Evaluating Imager Spectral Cloud Effective Radius Retrievals Against Multi-Angle Polarimetry and in Situ Cloud Probes

Cloud droplet effective radius (CER), defined as the ratio of the third moment of the droplet size distribution to the second moment, is a retrieved radiative quantity that has been widely used for studies of aerosol/cloud interactions and their impacts on the Earth’s radiation budget and hydrological cycle. CER commonly is retrieved simultaneously with cloud optical thickness (COT) from passive imager remote sensing observations using a bi-spectral technique pairing a non-absorbing visible or near-infrared spectral channel sensitive to COT with an absorbing shortwave infrared (SWIR) or mid-wave infrared (MWIR) spectral channel sensitive to CER. Multi-angle polarimetry also can be used to retrieve CER, along with the width of the droplet size distribution, from polarized reflectance observations across the cloud-bow. Examples of spaceborne imagers providing global CER retrievals are MODIS on NASA’s Terra and Aqua satellites, and VIIRS on SNPP and NOAA’s new generation of polar-orbiting meteorological satellites (NOAA-20/21+), among others. Spaceborne polarized cloud-bow retrievals have been limited primarily to CNES’s POLDER, though several upcoming spaceborne missions will include multi-angle polarimeters, notably ESA’s EarthCARE and NASA’s PACE and Atmosphere Observing System (currently in formulation) missions.

Evaluating remote sensing retrievals of CER, either spaceborne or airborne, typically is done via comparisons with CER derived from droplet size distributions measured in situ by airborne cloud probes. However, such seemingly straightforward comparisons in practice involve numerous confounding factors that have consequential implications on the interpretation of comparison results. For the remote sensing retrievals, these include imager radiometric calibration, spectral channels having differing vertical sensitivities, spectral above-cloud atmospheric corrections, and forward radiative transfer model assumptions such as plane-parallel atmospheres, assumed droplet size distributions, and the temperature dependence of the liquid water complex index of refraction, each of which can have impacts on the retrieved CER. For the cloud probes, confounding factors include the sampling strategy within the cloud, sensitivities to different portions of the size distribution, and known or unknown sizing uncertainties or errors.

In this presentation, we discuss results of an extensive comparison of airborne imager remote sensing retrievals of liquid cloud CER from the Enhanced MODIS Airborne Simulator (eMAS) against spatially and temporally co-located multi-angle polarimetric retrievals from the Research Scanning Polarimeter (RSP) and in situ cloud probes obtained for marine boundary layer stratocumulus during the NASA ORACLES field campaign. eMAS, a multi-spectral imager having spectral capabilities similar to MODIS and VIIRS, has multiple spectral channels in the CER-sensitive 1.6µm (2 channels) and 2µm (4 channels) SWIR window bands and the 3.7µm (1 channel) MWIR window band that enable a variety of bi-spectral retrieval channel pairings beyond those of any single spaceborne imager. Moreover, during ORACLES, on several occasions the aircraft flight heading was such that eMAS observed the backscatter glory in a portion of its swath; we show CER and droplet size distribution effective variance inferred from this single-scattering reflectance feature using a peak-matching technique similar to the polarized cloud-bow, and compare against both the bi-spectral and polarimetric cloud-bow retrievals. The impacts of numerous confounding factors are explored, including bi-spectral retrieval assumptions, and the broader implications of retrieval differences due to differing fundamental sensitivities of multi-spectral imagery and multi-angle polarimetry are discussed.