Validation of MODIS Bi-spectral Retrieved Liquid Cloud Droplet Effective Radius over the Philippines with CAMP2Ex Airborne Remote Sensing and In-Situ Measurements

Currently, our longest record (spanning nearly four decades) of observations for cloud optical and microphysical properties from space is derived from the bi-spectral approach. It simultaneously retrieves cloud optical depth and cloud effective radius (Re) from visible/near infrared and shortwave infrared radiances. This approach makes several critical assumptions, including 1-D radiative transfer, single-mode droplet size distribution, and cloud horizontal and vertical homogeneity. Deviations from these assumptions lead to systematic errors in the cloud retrievals that co-vary with the underlying scene heterogeneity. Using data collected during the Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) from August-October 2019, we present one of the first field validations of bi-spectral Re in heterogeneous cloud regions. Cross comparisons between Terra-MODIS bi-spectral Re, airborne remotely sensed Re from the Research Scanning Polarimeter (RSP) onboard the NASA P-3, and in-situ measurements from both the P-3 and the SPEC Learjet are applied to matching cumulus cloud fields. Re from the polarimetric technique is used for validating the bi-spectral approach because it is less sensitive to the assumptions of plane parallel and homogeneous clouds. RSPs ability to derive collocated bi-spectral and polarimetric Re allows for comparing Re retrievals from the two techniques with the same sampling. When compared to RSP polarimetric Re, on average, RSP bi-spectral Re overestimates by 6 m, and the MODIS Re overestimates by 7 m. By contrast, in-situ derived Re and our bias-adjusted MODIS Re (based on the angular variations in the retrieved cloud optical depth) are both within 2 m. Sensitivity analysis using RSP data reveals that small cumuli frequently exist in the sampled region (50% of the clouds sampled by RSP have transect lengths less than 0.6 km), and that clouds with the smallest transect length and retrieved optical depth are associated with the largest Re overestimates.The overestimates in bi-spectral Re ultimately affects derived liquid water path and number concentrations. Our results suggest a careful reexamination of past studies, particularly those which used passive satellite bi-spectral Re to study aerosol-cloud interaction estimates.