First results from the new SMHI atmosphere-ocean-ice model RCA4_NEMO

A new North Sea and Baltic sea coupled RCA4_NEMO model system has been successfully developed in the past few months. This new coupled model has been evaluated with various observation datasets. The preliminary results show that this coupled system is stable and the results is reasonable good.

With the motivation to improve the simulation of RCA4, the new regional atmosphere-ocean coupled model RCA4_NEMO has been developed in this study, which consists of the new Rossby Center regional climate model RCA4 , the regional ocean model NEMO 3.3.1 and the sea ice model LIM3.

The regional atmosphere model RCA4 runs in a horizontal resolution of 0.22° on a rotated latitude-longitude grid with 40 vertical levels (Figure 1). NEMO runs in a resolution of 2 minutes with 84 vertical levels. The active coupling region covers the Baltic Sea and North Sea (Figure 1). The coupling between RCA4 and NEMO is carried out using the OASIS3 coupler developed by CERFACS.

RCA4 domain and orography
Figure 1. The RCA4 domain and orography (red region is the coupling area).
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RCA4 calculates heat, freshwater, momentum fluxes, no-solar heat flux derivative (required by sea ice model LIM3 in NEMO) and receives SST, sea ice concentration, sea ice temperature and albedo from NEMO model for the coupling area. The atmosphere-ocean coupling frequency is set to 3 hours. Due to the limitation of current RCA4 model, there is no river routing available in RCA4 so far. The climatology runoff is used in NEMO for the present day coupling. To improve this issue, one river routing will be implemented into RCA4 model in the next stage.

This coupled model has been run for a 30 years period (1979-2008). RCA4 is initialized and driven by ERA-interim data and NEMO is initialized with Levitus climatology data.

Heat balance of the Baltic Sea

The monthly mean heat fluxes component is calculated and validated and the results show that this coupled model can capture the common characteristics of the ocean. The overall gain and loss of heat through the atmosphere-ice-ocean interface is captured well (Figure 2). Comparing with observations, some of the parameters also have variable bias. e.g. the sea ice extent is underestimated in the Baltic Sea.

RCA4 Mean radiation fluxes for the period 1988-1994
Figure 2. Mean radiation fluxes for the period 1988-1994. Positive fluxes are directed downward.
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Positive effect on RCA4 simulation results

Comparing with observation data and standalone run RCA4, this coupled model has some improvements for most of the variables. However, due to the complexity of the spatial distribution, there is no general pattern for the improvement over the whole model domain. As an example Figure 3 show the difference of T2m between the coupled and uncoupled run and the observation data in winter.
Generally, RCA4 has warm bias in the east Europe and cold bias over the rest of domain . The difference between the coupled run and uncoupled run indicates that the cold bias has been improved over East Scandinavia and Baltic Sea coast region and the bias ranges between ±0.5° for the whole domain. The preliminary validation shows that this coupled model is stable and some parameters have been slightly improved.

Differences of T2m between RCA4 and CRU
Figure 3. Differences of T2m between RCA4 and CRU (left) and coupled and uncoupled run (right) for the winter season.
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Rutgersson,A., A. Omstedt and Y. Chen, 2005: Evaluation of the heat balance components over the Baltic Sea using four gridded meteorological databases and direct observations, Nordic Hydrology, 36(4), 381-396.