Measurement Report: Differences in cloud optical and microphysical properties in the Arctic and Antarctic derived using thermal infrared spectroscopy

Abstract. Climate models struggle to accurately represent polar regions, largely due to the difficulty in modeling clouds. The uncertainty budget of polar radiation is dominated by cloud and cloud-aerosol interactions, but challenges in maintaining robust field observations mean that even basic knowledge such as cloud occurrence is not well known. Measurements of the thermal emission of Earth’s atmosphere can help close this knowledge gap due to the sensitivity of this spectral region to radiative properties of clouds. Measurements of the downwelling atmospheric emission (400 to 3000 cm⁻¹) have been collected at two polar field sites using Atmospheric Emitted Radiance Interferometer (AERI) instruments, two at the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka, Canada (80° N), for which we use data from 2006 to 2022, and another at McMurdo Station (77° S) in Antarctica as part of the Atmospheric Radiation Measurement (ARM) West Antarctica Radiation Experiment (AWARE) project in 2016. We analyze these spectra, with supplementary data from other instruments and models, to compare optical and microphysical properties of moderately thick clouds at Eureka and McMurdo Station, including optical depth, thermodynamic phase, liquid droplet and ice crystal effective scattering radii, and cloud boundaries. We find that the clouds sampled at McMurdo generally feature lower temperatures, smaller liquid droplets, a more concentrated distribution of ice effective radii, less seasonal variability in optical depth, and lower optical thickness because they consist of a higher proportion of ice than those at Eureka. Additionally, both locations have high occurrence rates of supercooled liquid and mixed-phase clouds and exhibit differences between single-phase and mixed-phase microphysics.