![]() ![]() MODIS Atmosphere Team Collection 6 (C6) Aqua Level-2 (L2), or pixel-level, reprocessing began in December 2013 and was completed in early May 2014 (data acquisition dates 4 July 2002 through 31 December 2013) Aqua forward processing began on 1 January 2014. Collection 5 (C5) was completed in calendar year 2006, while a reprocessing to C5.1 was completed in calendar year 2010. A major increment in the collection number denotes comprehensive changes to the instrument calibration and science algorithms. MODIS (re)processing streams are referred to as data collections. MOD06-specific heritage work is also described in and (microphysical retrievals using the AVHHR 3.7 μm channel), (1.6–2.1 μm retrievals over snow/ice surfaces), and thermodynamic phase retrievals. The basic physical principle behind the simultaneous retrieval of COT and CER is the bi-spectral solar reflectance method first described in and applied to airborne data. Relative to previous generation global imagers such as the Advanced Very High Resolution Radiometer (AVHRR), MODIS has a number of additional spectral channels, including window channels centered near 1.6 and 2.1 μm that, in addition to an AVHRR heritage channel near 3.7 μm, provide cloud microphysical information. The 1 km cloud optical and microphysical product algorithm makes primary use of six VIS, near-infrared (NIR), SWIR, and midwave-infrared (MWIR) MODIS channels, as well as several thermal IR channels. The cloud top properties algorithm, which relies on CO 2-slicing channels (13–14 μm spectral region) and two IR window channels –, has heritage with the High resolution Infrared Radiation Sounder (HIRS) (see ) spatial resolution is at both 5 km and 1 km for C6. The NASA operational cloud product (Earth Science Data Set names MOD06 and MYD06 for Terra and Aqua MODIS, respectively, though for simplicity the modifier MOD will subsequently be used for both Terra and Aqua since the respective algorithms are nearly identical) contains pixel-level retrievals of cloud top properties (pressure, temperature, and height during both day and night) and cloud optical and microphysical properties (cloud optical thickness, effective particle radius, and derived water path for both liquid water and ice cloud thermodynamic phases during daytime only) (see ). ![]() MODIS provides unique spectral and spatial capabilities for retrieving cloud properties. Designed to provide global observations of the Earth’s atmosphere, land, and oceans –, MODIS measures reflected solar and emitted thermal radiation in 36 spectral channels ranging from the visible (VIS) to the infrared (IR) at a native spatial resolution of 250 m (0.66 and 0.87 μm channels), 500 m (five channels including 3 shortwave-infrared ), and 1 km (all others). Since the launch of NASA’s Terra satellite on 18 December 1999, followed by Aqua on, the Moderate-resolution Imaging Spectroradiometer (MODIS) has become one of the most widely used satellite remote sensing platforms for Earth science investigations. While the emphasis is on the suite of cloud optical property datasets, other MODIS cloud datasets are discussed when relevant. Example Level-2 granule and Level-3 gridded dataset differences between the two collections are shown. The C6 algorithm changes collectively can result in significant changes relative to C5, though the magnitude depends on the dataset and the pixel’s retrieval location in the cloud parameter space. ![]() Notable C6 optical and microphysical algorithm changes include: (i) new ice cloud optical property models and a more extensive cloud radiative transfer code lookup table (LUT) approach, (ii) improvement in the skill of the shortwave-derived cloud thermodynamic phase, (iii) separate cloud effective radius retrieval datasets for each spectral combination used in previous collections, (iv) separate retrievals for partly cloudy pixels and those associated with cloud edges, (v) failure metrics that provide diagnostic information for pixels having observations that fall outside the LUT solution space, and (vi) enhanced pixel-level retrieval uncertainty calculations. Here we provide an overview of major C6 optical property algorithm changes relative to the previous Collection 5 (C5) product. Collection 6 (C6) reprocessing of the product was completed in May 2014 and March 2015 for MODIS Aqua and Terra, respectively. The MODIS Level-2 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).
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