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Is there added value of convection-permitting regional climate model simulations for storms over the German Bight and Northern Germany?

Schaaf B., Feser F.

Meteorology Hydrology and Water Management. Research and Operational Applications

2018

Vol. 6, Iss. 2

21--37

This study tackles the question: Do very high-resolution convective-permitting regional climate model (RCM) simulations add value compared to coarser RCM runs for certain extreme weather conditions, namely strong wind and storm situations? Ten strong storm cases of the last two decades were selected and dynamically downscaled with the RCM COSMO-CLM (24 and 2.8 km grid point distance). These cyclones crossed the high-resolution model domain, which encompasses the German Bight, Northern Germany, and parts of the Baltic Sea. One storm case study (storm Christian of October 2013) is discussed in more detail in order to analyze the smallscale storm features and the associated potential added value of the high-resolution simulation. The results indicate an added value for atmospheric dynamical processes such as convective precipitation or post-frontal cloud cover. The multiple storm analysis revealed added value for the high-resolution regional climate simulation for 10 m wind speed, mean sea level pressure, and total cloud cover for most storms which were examined, but the improvements are small. Wind direction and precipitation were already well simulated by the coarser RCM and the higher resolution could often not add any value for these variables. The analysis showed that the added value is more distinct for the synoptic comparisons than for the multiple storm study analyzed with statistical measures like the Brier Skill Score.

Quality assessment of atmospheric surface fields over the Baltic Sea from an ensemble of regional climate model simulations with respect to ocean dynamics

Meier H.E.M., Höglund A., Döscher R., Andersson H., Löptien U., Kjellström E.

Oceanologia

2011

No. 53 (1-TI)

193-227

Climate model results for the Baltic Sea region from an ensemble of eight simulations using the Rossby Centre Atmosphere model version 3 (RCA3) driven with lateral boundary data from global climate models (GCMs) are compared with results from a downscaled ERA40 simulation and gridded observations from 1980-2006. The results showed that data from RCA3 scenario simulations should not be used as forcing for Baltic Sea models in climate change impact studies because biases of the control climate significantly affect the simulated changes of future projections. For instance, biases of the sea ice cover in RCA3 in the present climate affect the sensitivity of the model's response to changing climate due to the ice-albedo feedback. From the large ensemble of available RCA3 scenario simulations two GCMs with good performance in downscaling experiments during the control period 1980-2006 were selected. In this study, only the quality of atmospheric surface fields over the Baltic Sea was chosen as a selection criterion. For the greenhouse gas emission scenario A1B two transient simulations for 1961-2100 driven by these two GCMs were performed using the regional, fully coupled atmosphere-ice-ocean model RCAO. It was shown that RCAO has the potential to improve the results in downscaling experiments driven by GCMs considerably, because sea surface temperatures and sea ice concentrations are calculated more realistically with RCAO than when RCA3 has been forced with surface boundary data from GCMs. For instance, the seasonal 2 m air temperature cycle is closer to observations in RCAO than in RCA3 downscaling simulations. However, the parameterizations of air-sea fluxes in RCAO need to be improved.