MOD21A2 v006

MODIS/Terra Land Surface Temperature/3-Band Emissivity 8-Day L3 Global 1 km SIN Grid


PI: Glynn C. Hulley

Description

A new suite of Moderate Resolution Imaging Spectroradiometer (MODIS) Land Surface Temperature and Emissivity (LST&E) products are available in Collection 6. The MOD21 Land Surface Temperatuer (LST) algorithm differs from the algorithm of the MOD11 LST products, in that the MOD21 algorithm is based on the ASTER Temperature/Emissivity Separation (TES) technique, whereas the MOD11 uses the split-window technique. The MOD21 TES algorithm uses a physics-based algorithm to dynamically retrieve both the LST and spectral emissivity simultaneously from the MODIS thermal infrared bands 29, 31, and 32. The TES algorithm is combined with an improved Water Vapor Scaling (WVS) atmospheric correction scheme to stabilize the retrieval during very warm and humid conditions.

The MOD21A2 dataset is an 8-day composite LST product that uses an algorithm based on a simple averaging method. The algorithm calculates the average from all the cloud free MOD21A1D and MOD21A1N daily acquisitions from the 8-day period. Unlike the MOD21A1 data sets where the daytime and nighttime acquisitions are separate products, the MOD21A2 contains both daytime and nighttime acquisitions as separate Science Dataset (SDS) layers within a single Hierarchical Data Format (HDF) file. The LST, Quality Control (QC), view zenith angle, and viewing time have separate day and night SDS layers, while the values for the MODIS emissivity bands 29, 31, and 32 are the average of both the nighttime and daytime acquisitions. MOD21A2 products are available two months after acquisition due to latency of data inputs. Additional details regarding the method used to create this Level 3 (L3) product are available in the Algorithm Theoretical Basis Document (ATBD).

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Characteristics

Product Maturity

Terra MODIS LST&E products are provisional and have not been validated for their science quality. Further details regarding MODIS land product validation for the MOD21 data products are available from the MODIS Land Team Validation site.

Collection and Granule

Collection

Characteristic Description
CollectionTerra MODIS
DOI10.5067/MODIS/MOD21A2.006
File Size~5.3 MB
Temporal ResolutionMulti-Day
Temporal Extent2000-02-18 to 2005-12-31
Spatial ExtentGlobal
Coordinate SystemSinusoidal
DatumN/A
File FormatHDF-EOS
Geographic Dimensions1200 km x 1200 km

Granule

Characteristic Description
Number of Science Dataset (SDS) Layers11
Columns/Rows1200 x 1200
Pixel Size1000 m

Layers

SDS Name Description Units Data Type Fill Value No Data Value Valid Range Scale Factor Additional Offset
LST_Day_1KM Day Land surface temperature Kelvin 16-bit unsigned integer 0 N/A 7500 to 65535 0.02 N/A
QC_Day Day Quality Control (QC) N/A 8-bit unsigned integer N/A N/A 0 to 255 N/A N/A
View_Angle_Day Day MODIS view zenith angle Degree 8-bit unsigned integer 255 N/A 0 to 130 N/A -65
View_Time_Day Time of MODIS observation for day Hours 8-bit unsigned integer 255 N/A 0 to 240 0.1 N/A
LST_Night_1KM Night Land Surface Temperature Kelvin 16-bit unsigned integer 0 N/A 7500 to 65535 0.02 N/A
QC_Night Night Quality Control (QC) N/A 8-bit unsigned integer N/A N/A 0 to 255 N/A N/A
View_Angle_Night Night view zenith angle Degree 8-bit unsigned integer 255 N/A 0 to 130 N/A -65
View_Time_Night Time of Observation for night Hours 8-bit unsigned integer 255 N/A 0 to 240 0.1 N/A
Emis_29 Average Day/Night Band 29 emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 0.002 0.49
Emis_31 Average Day/Night Band 31 emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 0.002 0.49
Emis_32 Average Day/Night Band 32 emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 0.002 0.49

Product Quality

The QC layer is stored in an efficient bit-encoded manner. The unpack_sds_bits executable from the LDOPE Tools is available to the user community to help parse and interpret the quality layer.

The final quality bit for the output product reflects the lowest quality values from all observations that went into the final average. The bit definition index for the QC layer is available in the User Guide on page 26.

Known Issues

Users of MODIS LST products may notice an increase in occurrences of extreme high temperature outliers in the unfiltered MxD21 Version 6 and 6.1 products compared to the heritage MxD11 LST products. This can occur especially over desert regions like the Sahara where undetected cloud and dust can negatively impact both the MxD21 and MxD11 retrieval algorithms.

In the MxD11 LST products, these contaminated pixels are flagged in the algorithm and set to fill values in the output products based on differences in the band 32 and band 31 radiances used in the generalized split window algorithm. In the MxD21 LST products, values for the contaminated pixels are retained in the output products (and may result in overestimated temperatures), and users need to apply Quality Control (QC) filtering and other error analyses for filtering out bad values. High temperature outlier thresholds are not employed in MxD21 since it would potentially remove naturally occurring hot surface targets such as fires and lava flows.

High atmospheric aerosol optical depth (AOD) caused by vast dust outbreaks in the Sahara and other deserts highlighted in the example documentation are the primary reason for high outlier surface temperature values (and corresponding low emissivity values) in the MxD21 LST products. Future versions of the MxD21 product will include a dust flag from the MODIS aerosol product and/or brightness temperature look up tables to filter out contaminated dust pixels. It should be noted that in the MxD11B day/night algorithm products, more advanced cloud filtering is employed in the multi-day products based on a temporal analysis of historical LST over cloudy areas. This may result in more stringent filtering of dust contaminated pixels in these products.

In order to mitigate the impact of dust in the MxD21 V6 and 6.1 products, the science team recommends using a combination of the existing QC bits, emissivity values, and estimated product errors, to confidently remove bad pixels from analysis. For more details, refer to this dust and cloud contamination example documentation.

For complete information about the MOD21 known issues please refer to the MODIS Land Quality Assessment website.


About the image

Terra MODIS land surface temperature (LST) data from the MOD21A2 product over India on January 5, 2000.

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Documentation

User Guide
Algorithm Theoretical Basis Document (ATBD)
File Specification

Using the Data

Access Data

Citation

DOI: 10.5067/MODIS/MOD21A2.006