VIIRS NASA Albedo Product Validation

(February 2021)

Version 1 Validation Statement

Validation at stage 2 has been achieved for the VIIRS albedo product (VNP43). The accuracy of the high quality gridded 500m and 1km VIIRS operational albedo is well less than 5% albedo at the majority of the validation sites studied thus far, and even those albedo values with low quality flags have been found to be primarily within 10% of the field data. Data for solar zenith angles greater than 70 degrees should be considered suspect for research efforts.

While the daily retrieval algorithm has been shown to capture rapid changes well, such as snow melt and greenup, the values associated with rapid change may be flagged as low quality as the algorithm can lag in response to abrupt reactions seen in field data. Therefore, it is vitally important that users refer to the associated quality flags in the product.
**Note in C1 of the product, the VIIRS Land Surface Reflectance product (VNP09) input consistently flags a large number of retrievals as "high aerosol" in the quality data set when compared to the heritage MODIS surface reflectance product. This is observed globally, but specifically impacts arid regions in the mid- to high-latitudes and vegetated areas of the tropics. Pixels inaccurately flagged as "high aerosol" in the VNP09 product directly impacts the quality of the downstream VIIRS BRDF Albedo product, causing the BRDF retrieval algorithm to reject these "high aerosol" observations. Consequently, this reduced number of valid observations results in an increased number of poorer quality "magnitude inversions" and fill values, relative to the higher quality "full inversions", due to both the reduction in total number of observations per pixel and the reduced angular distribution of these observations.

View a detailed description of the C1 Land Surface Reflectance (VNP09) issue here. This problem with VNP09 has been fixed in the next reprocessing (Collection 2) which will be available in 2021. Therefore, all users are encouraged to switch to C2 reprocessed data as soon as they become available.

 

Supporting Studies

Wang, Z., Schaaf, C., Lattanzio, A., Carrer, D., Grant, I., Román, M., Camacho, F., Yu, Y., Sánchez-Zapero, J. & Nickeson, J. (2019). Global Surface Albedo Product Validation Best Practices Protocol. Version 1.0. In Z. Wang, J. Nickeson & M. Román (Eds.), Best Practice for Satellite Derived Land Product Validation (p. 45): Land Product Validation Subgroup (WGCV/CEOS). doi:10.5067/DOC/CEOSWGCV/LPV/ALBEDO.001.

Liu, Y., Hill, M. J., Zhang, X., Wang, Z., Richardson, A. D., Hufkens, K., Filippa, G., Baldocchi, D. D., Ma, S., Verfaillie, J., & Schaaf, C. B. (2017). Using Data from Landsat, MODIS, VIIRS and PhenoCams to Monitor the Phenology of California Oak/grass Savanna and Open Grassland across Spatial Scales. Agricultural and Forest Meteorology, 237-238, 311-325, https://doi.org/10.1016/j.agrformet.2017.02.026