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Two-way nesting

Influence of regional scale information on the global circulation: a two-way nesting climate simulation

Philip Lorenz

To address a two-way-nesting (TWN) approach for coupling a regional atmospheric climate model with a global climate model, the Max Planck Institute for Meteorology models REMO and ECHAM4 were used. The ECHAM4 model is a global atmospheric general circulation model with a spectral representation of the prognostic variables except the water components. REMO is a regional hydrostatic atmospheric climate model, the set of physical parameterizations of this model is absorbed from the global ECHAM4 model.

In an (up to now used) one-way-nesting mode REMO is initialized and driven at the lateral boundaries using data from (Re)-Analysis products resp. global model output; there is no feedback from the regional model to the global model. Within the presented two-way-nesting approach, every time-step the prognostic variables of the ECHAM4 model are updated within the regional model domain by the corresponding results of the REMO model for this time-step; there is a feedback from the regional model to the global model. A schematic flow scheme of this two-way-nested system is shown in figure 1.

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Figure 1: Schematic flow scheme of the two-way-nested ECHAM4-REMO system

A two-way-nested and an ECHAM4-only run initialized at the 1st of January 1980 were integrated for 10 years using observed SST data (AMIP); the two-way-nested regional domain covers the Western Pacific / Indonesian Warm Pool.

A comparison between these runs for the 10-year seasonal zonal mean temperature shows especially for the boreal summer season a warming of the polar upper troposphere and a cooling of the upper tropical troposphere (Fig. 2). In the MPI-M-Report No. 218 (Roeckner et al., 1996) a comparison between ECHAM4-T42 and ECMWF analysis indicates a significant cold bias in the polar upper troposphere and a warm bias of the upper tropical troposphere of ECHAM4. These biases are reduced within the presented two-way nested ECHAM4-REMO simulation.

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Figure 2: 10-year seasonal zonal mean temperature difference ECHAM4(TWN)-ECHAM4(Ori) for boreal summer season JJA (top) and boreal winter season DJF (bottom)

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