Too cold, too saturated? Evaluating climate models at the gateway to the Arctic

Abstract. The Arctic wintertime energy and moisture budget are largely controlled by the advection of warm, moist air masses from lower latitudes, cooling and drying of these air masses inside the Arctic and the export of cold, dry air masses. Climate models have substantial difficulties in representing key processes in these air-mass transformations, including turbulence under stable stratification and mixed-phase cloud processes. Here, we use radiosonde profiles of temperature and moisture and surface radiation observations from Ny Ålesund, Svalbard (1993–2014), to assess the properties of air masses being imported into and exported from the central Arctic in CMIP6 climate models. In the free troposphere, models tend to be cold-biased especially for the coldest temperatures. Most models underestimate the frequency of occurence of supersaturation with respect to ice, and a sensitivity experiment suggests that this can be improved by using 2-moment microphysics, i.e. prognostic rather than prescribed ice number concentrations. Cold and dry biases are stronger in air masses being exported from the Arctic than those entering the Arctic. This suggests that previously reported cold biases and excess energy convergence in the Arctic in CMIP6 models are probably due to errors in local thermodynamic processes.