Sensitivity of simulated wintertime Arctic atmosphere to vertical resolution in the ARPEGE/IFS model

The current state of the art general circulation models, including several of those used by the IPCC, show considerable disagreement in simulating present day high latitude climate. This is of major concern and reduces the confidence in future model projections of high latitude climate. We here employ ideal vertical profiles of temperature and wind from turbulence resolving simulations to perform a priori studies of the first order eddy-viscosity closure scheme employed in the ARPEGE/IFS model. This reveals that the coarse vertical resolution (31 layers) of the model cannot be expected to realistically resolve the Arctic stable boundary layer. The curvature of the Arctic inversion and thus also the vertical turbulent exchange processes cannot be reproduced by the coarse vertical mesh employed. Correct representation of boundary layer turbulent exchange processes is a critical factor in climate simulations. To investigate how turbulent vertical exchange processes in the Arctic boundary layer are represented by the model parameterization a simulation with high vertical resolution (90 layers) in the lower part of the atmosphere is performed. Results from the model simulations are validated against data from the ERA-40 reanalysis and from in situ data from the SHEBA project. The dependence of the surface air temperature on surface winds, surface energy fluxes, inversion stability and boundary layer height is investigated. The coarse resolution run reveals considerable biases in these parameters, and in their physical relations to surface air temperature. In the simulation with fine vertical resolution these biases are clearly reduced. The physical relation between governing parameters for the vertical turbulent exchange processes becomes more realistic. The coarse resolution run shows considerable biases in representing the Arctic inversion. By improving the vertical resolution in the lower part of the atmosphere we achieve a realistic simulation of the Arctic inversion. A correct representation of the inversion is important in order to achieve a realistic representation of radiation and cloud processes in the Arctic.

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