Retrieval of microphysical cloud parameters from EM-FTIR spectra measured in Arctic summer 2017

Abstract. Infrared spectral radiances of optically thin clouds show high sensitivity to changes of the microphysical cloud parameters. Therefore, measurements of infrared spectral radiance of clouds in the spectral range from 770.9 cm⁻¹ to 1163.4 cm⁻¹ using a mobile Fourier Transform Infrared spectrometer were performed on the German research vessel Polarstern in the Arctic in summer 2017.

A new retrieval for microphysical cloud parameters of optically thin clouds called Total Cloud Water retrieval, designed to retrieve cloud water optical depth τ_cw, total effective droplet radius r_total and condensed water path CWP from infrared spectral radiances without the incorporation of spectral radiances in the far-infrared below 600cm⁻¹, has been developed for application on radiances from the measurement campaign. Validation is performed against derived quantities from a combined cloud radar, lidar and microwave radiometer measurement synergy retrieval, called Cloudnet, performed by the Leibnitz Institute for Trospheric Research.

Applied to spectral radiances of synthetic testcases, Total Cloud Water retrieval shows a high ability to retrieve τ_cw with a correlation of |r| = 0.98, as well as to retrieve CWP with |r| = 0.95 and r_total with |r| = 0.86. Using the dataset from the campaign, a comparison between CWP from Total Cloud Water retrieval and Cloudnet was performed and showed a correlation of |r| = 0.81. In conclusion, the comparison to artificial clouds and the validation using Cloudnet showed that Total Cloud Water retrieval is able to retrieve the condensed water path from clouds for optically thin clouds and makes it a useful complementation for thin clouds to existing microwave-based measurements.