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Introducing sea ice in the COSMO model

Introducing sea ice in the COSMO model. Jan-Peter Schulz Deutscher Wetterdienst COSMO General Meeting, 6 - 10 Sep. 2010, Moscow. Motivation.

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Introducing sea ice in the COSMO model

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  1. Introducing sea ice in the COSMO model Jan-Peter Schulz Deutscher Wetterdienst COSMO General Meeting, 6 - 10 Sep. 2010, Moscow 6 Sep. 2010

  2. Motivation The presence of sea ice on the ocean's surface has a significant impact on the air-sea interactions. Compared to an open water surface the sea ice completely changes the surface characteristics in terms of albedo and roughness, and therefore substantially changes the surface radiative balance and the turbulent exchange of momentum, heat and moisture between air and sea. In order to deal with these processes the operational GME model includes a sea ice module which was now also implemented in the COSMO model code. 6 Sep. 2010

  3. The sea ice model It is a one-layer thermodynamic model which basically computes the energy balance at the ice's surface. From this the evolution of the ice surface temperature and the ice thickness are deduced. In particular, the model allows for very low surface temperatures which can be significantly lower than the water temperature below the ice. 6 Sep. 2010

  4. The sea ice model QA Tice Tice : Sea ice surface temperature Hice : Sea ice thickness Sea ice QI Hice Tmelt = -1.7oC Ocean 6 Sep. 2010

  5. Sea ice distribution As input the model needs the sea ice distribution which is provided by the operational analysis of sea surface temperature (SST). The remote sensing based sea ice mask of NCEP is provided by the SST analysis to the DWD global model GME. This GME sea ice mask is then interpolated to the COSMO-EU grid again by its SST analysis. 6 Sep. 2010

  6. Sea ice in COSMO-EU Currently the GME sea ice surface temperature is used to “emulate” the existence of sea ice in COSMO-EU by providing a realistic temperature at water points which are regarded as being ice covered. Using the threshold value Tmelt = -1.7oC the COSMO-EU water points are distinguished between open water or ice covered. The albedo and the roughness length are set accordingly. Now with the sea module implemented in the COSMO model the sea ice can actually be simulated. There are two new prognostic variables, the sea ice surface temperature Tice and the sea ice thickness Hice. They allow for a better thermodynamically coupled treatment of sea ice in the COSMO model as lower boundary condition for the atmosphere. 6 Sep. 2010

  7. Experiments The sea ice model together with the modified SST analysis shall be tested in experiments with COSMO-EU for the winter 2009/2010. Two experiments were started, one with sea ice and one without as reference. The start date is 02 Feb. 2010, 21 UTC, with the assimilation cycle, the sea ice is introduced with the SST analysis on 03 Feb. 2010, 00 UTC. 6 Sep. 2010

  8. Cold start from GME using the SST analysis … 6 Sep. 2010

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  13. GME T_ICE COSMO-EU T_SO(0) without sea ice COSMO-EU:T_SO(0) = 0.5 (T_ICE + Tmelt)Tmelt = -1.7oC 6 Sep. 2010

  14. GME T_ICE COSMO-EU T_SO(0) without sea ice COSMO-EU:If GME=ice and BSH=ice:use same formula as in first case. 6 Sep. 2010

  15. GME T_ICE COSMO-EU T_SO(0) without sea ice COSMO-EU:If GME=no ice and BSH=ice:create new ice point.Initialize: T_SO(0) = Tmelt – epsThen: Search in the neighbourhood for the warmest ice point which originates from GME. Use this temperature for T_SO(0).eps = 0.05oC 6 Sep. 2010

  16. GME T_ICE COSMO-EU T_SO(0) without sea ice COSMO-EU:If GME=ice and BSH=no ice:remove ice point, create water.Initialize: T_SO(0) = Tmelt + eps 6 Sep. 2010

  17. GME T_ICE COSMO-EU T_SO(0) with sea ice COSMO-EU:At all sea ice points: T_SO(0) = TmeltWater temperature below the ice. 6 Sep. 2010

  18. GME T_ICE COSMO-EU T_ICE with sea ice COSMO-EU:GME values of T_ICE are interpolated to the COSMO-EU grid. 6 Sep. 2010

  19. GME T_ICE COSMO-EU T_ICE with sea ice COSMO-EU:If GME=no ice and BSH=ice:create new ice point.Initialize: T_ICE = Tmelt – eps 6 Sep. 2010

  20. GME H_ICE COSMO-EU H_ICE with sea ice COSMO-EU:GME values of H_ICE are interpolated to the COSMO-EU grid. 6 Sep. 2010

  21. GME H_ICE COSMO-EU H_ICE with sea ice COSMO-EU:If GME=no ice and BSH=ice:create new ice point.Initialize: H_ICE = 0.2 m 6 Sep. 2010

  22. The first forecast run … 6 Sep. 2010

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  29. Conclusions • The GME sea ice module was implemented in the COSMO model code. • The code of the external SST analysis and the operational shell scripts for running the COSMO model at DWD were adapted to the sea ice model (thanks to Martin Lange and Thomas Hanisch at DWD). • Experiments with COSMO-EU were started on 02. Feb. 2010. The cold start from GME sea ice fields via the SST analysis works fine. It is planned to simulate basically a full cycle of freezing and thawing of sea ice in the Baltic Sea from February until May 2010. • The new prognostic variables, sea ice surface temperature and sea ice thickness, immediately started to adapt to the atmospheric conditions in COSMO-EU. Thus, they will provide a much better fully coupled lower boundary condition for COSMO-EU than the interpolated temperature from GME which is kept constant. • Operational implementation is planned for autumn 2010. 6 Sep. 2010

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