tile is assigned a unique positive pointer value, while those that are not are assigned a value of -1. To determine the swath origin of a cell observation, link all the pointers in GRANULEPOINTERARRAY (by index) to the corresponding list of dates and times in GRANULEBEGINNINGDATETIMEARRAY. Then locate the granule in GRANULEPOINTERARRAY with the pointer value contained in
granule_pnt and use its index to extract the date and beginning-time string from GRANULEBEGINNINGDATETIMEARRAY.