JPSS VIIRS BRDF Products

Team Members: Jingjing Peng and Zhen Song

Product Description

The Bidirectional Reflectance Distribution Function (BRDF) describes how surface reflectance varies with solar and viewing geometry. From BRDF, two types of surface albedo are derived: black-sky albedo (BSA), representing albedo under direct solar illumination, and white-sky albedo (WSA), representing albedo under fully diffuse illumination. The product also provides nadir BRDF-adjusted reflectance (NBAR), which normalizes surface reflectance to a standard viewing geometry—local solar noon in this product. Together, BRDF, BSA, WSA, and NBAR offer a consistent set of variables that are essential for quantifying land surface anisotropy, monitoring surface changes, and assessing their impacts on Earth’s radiation balance and climate system.

Key Characteristics

• Spectral Bands & Resolution: Retrievals are generated for nine VIIRS bands (M1, M2, M3, M4, M5, M7, M8, M10, M11) as well as for broadband visible, near-infrared (NIR), and shortwave ranges, at 1 km resolution using a 16-day observation window.
• Primary Outputs:
  • BRDF model parameters (f_iso, f_vol, f_geo)
  • Black-sky albedo (BSA)
  • White-sky albedo (WSA)
  • Nadir BRDF-adjusted reflectance (NBAR)
  • Associated uncertainty layers and quality flags

Algorithm Overview

The BRDF/Albedo processing chain consists of three main units:

1. Mapping to Global Tiles (Unit 1):

   
   VIIRS L2 Surface Reflectance (750 m) granules, with associated quality flags, solar/view angles, and snow mask, are reprojected onto a global 1 km equal-area sinusoidal grid. The grid is organized into 72 × 72 non-overlapping tiles.

2. BRDF Retrieval and Albedo/NBAR Generation (Unit 2):

   
   Clear-sky observations within the accumulation window are used to estimate BRDF parameters at the pixel level. The retrieval method adapts according to the number of valid observations. Missing data are first filled temporally (using results from the past 7 days) and then spatially using climatology and similar land cover. The completed BRDF fields are subsequently integrated to generate black-sky albedo (BSA), white-sky albedo (WSA), and nadir BRDF-adjusted reflectance (NBAR).

Theoretical Basis

• Kernel-Driven Model: A three-kernel linear model is used, combining:
  • Isotropic kernel (f_iso)
  • Ross-Thick volumetric scattering kernel (f_vol)
  • Li-Sparse Reciprocal geometric-optical kernel (f_geo)
• Observation Strategy: Multi-satellite synergy (S-NPP, NOAA-20, NOAA-21) shortens accumulation time to as least as 10 days, ensuring sufficient angular sampling while limiting the retrieval window to 16 days.

Input Data

• VIIRS L2 Surface Reflectance (750 m) granules with geometry, snow mask, and quality flags
• Constraints: Observations with solar zenith angle ≥ 70° are excluded

Output Products

• Daily tile-based files include:
  • BRDF model parameters
  • Black-sky albedo (BSA) for spectral bands (1, 2, 3, 4, 5, 7, 8, 10, and 11), the visible (0.4-0.7 µm), near-infrared NIR (0.7-4.0 µm), and shortwave (0.4-4.0 µm) broadbands
  • White-sky albedo (WSA) for spectral bands (1, 2, 3, 4, 5, 7, 8, 10, and 11), the visible (0.4-0.7 µm), near-infrared NIR (0.7-4.0 µm), and shortwave (0.4-4.0 µm) broadbands
  • Nadir BRDF-adjusted reflectance (NBAR)
  • Pixel-level uncertainties and quality flags

The current products are experimental and non-operational. If you are interested in accessing the experimental data, please contact Zhen Song. Users are encouraged to refer to the quality flags for information on retrieval methods and observation conditions across different regions.