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Abstract. In this case study, measured solar irradiances above and below Arctic cirrus are compared to simulations of the European Centre for Medium-Range Weather Forecasts’ Integrated Forecasting System (IFS) making use of offline runs of the operational ecRad radiation scheme. Independent of the solar irradiances, cirrus properties are derived from active remote sensing and used to evaluate the optical and microphysical parameterizations in ecRad. The data set was collected in the central Arctic over sea ice (81°–90° North) with the High Altitude LOng range research aircraft (HALO) during a campaign in March and April 2022. HALO was equipped with broadband radiation and remote sensing instrumentation, including upward and downward-looking pyranometers (solar irradiance), a cloud radar, and a multi-wavelength water vapor differential absorption lidar. Flight legs above and below single-layer cirrus were performed. Measurements of solar irradiance are used to evaluate the ecRad radiation scheme in two case studies of optically thin and thick cirrus. The optically thin cirrus had a mean transmissivity of 0.9, while the optically thick cirrus had a transmissivity of about 0.6. Different ice optics parameterizations optionally available within ecRad are tested to improve the match between simulation and measurements. Furthermore, the IFS predicted ice water content and ice effective radius were replaced by values retrieved with the radar and lidar. The choice of ice optics parameterizations does not significantly improve the model-measurement agreement. However, introducing the retrieved ice microphysical properties brings measured and modelled irradiances into closer agreement for the optical thin cirrus, while the optically thick cirrus case is now simulated as too thick. From the comparison of the different ice optics parametrizations in the original and in the forced setup, it can be concluded that the ice water content forcasted by the IFS is realistic. The missmatch between observed and simulated irradiances mostly originates from the assumed or parameterized ice effective radius.
Röttenbacher et al. (Wed,) studied this question.