Benefit from using atmosphere-ocean coupled systems for summer dry bias of regional climate models over Central Europe


A precipitation dry bias over major mid-latitude continents is a common problem of many atmospheric models. For Europe, several studies pointed out the dry bias problem of many regional climate models (RCMs) in summer. In the present study we investigated the benefit of using atmosphere-ocean coupled systems for the summer drying problem of regional climate models over Central Europe for two RCMs, the COSMO-CLM (hereafter CCLM) and the RCA4. For CCLM, the atmosphere-only experiment at a resolution of 0.44o is compared with an experiment of the coupled system COSTRICE in which CCLM is coupled to an ocean and a sea-ice model, which are specifically designed for the North and Baltic Sea regions. The coupling ocean domain of the COSTRICE coupled system covers not only the Baltic Sea and the North Sea as other coupled systems but also a part of the North Atlantic Ocean. For RCA4, the atmosphere-only experiment at a resolution of 0.22o is compared with the coupled simulation of RCA4-NEMO. The coupling domain of RCA4-NEMO restricts the North Sea to a northern boundary set at 60oN but covers the entire Baltic Sea. CCLM and RCA4 are set up for the EURO-CORDEX domain. The coupling impact on the dry bias is analyzed for summer (JJA) seasonal means of 30 years (1979-2009) and especially for summer extreme precipitation. The E-OBS data on the 0.5o grid are used to evaluate the model performance. The comparisons show that benefit of the coupling is different for the considered time-scales. For example, if we look at the long-term seasonal mean, the coupled and uncoupled simulations are mostly identical, which is the case for CCLM and RCA4 as well. However, if extreme pre- cipitation is considered, especially for the Northerly circulation type (when the air flow from the North Atlantic Ocean passes the coupling domain – the North Sea), the simulations of the COSTRICE coupled systems are generally improved compared to the CCLM atmosphere only run. However, for RCA4, the benefit of coupling is unclear. The difference between RCA4 and RCA4-NEMO is insignificant also for extreme precipitation. The different behaviour of these two RCMs may be related to different sensitivities of the atmospheric model components to the change of SST over the coupling domain. The size and exact location of the coupling domain may be also an important issue, which should be investigated in a future study, e.g. by using a larger coupling domain for RCA4-NEMO
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