VIIRS NASA Albedo Product Validation

(October 2019)

Version 1 Validation Statement

Validation at stage 1 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.

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 that as of summer 2019, the VIIRS Land Surface Reflectance product (VNP09) input seems to consistently flag 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 Land Surface Reflectance (VNP09) issue here. This probelm with VNP09 will be fixed in the next reprocessing (Collection 2) that will begin in the last quarter of 2019.

 

Supporting Studies

Liu, Y., Wang, Z., Sun, Q., Erb, A. M., Li, Z., Schaaf, C. B., Zhang, X., Román, M. O., Scott, R. L., Zhang, Q., Novick, K. A., Syndonia    Bret-Harte, M., Petroy, S., & San Clements, M. (2017). Evaluation of the VIIRS BRDF, Albedo and NBAR Products Suite and an    Assessment of Continuity with the Long Term MODIS Record. Remote Sensing of Environment, 201, 256–274.