Identification of Precipitating Marine Low-Altitude Water Clouds by CALIPSO: Observations and Detections

Abstract

Marine boundary layer (MBL) clouds and their precipitation are crucial in regulating the Earth's radiation budget. In this study, we use measurements from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) to investigate how the microphysical and optical properties of MBL clouds change at the cloud top when precipitation forms. These changes include (a) broadening of particle size distributions when transitioning from cloud droplets to raindrops, as evidenced by a pronounced decrease in the cloud extinction-to-backscatter ratios; (b) lower droplet number concentrations, resulting in reduced in-cloud signal attenuation, smaller backscatter coefficients, lower depolarization ratios, and deeper signal penetration into clouds; and (c) increased cloud inhomogeneity, arising from significant spatial variability in droplet size and number concentrations that yield corresponding variations in lidar backscatter signal intensities. The distinct differences observed in CALIOP measurements of precipitating and non-precipitating clouds allow for effective discrimination between the two states. Independent detections of precipitating clouds from space-borne lidar are expected to provide new insights into cloud life cycles and enhance the existing A-Train data record by filling many of the existing gaps in global-scale light precipitation detection.