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 biases in the simulated present day high-latitude climate compared to observations and reanalysis data. These biases are most pronounced during the winter season. 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. 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 total) in the lower troposphere is performed. Results from the model simulations are validated against data from the ERA-40 reanalysis. The dependence of the surface air temperature on surface winds, surface energy fluxes, free atmosphere 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 improves in comparison with ERA-40 data.     

Application of a Large-Eddy Simulation Database to Optimisation of First-Order Closures for Neutral and Stably Stratified Boundary Layers

Large-eddy simulation (LES) is a well-established numerical technique, resolving the most energetic turbulent fluctuations in the planetary boundary layer. By averaging these fluctuations, high-quality profiles of mean quantities and turbulence statistics can be obtained in experiments with well-defined initial and boundary conditions. Hence, LES data can be beneficial for assessment and optimisation of turbulence closure schemes. A database of 80 LES runs (DATABASE64) for neutral and stably stratified planetary boundary layers (PBLs) is applied in this study to optimize first-order turbulence closure (FOC). Approximations for the mixing length scale and stability correction functions have been made to minimise a relative root-mean-square error over the entire database. New stability functions have correct asymptotes describing regimes of strong and weak mixing found in theoretical approaches, atmospheric observations and LES. The correct asymptotes exclude the need for a critical Richardson number in the FOC formulation. Further, we analysed the FOC quality as functions of the integral PBL stability and the vertical model resolution. We show that the FOC is never perfect because the turbulence in the upper half of the PBL is not generated by the local vertical gradients. Accordingly, the parameterised and LES-based fluxes decorrelate in the upper PBL. With this imperfection in mind, we show that there is no systematic quality deterioration of the FOC in the strongly stable PBL provided that the vertical model resolution is better than 10 levels within the PBL. In agreement with previous studies, we found that the quality improves slowly with the vertical resolution refinement, though it is generally wise not to overstretch the mesh in the lowest 500 m of the atmosphere where the observed, simulated and theoretically predicted stably stratified PBL is mostly located. The submission to a special issue of the “Boundary-Layer Meteorology” devoted to the NATO advanced research workshop “Atmospheric Boundary Layers: Modelling and Applications for Environmental Security”.     

The importance of wind forcing in fjord wave modelling

Ocean Dynamics - Accurate predictions of surface ocean waves in coastal areas are important for a number of marine activities. In complex coastlines with islands and fjords, the quality of wind...     

Sensitivity of last glacial maximum climate to sea ice conditions in the Nordic Seas

Published reconstructions of last glacial maximum (LGM) sea surface temperatures and sea ice extent differ significantly. We here test the sensitivity of simulated North Atlantic climates to two different reconstructions by using these reconstructions as boundary conditions for model experiments. An atmospheric general circulation model has been used to perform two simulations of the (LGM) and a modern-day control simulation. Standard (CLIMAP) reconstructions of sea ice and sea surface temperatures have been used for the first simulation, and a set of new reconstructions in the Nordic Seas/Northern Atlantic have been used for the second experiment. The new reconstruction is based on 158 core samples, and represents ice-free conditions during summer in the Nordic Seas, with accordingly warmer sea surface temperatures and less extensive sea ice during winter as well. The simulated glacial climate is globally 5.7 K colder than modern day, with the largest changes at mid and high latitudes. Due to more intense Hadley circulation, the precipitation at lower latitudes has increased in the simulations of the LGM. Relative to the simulation with the standard CLIMAP reconstructions, reduction of the sea ice in the North Atlantic gives positive local responses in temperature, precipitation and reduction of the sea level pressure. Only very weak signatures of the wintertime Icelandic Low occur when the standard CLIMAP sea surface temperature reconstruction is used as the lower boundary condition in LGM. With reduced sea ice conditions in the Nordic Seas, the Icelandic Low becomes more intense and closer to its present structure. This indicates that thermal forcing is an important factor in determining the strength and position of the Icelandic Low. The Arctic Oscillation is the most dominant large scale variability feature on the Northern Hemisphere in modern day winter climate. In the simulation of the LGM with extensive sea ice this pattern is significantly changed and represents no systematic large scale variability over the North Atlantic. Reduction of the North Atlantic sea ice extent leads to stronger variability in monthly mean sea level pressure in winter. The synoptic variability appears at a lower level in the simulation when standard reconstructions of the sea surface in the LGM are used. A closer inspection of storm tracks in this model experiment shows that that the synoptic lows follow a narrow band along the ice edge during winter. The trajectories of synoptic lows are not constrained to the sea ice edge to the same degree when the sea ice extent is reduced. Seasonally open waters in the Nordic Seas in the new reconstruction apparently act as a moisture source, consistent with the current understanding of the rapid growth of the Fennoscandian and Barents Ice Sheets, during the LGM. The signal from the intensified thermal forcing in the North Atlantic in Boreal winter is carried zonally by upper tropospheric waves, and thus generates non-local responses to the changed sea ice cover.