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ATBD - Overview

Overview

An Agorithm Theoretical Basis Document (ATBD) is a fairly complete description of the algorithm(s) used to create the satellite product(s) under discussion. As such it is intended for an advanced audience.  A more simplified discussion and explanation of the dark target products and how they are created can be found in the algorithm and product pages of our website. The content of this ATBD has been modified slightly for web display so it is may not identical to the full document which can be downloaded from this site.

The MOderate resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra and Aqua satellites retrieves the aerosol optical depth (AOD) on a near-daily basis over ocean and vegetated land, as well as proxies for aerosol size (e.g. Fine-Weighting or Ångström Exponent) over ocean. These aerosol products are primarily intended for radiative budget and climate applications, but are also relevant for hydrological, oceanographic and air quality applications. The combined ocean/land algorithm takes advantage of the MODIS wide spectral range (0.41 – 14.4 mm) and fine spatial resolution (between 0.25 and 1 km depending on channel), with near-daily global coverage. These characteristics of MODIS allow excellent cloud screening (rejection) while maintaining statistics of cloud free pixels. The wide spectral range permits resolution of fine and coarse aerosols.

This Algorithm Theoretical Basis Document (ATBD-16) describes the Collection 6 (C6) version of the “Dark-Target” (DT) algorithms for retrieving aerosol properties over global land and ocean, as applied to spectral reflectance observed by Moderate resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra and Aqua satellites.

ATBD-16 updates and simplifies previous versions, including the 1996 pre-launch version (ATBD-96) and the most-recent 2009 Collection 5 version-Revision 2: (ATBD-09). Both land and ocean algorithms have experienced modest updates since ATBD-09, so this document focuses on describing the current algorithm and notable upgrades. Changes include updated cloudmasking, the addition of wind speed dependent look up tables, and updated QA logic. The gas (ozone, water vapor and carbon dioxide) absorption corrections, central wavelength assumptions, and assumed Rayleigh optical depths have also been updated. Co-located sunphotometer validation for an eight-month testbed of data is provided for both land and ocean products. In addition to the standard 10 km resolution product, C6 introduces a new, additional MOD04/MYD04 product with a 3 km resolution. Lastly, we include a section describing the structure of the operational product files (i.e. MOD04/MYD04-C6).  Please note that unless otherwise specified this document refers to the “MODIS DT” algorithm.  Although the “Deep Blue” (DB) retrieval (Hsu et al., 2006, Hsu et al., 2013, Sayer et al., 2013) is a component of the MOD04/MYD04 product, it is not detailed in this ATBD. However, we provide a short description of a new DB/DT merge product that is part of the C6 release.