The NASA MODIS-VIIRS Continuity Cloud Optical Properties Products

Abstract

The NASA Aqua MODIS and Suomi National Polar-Orbiting Partnership (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) climate data record continuity cloud properties products (CLDPROP) were publicly released in April 2019 with an update later that year (Version 1.1). These cloud products, having heritage with the NASA Moderate-resolution Imaging Spectroradiometer (MODIS) MOD06 cloud optical properties product and the NOAA GOES-R Algorithm Working Group (AWG) Cloud Height Algorithm (ACHA), represent an effort to bridge the multispectral imager records of NASA’s Earth Observing System (EOS) and NOAA’s current generation of operational weather satellites to achieve a continuous, multi-decadal climate data record for clouds that can extend well into the 2030s. CLDPROP offers a “continuity of approach,” applying common algorithms and ancillary datasets to both MODIS and VIIRS, including utilizing only a subset of spectral channels available on both sensors to help mitigate instrument differences. The initial release of the CLDPROP_MODIS and CLDPROP_VIIRS data records spans the SNPP observational record (2012-present). Here, we present an overview of the algorithms and an evaluation of the intersensor continuity of the core CLDPROP_MODIS and CLDPROP_VIIRS cloud optical property datasets, i.e., cloud thermodynamic phase, optical thickness, effective particle size, and derived water path. The evaluation includes analyses of pixel-level MODIS/VIIRS co-locations as well as spatial and temporal aggregated statistics, with a focus on identifying and understanding the root causes of individual dataset discontinuities. The results of this evaluation will inform future updates to the CLDPROP products and help scientific users determine the appropriate use of the product datasets for their specific needs.

Sensitivity of Multispectral Imager Liquid Water Cloud Microphysical Retrievals to the Index of Refraction

“…This study uses version 1.1 of NASA's MODIS/VIIRS continuity cloud optical property algorithm (product filename designation CLDPROP). This algorithm, publicly released in 2019, closely follows that of the NASA MODIS Collection 6.1 (C6.1) product (filenames MOD06 and MYD06 for MODIS Terra and Aqua, respectively [18]) but modifies the cloud mask and cloud-top property algorithms to use only the spectral channels that are common to the two imagers [31,38]. Cloud mask and optical property retrievals are, at nadir, 750 m for VIIRS (M-bands) and 1 km for MODIS (higher spatial resolution channels aggregated into a merged 1 km L1B file).…”

“…Mie calculations of droplet single scattering co-albedo (1-SSA) as a function of CER are shown in Figure 3. Calculations at each CER are made using a modified gamma droplet size distribution with an effective variance of 0.10, consistent with the forward modeling assumption in the NASA MODIS and VIIRS cloud algorithms [18,31]. The dashed lines in each panel represent the index of refraction dataset used in the MODIS standard cloud product algorithm, while the two solid lines indicate candidate datasets examined in detail in this study.…”

“…The dashed lines in each panel represent the index of refraction dataset used in the MODIS standard cloud product algorithm, while the two solid lines indicate candidate datasets examined in detail in this study. The thick solid lines are the supercooled (265 K) temperature datasets adopted for the MODIS/VIIRS cloud continuity optical property algorithm as shown in Section 3.2 [31], with the exception that the MWIR scattering properties are shown for the 269 K laboratory measurements (for comparison with Figure 2) instead of a 265 K interpolated value. While the plots provide a CER lower limit of 2 µm, the retrieval algorithms of Section 3 (and all CLDPROP and MYD06 production code) reject droplet retrievals less than 4 µm.…”

“…However, resulting intra-sensor channel-to-channel CER changes can be substantial between the datasets. Note that other radiative modeling error sources that can contribute to spectral CER differences are discussed in [31] (see Section 4), including sensor radiometric calibration biases and cloud microphysical vertical heterogeneity.…”

“…The study was largely motivated, as further discussed in Section 3.2, by the development of the NASA Aqua MODIS and Suomi National Polar-orbiting Partnership (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) cloud continuity products (CLDPROP) continuity cloud product and the challenges encountered in achieving CER retrieval consistency in the spectrally mismatched 2.2 µm windows (central wavelengths at 2.13 µm for MODIS and 2.25 µm for VIIRS) when using the otherwise common algorithm and accounting for potential radiometric differences [30,31]. As a result of the study described here, the CLDPROP algorithms use the index of refraction datasets of [25] at 265 K for the SWIR channel cloud radiative models and, for consistency in the MWIR, the dataset of [26] interpolated to 265 K (from available tabulated data at 258 K and 269 K).…”