Abstract
A cloud property retrieved from multispectral imagers having spectral channels in the shortwave infrared (SWIR) and/or midwave infrared (MWIR) is the cloud effective particle radius (CER), a radiatively relevant weighting of the cloud particle size distribution. The physical basis of the CER retrieval is the dependence of SWIR/MWIR cloud reflectance on the cloud particle single scattering albedo, which in turn depends on the complex index of refraction of bulk liquid water (or ice) in addition to the cloud particle size. There is a general consistency in the choice of the liquid water index of refraction by the cloud remote sensing community, largely due to the few available independent datasets and compilations. Here we examine the sensitivity of CER retrievals to the available laboratory index of refraction datasets in the SWIR and MWIR using the retrieval software package that produces NASA’s standard Moderate Resolution Imaging Spectroradiometer (MODIS)/Visible Infrared Imaging Radiometer suite (VIIRS) continuity cloud products. The sensitivity study incorporates two laboratory index of refraction datasets that include measurements at supercooled water temperatures, one in the SWIR and one in the MWIR. Neither has been broadly utilized in the cloud remote sensing community. It is shown that these two new datasets can significantly change CER retrievals (e.g., 1–2 µm) relative to common datasets used by the community. Further, index of refraction data for a 265 K water temperature gives more consistent retrievals between the two spectrally distinct 2.2 µm atmospheric window channels on MODIS and VIIRS. As a result, 265 K values from the SWIR and MWIR index of refraction datasets were adopted for use in the production version of the continuity cloud product. The results indicate the need to better understand temperature-dependent bulk water absorption and uncertainties in these spectral regions.
Highlights
Clouds are the strongest modulator of the Earth’s shortwave and longwave radiative budget with direct importance to the hydrological cycle
We show the sensitivity of liquid water scattering properties in the relevant Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS) effective radius retrieval channels to the candidate index of refraction datasets discussed in Section 2, along with the resulting temporally/spatially gridded cloud droplet retrieval sensitivities for both sensors
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
Summary
Clouds are the strongest modulator of the Earth’s shortwave and longwave radiative budget with direct importance to the hydrological cycle. Their feedbacks in response to global warming are not well understood, either regionally or globally, constituting the largest source of inter-model uncertainty in climate sensitivity (e.g., [1,2,3]), in particular for low liquid water marine clouds [4,5]. CER, the ratio of the third to second moment of the cloud particle size distribution, is fundamentally a radiative quantity as retrieved from imager spectral reflectance measurements in the shortwave infrared (SWIR) 1.6 and 2.2 μm and/or the midwave infrared (MWIR) 3.7 μm atmospheric windows. The SSA of a spherical liquid water droplet depends on both the droplet size and the imaginary index of refraction of bulk liquid water
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
