Coupling with Ocean-Ice Model
The uncertainty still existing in modeling present-day Arctic sea-ice conditions and its implications for climate projections indicates the need for improved descriptions of physical processes involved in atmosphere-ice-ocean feedbacks. A realistic simulation of these feedbacks is one of the major challenges in Arctic climate modeling, hampered by the scarcity of available observational data from polar regions and the resulting fragmentary knowledge of the underlying mechanisms of feedbacks. In order to gain deeper insight into Arctic climate processes and feedback mechanisms acting at the interface between atmosphere, ocean, and sea ice, a pan-Arctic coupled regional atmosphere-ocean-ice model has been developed at the Alfred Wegener Institute by coupling the regional atmosphere model HIRHAM with the high-resolution regional ocean-ice model NAOSIM.
The model domains of the atmosphere and the ocean-ice component are shown in Figure 1. In the present model configuration, the atmosphere and ocean-ice computations are implemented on different grids with different resolutions and necessitate a complex coupling procedure for the exchange of variables between the grids. The coupling is made directly (i.e. without using external coupling software) and comprises five steps:
1. Step: Construction of an auxiliary grid by extending the coarser grid (here the atmosphere grid) in such a way that it encloses the whole area of the finer grid (see Figure 2).
2. Step: Successive extrapolation of the values from the atmosphere grid onto the auxiliary grid as well as from the sea grid cells of the ocean grid onto adjacent land grid cells.
Fig.2 Schematic diagram of the different grids.
Fig.1 Geographical position of the model domains of HIRHAM and NAOSIM.
3. Step: Each grid cell of the finer grid (here the ocean grid) is subdivided into 3 times 3 sub-grid cells, and each of these sub-grid cells is assigned to a grid cell of the auxiliary grid (see Figure 3).
4. Step: (a) Ocean -> Atmosphere: Averaging of the values of all sub-grid cells that have been assigned to the current grid cell of the auxiliary grid. (b) Atmosphere -> Ocean: Averaging of the respective values of the auxiliary grid cells that have been assigned to the 9 sub-grid cells of the current ocean grid cell.
5. Step: All sea grid cells which lie outside of the overlap area of the two model domains are marked as uncoupled domain (shaded area) and afterwards treated as in uncoupled mode (forcing is taken from global reanalysis data like ERA-40).
Fig. 4 Schematic diagram of the differentiation between coupled (white area) and uncoupled domain (shaded area).
Fig. 3 Schematic diagram of the subdivision of the ocean grid cells within the coupling procedure
References
More information on the coupled atmosphere-ocean-ice model, including results from a series of sensitivity experiments, is given in the following papers:
- Rinke, A., R. Gerdes, K. Dethloff, T. Kandlbinder, M. Karcher, F., Kauker, S. Frickenhaus, C. Köberle, and W. Hiller (2003): A case study of the anomalous Arctic sea ice conditions during 1990: Insights from coupled and uncoupled regional climate model simulations, J. Geophys. Res., 108, 4275, doi:10.1029/2002JD003146
- Dorn, W., K. Dethloff, A. Rinke, S. Frickenhaus, R. Gerdes, M., Karcher, and F. Kauker (2007): Sensitivities and uncertainties in a coupled regional atmosphere-ocean-ice model with respect to the simulation of Arctic sea ice, J. Geophys. Res., 112, D10118, doi:10.1029/2006JD007814.