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Abstract

Cirrus clouds play an important role in the Earth's energy budget by reflecting solar radiation and trapping long wave radiation from the surface. While globally, they are believed to have a net warming effect on the atmosphere, the regional differences in the cirrus cloud radiative effect (CRE) can be significant. In the polar regions, where there is restricted solar radiation or high surface albedo, cirrus clouds have a strong warming effect on the atmosphere. However, the uncertainties in calculating the cirrus cloud CRE remains significant, partly due to the incomplete understanding of ice crystal optical properties. To address this issue, in-situ observations of Arctic cirrus ice crystal microphysical and optical properties were made using the Particle Habit Imaging and Polar Scattering (PHIPS) probe during the CIRRUS-HL airborne measurement campaign in the summer of 2021. The observations are used to generate a new parameterisation of ice crystal single scattering properties, which is tested in radiative transfer simulations to determine the CRE of Arctic cirrus during the polar day. The study highlights the key microphysical properties that define the magnitude of the ice cloud asymmetry parameter and provides a more accurate estimate of the CRE of cirrus clouds, particularly in the polar regions. The findings have significant implications for climate modelling and weather forecasting, and they demonstrate the importance of using direct in-situ observations to improve our understanding of the optical properties of cirrus clouds.