The MODIS Cloud Optical and Microphysical Products: Collection 6 Updates and Examples From Terra and Aqua

TLDR

MODIS Level‑2 cloud products (MOD06/MYD06) deliver pixel‑level cloud top and optical properties—including pressure, temperature, height, optical thickness, effective radius, and water path for liquid and ice phases—while other MODIS cloud datasets are referenced as needed. The study provides an overview of the major Collection 6 optical property algorithm changes relative to Collection 5. C6 introduces new ice‑cloud optical models, an expanded radiative‑transfer lookup table, improved short‑wave thermodynamic phase skill, separate effective‑radius datasets per spectral band, distinct retrievals for partly cloudy and edge pixels, failure‑metric diagnostics, and enhanced pixel‑level uncertainty estimates. C6 reprocessing finished in May 2014 (Aqua) and March 2015 (Terra); the algorithm updates produce notable differences from C5, with magnitude varying by dataset and pixel location, as illustrated by L2 granule and L3 gridded data comparisons.

Abstract

The Moderate-Resolution Imaging Spectroradiometer (MODIS) level-2 (L2) cloud product (earth science data set names MOD06 and MYD06 for Terra and Aqua MODIS, respectively) provides pixel-level retrievals of cloud top properties (day and night pressure, temperature, and height) and cloud optical properties (optical thickness, effective particle radius, and water path for both liquid water and ice cloud thermodynamic phases-daytime only). Collection 6 (C6) reprocessing of the product was completed in May 2014 and March 2015 for MODIS Aqua and Terra, respectively. Here we provide an overview of major C6 optical property algorithm changes relative to the previous Collection 5 (C5) product. Notable C6 optical and microphysical algorithm changes include: 1) new ice cloud optical property models and a more extensive cloud radiative transfer code lookup table (LUT) approach; 2) improvement in the skill of the shortwave-derived cloud thermodynamic phase; 3) separate cloud effective radius retrieval data sets for each spectral combination used in previous collections; 4) separate retrievals for partly cloudy pixels and those associated with cloud edges; 5) failure metrics that provide diagnostic information for pixels having observations that fall outside the LUT solution space; and 6) enhanced pixel-level retrieval uncertainty calculations. The C6 algorithm changes can collectively result in significant changes relative to C5, though the magnitude depends on the data set and the pixel's retrieval location in the cloud parameter space. Example L2 granule and level-3 gridded data set differences between the two collections are shown. While the emphasis is on the suite of cloud optical property data sets, other MODIS cloud data sets are discussed when relevant.