Representation of Arctic mixed-phase clouds in ECMWF forecasts during ACLOUD

Abstract

The representation of Arctic clouds in numerical weather prediction models is challenging, especially for mixed-phase clouds with both a liquid and ice phase present. We compare measurements conducted during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign, which took place in May/June 2017 northwest of Svalbard, Norway, with the operational ‘Atmospheric Model high resolution’ configuration (HRES) of the Integrated Forecasting System (IFS), operated by the European Centre for Medium-Range Weather Forecasts (ECMWF). Instead of using cloud retrieval products from airborne remote sensing, the comparison is performed in the observational space of spectral solar irradiances reflected by the clouds. To allow such an analysis along the flight track at flight level, the operational ecRad radiation scheme of the IFS is used in offline mode. Besides the HRES model output, vertical profiles of concentrations of trace and greenhouse gases provided by the ECMWF Atmospheric Composition Reanalysis 4 serve as the input for ecRad. The ability of the IFS to realistically represent the airborne radiation measurements collected during ACLOUD is evaluated for flight sections above sea ice and open ocean. Inconsistencies between the upward irradiance observed during ACLOUD and the simulations by ecRad are found and may originate from uncertainties introduced by the cloud fraction, the cloud phase, the sea ice albedo, and the ice optics parameterization. Our analysis aims to separate the influence of the different macro- and microphysical parameters on the upward irradiance. To disentangle the impact of these parameters, the spectral irradiance is analyzed where e.g. the impact of liquid and ice phase can be separated. Different case studies give insight into a sub-grid cloud cover variability that is not seen by the IFS above open ocean and an overestimation of the measurements by ecRad above sea ice that can be explained by the lack of cloud brightness. EcRad is additionally run with improved ice optics parameterizations. The choice of the applied ice optics becomes more important with an increasing ice water path of the clouds and is investigated in detail within the near-infrared bands of ecRad.