PACE Mission Instruments Show Varied Cloud Droplet Size Retrievals, Highlighting Measurement Discrepancies
An intercomparison study of liquid cloud droplet size retrievals from three instruments aboard the Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission—SPEXone, HARP-2, and OCI—has revealed differences in their measurements. SPEXone and HARP-2, which use hyperspectral and hyper-angular polarization observations respectively, showed the closest agreement in retrieving cloud droplet effective radius (CER) and cloud effective variance (CEV). OCI, which relies on radiometric measurements in the visible and shortwave infrared, generally retrieved larger CER values compared to both polarimeters. Over a one-year period (June 2024 - May 2025), the mean difference between SPEXone and HARP-2 CER was 1.45 µm over land and 0.81 µm over ocean. The mean difference between SPEXone and OCI was -2.06 µm over land and -2.67 µm over ocean, while the difference between HARP-2 and OCI was -3.51 µm over land and -3.47 µm over ocean. Globally, the annual CER statistical distribution peaked near 7 µm for SPEXone and HARP-...