VNP21A2 v001

VIIRS/NPP Land Surface Temperature and Emissivity 8-Day L3 Global 1 km SIN Grid


PI: Glynn Hulley

Description

The NASA Suomi National Polar-Orbiting Partnership (Suomi NPP) Visible Infrared Imaging Radiometer Suite (VIIRS) Land Surface Temperature and Emissivity (LST&E) 8-day product (VNP21A2) combines the daily VNP21A1D and VNP21A1N products over an 8-day compositing period into a single product.

The VNP21A2 dataset is an 8-day composite LST&E product at 1 kilometer resolution that uses an algorithm based on a simple-averaging method. The algorithm calculates the average from all the cloud-free VNP21A1D and VNP21A1N daily acquisitions from the 8-day period. Unlike the VNP21A1 datasets where the daytime and nighttime acquisitions are separate products, the VNP21A2 contains both daytime and nighttime acquisitions as separate science dataset (SDS) layers within a single Hierarchical Data Format (HDF) file.

The VNP21A2 product is developed synergistically with the Moderate Resolution Imaging Spectroradiometer (MODIS) LST&E Version 6 product (MOD21A2) using the same input atmospheric products and algorithmic approach. The overall objective for NASA VIIRS products is to ensure the algorithms and products are compatible with the MODIS Terra and Aqua algorithms to promote the continuity of the Earth Observation System (EOS) mission. Additional details regarding the method used to create this Level 3 (L3) product are available in the Algorithm Theoretical Basis Document (ATBD). VIIRS LST&E products are available 2 months after acquisition due to latency of data inputs.

The VNP21A2 product contains 11 Science Datasets (SDS): LST, quality control, view zenith angle, and time of observation for both day and night observations along with emissivity for bands M14, M15, and M16. Low-resolution browse images for day and night LST are also available for each VNP21A2 granule.

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Characteristics

Collection and Granule

Collection

Characteristic Description
CollectionSuomi NPP VIIRS
DOI10.5067/VIIRS/VNP21A2.001
File Size~2.05 MB
Temporal ResolutionMulti-Day
Temporal Extent2012-01-25 to Present
Spatial ExtentGlobal
Coordinate SystemSinusoidal
DatumN/A
File FormatHDF-EOS5
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 for LST and emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 N/A N/A
View_Angle_Day Day View zenith angle of LST Degree 8-bit unsigned integer 255 N/A 0 to 130 N/A -65
View_Time_Day Day Time of LST observation Hours 8-bit unsigned integer 255 N/A 0 to 240 0.1 N/A
LST_Night_1KM Night 1 km Land Surface Temperature Kelvin 16-bit unsigned integer 0 N/A 7500 to 65535 0.02 N/A
QC_Night Night Quality control for LST and emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 N/A N/A
View_Angle_Night Night View zenith angle of LST Degree 8-bit unsigned integer 255 N/A 0 to 130 N/A -65
View_Time_Night Night Time of LST observation Hours 8-bit unsigned integer 255 N/A 0 to 240 0.1 N/A
Emis_14 M14 emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 0.002 0.49
Emis_15 M15 emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 0.002 0.49
Emis_16 M16 emissivity N/A 8-bit unsigned integer 0 N/A 1 to 255 0.002 0.49

Product Quality

The final quality bit for the output product reflects the lowest quality values from all observations that went into the final average. The bit flags for the QC layer are defined in Table 9 on page 25 of the User Guide.

For additional information on product quality, refer to the VIIRS Land Product Quality website.

Known Issues

Users of VIIRS and MODIS LST products may notice an increase in occurrences of extreme high temperature outliers in the unfiltered VNP21 and MxD21 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 MxD11, MxD21, and VNP21 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 VNP21 and 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 VNP21 and 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 VNP21 and MxD21 LST products. Future versions of the VNP21 and MxD21 products 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.

To mitigate the impact of dust in the VNP21 and MxD21 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 known issues please refer to the MODIS/VIIRS Land Quality Assessment website.


About the image

LST, April 15 - April 23, 2019 over the Western USA from VNP21A2.

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Documentation

User Guide
Algorithm Theoretical Basis Document (ATBD)
File Specification

Using the Data

Access Data

Citation

DOI: 10.5067/VIIRS/VNP21A2.001