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VIIRS User Guide Collection 1

This Guide is a living document that describes the VIIRS BRDF and Albedo Products. The purpose of the document is to give the potential user an understanding of the VNP43 products and the current state of the data in those products.

Note: Users are urged to use the band specific quality flags to isolate the highest quality full inversion results for their own science applications. 

Albedo is defined as the ratio of upwelling to downwelling radiative flux at the surface. Downwelling flux may be written as the sum of a direct component and a diffuse component. Black-sky albedo (directional hemispherical reflectance) is defined as albedo in the absence of a diffuse component and is a function of solar zenith angle. White-sky albedo (bihemispherical reflectance) is defined as albedo in the absence of a direct component when the diffuse component is isotropic. Black-sky albedo and white-sky albedo mark the extreme cases of completely direct and completely diffuse illumination. Actual albedo is a value which is interpolated between these two as a function of the fraction of diffuse skylight which is itself a function of the aerosol optical depth [1], [4], [8]. The underlying assumption of an isotropic distribution for the diffuse skylight is approximate but avoids the expense of an exact calculation while capturing the major part of the phenomenon [2]. However, for large angles and bright surfaces it’s more appropriate to use the full anisotropic expression [8].

The VNP43IA3 and VNP43MA3 Albedo Product (VIIRS/Suomi-NPP Albedo Daily L3 Global 500m/1km SIN Grid) provides both the white-sky albedos and the black-sky albedos (at local solar noon). VNP43IA3 provides information at the three VIIRS Imagery bands (I1, I2, and I3) at a 500m scale, and VNP43MA3 provides information at the nine Moderate bands (M1- M5, M7-M8, M10-M11) as well as for the three broad bands (0.3-0.7µm, 0.7-5.0µm, and 0.3-5.0µm). While the total energy reflected by the Earth's surface in the shortwave domain is characterized by the shortwave (0.3-5.0µm) broadband albedo, the visible (0.3-0.7µm) and near-infrared (0.7-5.0µm) broadband albedos are often also of interest due to the marked difference of the reflectance of vegetation in these two spectral regions. The same approaches used by Liang et al. [3] and Stroeve et al. [9] are used to produce the narrowband to broadband (NTB) conversion coefficients for VIIRS. The coefficient used by the operational algorithm are in the following tables (Liu et al., 2017).

Table 1. Narrowband to broadband conversion coefficient-snow free.

Science Data Sets

The albedo product includes two Science Data Sets (SDS) for each pixel in the tile. These SDSs are Albedo (containing both black-sky and white-sky values for 12 bands at 1km scale—9 spectral and 3 broad, and 3 spectral bands at 500m scale) and BRDF_Albedo_Band_Mandatory_Quality (containing 1 byte of quality flags).

The Albedo SDS is specified as:

• Data Field Name: INT16 Albedo ("YDim", "XDim", "Num_Albedo_Bands", "Num_Albedos")
• Description: Black-Sky Albedo (at local solar noon) and white-sky albedo for spectral bands, and the vis, NIR and SW broadbands
• Data conversions:
  •  file data = ( Albedo / scale_factor ) + add_offset
  •  Albedo = ( file data - add_offset ) * scale_factor

Note that the Collection 1 VIIRS VNP43 products are retrieved daily and represent the best BRDF possible based on 16 days worth of inputs with the day of interest emphasized. The date associated with each daily retrieval is the center of the moving 16 day input window.

File Specifications

The File Specifications for VNP43IA3 and VNP43MA3 are at https://ladsweb.modaps.eosdis.nasa.gov/filespec/VIIRS/collection1

Local Attributes

In addition to the actual SDS data values produced at each pixel in a tile, each SDS can be associated with a number of standard metadata Local Attributes that apply to the data.