Influence of the ice crystal shape on radiative effects of Arctic cirrus: Observations and representation in numerical weather prediction models

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Based on the sensitivity of the ECMWF radiation scheme to the parametrization of ice crystal radiative properties observed in the completed project for ice clouds in mid-latitudes, the continuation of the project aims to extends this model evaluation for the radiative effects of cirrus in high-latitudes. For Arctic cirrus, the analysis needs to be extended to the thermal-infrared radiation budget, which dominates due to the lag of solar radiation and depends on cloud altitude, thickness and ice crystal properties. Therefore, the project is embedded in the proposed HALO missions Cirrus-HL (High Latitude) and HALO-(AC)³ (ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms), which both aim to investigate Arctic clouds by state of the art airborne remote sensing (active and passive) and cloud microphysical in situ observations.Within this project, measurements of the cloud-reflected solar and emitted thermal infrared radiance and irradiance with a new broadband radiometer system, a spectral albedometer, and a thermal-infrared imager are proposed to quantify the radiative energy budget above and below Arctic cirrus. Based on the observations, the cirrus radiative effect will be derived and evaluated with respect to its dependence on cloud macrophysical and microphysical properties, and the special Arctic environment (sea ice, persistent low clouds). We will evaluate how well the cirrus and their radiative effects are represented in numerical weather prediction models. The comparison will be performed in the observational space of irradiances and radiances instead of cloud properties. Therefore, the output of the numerical weather prediction (NWP) models will be converted by radiative transfer models into the observed radiation quantities. Operational and experimental radiation schemes will be tested and compared to the observed radiation quantities to identify the reasons of potential differences between model and observation. The airborne observations and the radiative transfer simulations will be used to corroborate the hypothesis: “The radiative effects of Arctic cirrus, which significantly depend on their macrophysical and microphysical properties such as the ice crystal shape, can be used to validate numerical weather prediction models.” To address this hypothesis, the proposed study will focus on five specific science questions: (A) How variable are the radiative effects by Arctic cirrus on different horizontal scales (e.g., contrail cirrus, cirrus in air mass transformation)? (B) How strong the radiative effects depend on the presence of sea ice and low clouds? (C) Do observed ice crystal shapes of Arctic cirrus lead to a significant change of cloud radiative effects? (D) Do NWP models realistically represent the radiative effects of Arctic cirrus? (E) Can we use spectral solar and thermal-infrared radiation measurements to constrain potential uncertainties of NWP models?