An Investigation of Optically Very Thin Ice Clouds from Ground-Based ARM Raman Lidars

Optically very thin ice clouds have the potential to not be detected by the space-based Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), a lidar system on board Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observation (CALIPSO) because of the instrument sensitivity. Following the instrument specifications, 0.01 represents the lower bound for optical depths retrieved from CALIOP. Higher sensitivity ground-based Raman lidars (RL) operated by the Department of Energy (DOE) Atmospheric Radiation Measurement Program (ARM) at the Southern Great Plains (SGP) site in Lamont, Oklahoma and the Tropical Western Pacific (TWP) site in Darwin, Australia provide the opportunity to detect optically thin ice clouds and assess the implications to space-based observations and the radiation budget. Ice cloud column optical depths retrieved from the RLs indicate around a quarter (~23%) of the ice cloudy column optical depth distribution to be optically very thin ice clouds (optical depths below 0.01) at both the SGP and TWP sites. The majority (~64-73%) of the optically very thin ice clouds are adjacent to ice cloud optical depth above 0.01, occurring at the cloud edges of or in-between optically thicker ice clouds. Ice cloud column optical depth time series indicate an increase from very thin ice cloud optical depths to thicker ice cloud optical depths and the reverse, providing support for a continuum in the cloud field from clear sky to cloudy sky. Comparisons between RL and CALIPSO ice cloud products will be analyzed through case studies when optically very thin ice clouds are observed by the RLs to assess the implications on CALIPSO ice cloud products. Additionally, the radiative impacts of the optically very thin ice clouds will be quantified using the NASA Langely Fu-Liou Radiative Transfer Model by comparing the resultant radiation budget and radiative heating profile with and without considering optically very thin ice clouds.