BRDF, ALBEDO and NADIR Reflectance Products of New England from VIIRS

 

With the launch of Suomi-NPP satellite, the Visible Infrared Imaging Radiometer Suite (VIIRS)   can continue providing Bidirectional Reflectance Distribution Function (BRDF), albedo and Nadir BRDF-Adjusted surface Reflectances (NBAR) and quality information for research and operational users. The aim is to extend the long term consistent MODIS standard data record into the VIIRS era.

The New England region (Tile H12V04) is used for this prototyping exercise to demonstrate the retrieval of bidirectional reflectance distribution functions (BRDFs) and albedo measures with VIIRS reflectance data.

The retrieval utilizes the same approach with the Collection V006 daily MODIS Bidirectional Reflectance Distribution Function (BRDF)/Albedo product. Multidate, multispectral, cloud-cleared, atmospherically-corrected surface reflectances are used to fit the Ross-Thick/Li-Sparse-Reciprocal (RTLSR) semi-empirical BRDF model.

 

   (1)

 

Where θ, υ and ϕ are solar zenith, view zenith and relative azimuth angles; iso, vol and geo mean isotropic, volumetric and geometric. K_vol is the volumetric kernel derived from Ross-Thick volume scattering radiative model, and K_geo is the geometric kernel derived from Li-Sparse geometric shadow casting theory. f_iso, f_vol and f_geo are the weights, given to the BRDF model parameters.  A Least-Squares Error function (LSE) is used to establish the analytical solutions for the model parameters f_k.

To do so, first, cloudy or low quality observations are deleted. Clear sky observations are weighted based on their quality (shadowed or affected by aerosol), observation footprint, and proximity to the production date of interest. Then the model parameters f_k are retrieved using the weighted reflectances. When a high quality full inversion is not possible, the latest full inversion retrieved is used as a priori information to produce a lower quality magnitude inversion.  The model parameters f_k are then utilized to calculate albedo and NBAR. The QA flags provided include the retrieval quality, the uncertainty, and the dates actually used for the retrieval.

In comparisons with MODIS results of New England, the results demonstrate that VIIRS can provide similar BRDF, albedo and nadir reflectance products that will assure data continuity for land surface climate and biosphere models. Figure 1 is a true color image of BSA (Black Sky Albedo) of New England from VIIRS. As 80 percent of New England is covered by forest or thick woods these  data can be used are generally characterizing the anisotropy and structural variability of the forest canopies and providing measures of the forest surface energy budget.

Figure 1 True color image of BSA of New England