The physics of turbulence over Antarctic leads and polynyas and its parameterization: a joint study using observations, LES and a micro-/mesoscale model

In regions with large sea ice concentrations there are always open water areas called leads or polynyas depending on their shape. The length of leads varies between a few kilometres and tens of kilometres and their width ranges from several meters to kilometres. The strong sensible and latent heat fluxes caused by the large temperature difference between air and water can trigger convection which significantly modifies the atmospheric boundary layer. For this reason leads are supposed to have a large influence on the energy exchange between the polar ocean and the atmosphere which is still insufficiently considered in weather and climate models.

The goals of this project are to get a better understanding of the still not well understood effects of leads and polynyas on the boundary layer turbulence and to clarify their importance for the energy budget and structure of the ABL.

High-resolution results of the parallelized LES model PALM and new helicopter based observations of turbulence over leads will be used to (further) develop parameterizations of the lead effect which can be used in models with different grid sizes ranging from microscale non eddy resolving models to weather forecast and regional climate models. The parameterizations are tested in the nonhydrostatic microscale model METRAS.

So far, only linear leads of infinite length with a lead-normal flow were considered. Simulations with different lead width, horizontal wind speed and thermal stratification were performed and show a clear dependence of the lead induced convective plume on these parameters. An LES sensitivity study with respect to grid resolution revealed that with a resolution of 0.5 m and less it is now possible to resolve turbulence above most parts of a 100 m wide lead where the height of the convective boundary layer is only a few meters.

Further studies comprise variations of more parameters as the flow direction and a non-linear geometry of the lead. To obtain a better estimation of the lead temperature, a coupled atmosphere-ocean LES will be performed for the first time. The water temperature will then no longer be constant but determined by the interaction between ocean and atmosphere. To improve the parameterizations for mesoscale models, different sea ice scenarios will be prescribed which refer to idealized as well as realistic cases based on satellite observations.

Scientists

Dr. Siegfried Raasch
Institute for Meteorology and Climatology
Leibniz University Hannover

Dr. Christof Lüpkes
Alfred-Wegener-Institute Bremerhaven

Dipl.-Met. Björn Witha
Institute for Meteorology and Climatology
Leibniz University Hannover

Cand.-Met. Tim Gollnik
Alfred-Wegener-Institute Bremerhaven

Research areas

Antarctic marginal sea ice zone

Publications

Lüpkes C, Vihma T, Birnbaum G, Dierer S, Garbrecht T, Gryanik VM, Gryschka M, Hartmann J, Heinemann G, Kaleschke L, Raasch S, Savijärvi H, Schlünzen KH, Wacker U, 2010. Mesoscale modelling of the Arctic atmospheric boundary layer and its interaction with sea ice, Chapter 7 in: ARCTIC Climate Change - The ACSYS Decade and Beyond (edited by Peter Lemke), Springer, Atmospheric and Oceanographic Sciences Library, 29 pp.

Lüpkes C, Gryanik VM, Witha B, Gryschka M, Raasch S, Gollnik T, 2008. Modeling convection over arctic leads with LES and a non-eddy-resolving microscale model. J. Geophys. Res., 113, C09028., doi:10.1029/2007JC004099.

Lüpkes C, Vihma T, Birnbaum G, Wacker U, 2008. The Influence of Leads in Sea Ice on the Temperature of the Atmospheric Boundary Layer During Polar Night, Geophysical Research Letters, 35, L03805, doi:10.1029/2007GL032461.

Homepage

Find more about our research on leads and polynas at our Homepage

Research funding organisation

German Research Foundation

Project numbers: RA 617/14-1,2, LU 818/1-1,2
Funding period: 2008 - 2010