Evaluation of ice optic parameterization to describe the radiative effect of Arctic Cirrus in numerical weather prediction models

We use airborne radiation measurements from the research campaign on ice clouds in high latitudes (CIRRUS-HL, 31 May – 29 July 2021) collected with the Spectral Modular Airborne measuRement sysTem (SMART) to evaluate the performance of the radiative transfer scheme ecRad of the Integrated Forecasting System (IFS) numerical weather prediction model developed by the European Center for Medium-Range Weather Forecast (ECMWF). We focus on the radiative properties and effects of Arctic cirrus, which compared to mid-latitude cirrus is known to have a stronger warming effect. We run ecRad in an offline mode using the current forecast of the operational Atmospheric Model High Resolution configuration (HRES) of the IFS to simulate the iraddiances. Simulated spectral irradiances are compared to the measurements onboard the High Altitude Long Range Research Aircraft (HALO). We test different ice crystal parameterization schemes in ecRad to quantify the influence of the parameterization scheme on the radiative transfer calculations. However, large differences between observations and simulations are found, and could be linked to the misrepresentation of liquid cloud layers in IFS, which were present below the cirrus. Thus, it is concluded, that for the case study presented here, the cloud situation is too complex to isolate the influence of the effect of the parameterization on the radiative transfer simulation. Therefore, further case studies such as provided by the HALO-(AC)3 campaign ((AC)3 - ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms) in Spring 2022 will be investigated.