An Observational-Based Approach for Studying Cirrus Microphysical Properties with the Combined Use of Lidar and Polarimeter Observations

Cirrus clouds permanently cover almost half of the Earth’s surface and impact the global climate system through their role in the radiative budget. A better understanding of cirrus microphysical properties, especially the shape and size of ice crystals with observing conditions, is necessary to more accurately quantify their effects on the climate system. However, the shape of ice crystals remains a major uncertainty, due to the infinite possibilities. As such, models generally assume a random orientation of hexagonal or columnar ice crystals.

This study aims to quantify microphysical properties of cirrus clouds utilizing coincident lidar and polarimeter aircraft data collected during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling (SEAC⁴RS), Aerosol Characterization from Polarimeter and Lidar (ACEPOL), and Polarimeter Definition Experiment (PODEX) campaigns. TheCloud Physics Lidar (CPL) flown onboard NASA’s ER-2 aircraft uses backscatter and depolarization products at multiple wavelengths to classify clouds and aerosols with high temporal and spatial resolution. Additionally, airborne polarimeters such as the Research Scanning Polarimeter (RSP) enable the retrieval of cirrus microphysical properties by measuring the radiance and state of polarization of light. Using coincident observations from lidar and polarimeter, the relationships between depolarization ratio, effective radius and environmental parameters such as temperature and relative humidity are studied and are further inter-compared with radiative transfer model-based analyses.