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https://doi.org/10.5194/egusphere-2025-546
https://doi.org/10.5194/egusphere-2025-546
13 Feb 2025
 | 13 Feb 2025

Retrieving Vertical Profiles of Cloud Droplet Effective Radius using Multispectral Measurements from MODIS: Examples and Limitations

Andrew John Buggee and Peter Andrew Pilewskie

Abstract. With the coming launch of the Climate Absolute Radiance and Refractivity Earth Observatory (CLARREO) Pathfinder (CPF) comes an opportunity to develop a new cloud retrieval from spectral reflectance measurements. With continuous coverage across the shortwave spectrum and a factor of 5 to 10 lower radiometric uncertainty than the Moderate Resolution Imaging Spectroradiometer (MODIS), CPF facilitates the retrieval of a vertical profile of droplet size, providing insight into the internal structure of a cloud. Measurements from MODIS coincident with in situ observations provide the foundation for developing a constrained optimal estimation technique, ensuring a solution consistent with forward model assumptions. The limited unique information in the MODIS bands used in this analysis led to a non-unique solution, with many droplet profiles leading to convergence. Droplet size at cloud bottom is difficult to constrain because visible and near-infrared reflectances have an average penetration depth near cloud top. The region of convergence within the solution space decreased along the cloud bottom radius dimension by 2 μm when increasing the number of wavelengths used in the retrieval from seven to 35, and by 5 μm when reducing the measurement uncertainty from 2 % to 0.3 %. The enhanced accuracy and, to a lesser degree, the enhanced spectral sampling provided by CPF measurements are essential to extracting vertically resolved droplet size information from moderately thick, warm clouds.

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A constrained optimal estimation technique was developed to utilize space-borne hyperspectral...
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