The physics of turbulence over Antarctic leads and polynyas and its parameterization: a joint study using observations, LES, and a micro-/mesoscale model
Zusammenfassung der Projektergebnisse
In polar regions with large sea ice cover there is always an open water fraction arranged in small channels called leads. The large temperature difference between the lead surface and the near-surface air causes strong turbulent heat fluxes with convective plumes penetrating into the atmospheric boundary layer. The goals of this project, carried out as a joint work at the Alfred Wegener Institute for Polar and Marine Research (AWI) in Bremerhaven and at the Institute for Meteorology and Climatology (IMUK) at the Leibniz University Hannover, were to get a better understanding of lead effects on the ABL and to develop turbulence parameterizations for models with different grid sizes applied to the polar atmosphere over sea ice with leads. The basic tool for this investigation was the large eddy simulation (LES) model PALM run by IMUK on a massively parallel computer which allowed a very high resolution. It was used to resolve turbulent eddies over leads with grid spacings between 0.25 m and 10 m. Parameterizations of the turbulent processes over leads were developed and tested in the mesoscale model METRAS which was run with different resolutions but in most studies with a grid spacing of 100 m, allowing to resolve the internal ABL over leads. The development of a turbulence parameterization for this microscale led to a deepened insight into the governing processes over leads. It can be seen as an intermediate step towards the development of a parameterization of the lead impact in climate models. LES runs helped first to identify the necessary resolution for the modelling of turbulence over leads. It was found that much smaller grid spacings are necessary than those used in the literature before the current project. A large number of case studies with extremely high resolution was only possible by implementing a new initialization procedure in PALM allowing inflow with already developed turbulence. This procedure has proven to be a powerful method for the future modelling of similar cases. Our investigations showed that convective roll-like structures near the surface, described in earlier publications, occur as a numerical artefact when the turbulence is not fully resolved. Another important part of the LES studies addressed the dependence of heat fluxes on lead width, wind speed and domain size. The result of this study was that a non-monotonic dependence of heat fluxes on lead width λ with a maximum flux at λ = 1 km is also only a numerical artefact, circumvented by a very high resolution. The new results shows that fluxes decrease monotonically with increasing lead size. This finding agrees better with observations. A new closure for the microscale was developed. It consists of a combination of a nonlocal and local closure, allowing countergradient transport in the core of the convective region over leads and local transport in the outer region governed by the local vertical gradients of temperature and wind. The new closure accounts for the dependence of turbulence downstream of leads on the conditions over the leads which were ignored in all closures being available before this project. Other project studies aimed to analyze the lead impact as obtained by climate models. First, a coupled sea ice atmosphere 1D version of METRAS was used accounting for leads as climate models are doing. Results showed that the lead impact can be larger than known from previous studies. For winterly clear-sky conditions the near-surface temperature is modified by up to 3.5 K per 1 % change in sea ice concentration. Finally, the microscale model was applied with the new closure in 2D to large domains with several leads. It was found that for a given domain averaged sea ice concentration the domain averaged fluxes depend on the lead width and even on the distance between leads. Another result was that climate models underestimate the turbulent fluxes of heat and overestimate the fluxes of momentum, when they are used with the traditional turbulence closures over sea ice with leads. Results obtained by our studies with LES and the microscale model give a line to improve parameterizations in climate models. They could be constructed by accounting for the fetch dependence of fluxes and by a modification of characteristic surface layer values.
Projektbezogene Publikationen (Auswahl)
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(2010), Der turbulente Wärmeaustausch über Eisrinnen - hochauflösende Simulationen mit dem LES-Modell PALM, DACH Meteorologentagung, 20-24 September 2010, Bonn
Witha, B. and S. Raasch
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(2007), Der Einfluss von Eisrinnen auf die Grenzschichttemperatur über dem Meereis der zentralen Polargebiete, DACH Meteorologentagung, 11-14 September 2007, Hamburg, 6 pp.
Lüpkes, C., G. Birnbaum, U. Wacker and S. Raasch
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(2007), Impact of leads on processes in the polar atmospheric boundary layer, SEARCH DAMOCLES workshop, Paris, 29 October 2007
Lüpkes, C., A. B. Herold and T. Vihma
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(2007), Model results and observations of the polar atmospheric boundary layer above inhomogeneous sea ice, DAMOCLES workshop in Copenhagen 22-23 May 2007
Lüpkes, C., V. M. Gryanik, A. B. Herold, J. Hartmann and W. Dierking
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(2007), Polar atmospheric boundary layer processes and their dependence on the lead fraction, DAMOCLES, General Assembly, Oslo, 27 November 2007
Lüpkes, C.
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(2007), The physics of turbulence over Antarctic leads and polynyas and its parameterization: a joint study using observations, LES, and a micro-/mesoscale model, DFG Koordinations-Workshop, SPP 1158, 11-12 October 2007, Bremerhaven, AWI
Witha, B., C. L¨pkes, V. M. Gryanik, S. Raasch, M. Gryschka and T. Gollnik
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(2007), Untersuchung des Einflusses von Eisrinnen auf die antarktische Grenzschicht, DACH Meteorologentagung, 11-14 September 2007, Hamburg
Witha, B., S. Raasch and C. Lüpkes
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(2008), Air-sea-ice interaction in the presence of leads, Workpackage 5.2 workshop (Arctic Ocean Variability - advection and external forcing encounter local processes and constraints) held at DAMOCLES General Assembly in Oslo November 2007, Summary and Extended abstracts compiled by B. Rudels, University of Helsinki and Finnish Institute of Marine Research, 31-36
Lüpkes, C., J. Hartmann, V. M. Gryanik, A. B. Herold and T. Vihma
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(2008), Influence of leads in sea ice on the temperature of the atmospheric boundary layer during polar night, Geophys. Res. Lett., 35, L03805
Lüpkes, C., T. Vihma, G. Birnbaum and U. Wacker
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(2008), Modelling convection over arctic leads with LES and a non-eddy-resolving microscale model, J. Geophys. Res., 113, C09028
Lüpkes, C., V. M. Gryanik, B. Witha, M. Gryschka, S. Raasch and T. Gollnik
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(2008), Processes in the atmospheric boundary layer over the Arctic Ocean and their parameterization, Second Workshop on the Evaluation of Improved Numerical Weather, Ocean and Sea Ice Prediction during Damocles, Reykjavik, Iceland, 11-12 February 2008: Workshop report / Per Kallberg, ed., 23-26
Lüpkes, C., G. Birnbaum, J. Hartmann and V. M. Gryanik
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(2008), The effect of leads in the sea-oce on the Antarctic boundary layer - a high resolution study using a parallelized LES model, 18th Symposium on Boundary Layers and Turbulence, 09-13 June 2008, Stockholm
Witha, B. and S. Raasch
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(2009), The effect of leads in the sea-ice on the Antarctic boundary layer - a high resolution study using a parallelized LES model with turbulent inflow, EGU General Assembly 2009, 19-24 April 2009, Wien
Witha, B. and S. Raasch
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(2009), Turbulent mixing in the Arctic atmosphere, The Arctic climate cystem, its present status, future evolution and potential, conf. organized by the DAMOCLES Steering Committee, 10-12 November 2009, Brussels, Belgium
Lüpkes, C.
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(2010), Messung und Modellierung von Grenzschichtprozessen über dem polaren Meereis, DACH Meteorologentagung, 20-24 September 2010, Bonn
Lüpkes, C., J. Hartmann and G. Birnbaum
