Description: Extreme weather events can have serious impacts on human society as well as on ecosystems. Future and recent changes, as well as the underlying atmospheric mechanisms of the extremes, are usually estimated from climate model simulations that employ atmospheric general circulation models (AGCMs) of relatively coarse horizontal resolution (of the order of a few hundred kilometers). This coarse resolution has an impact on simulated extreme events, particularly precipitation extremes. Since heavy precipitation events are highly variable in space, precipitation extremes are expected to decrease with bigger grid size due to the effect of averaging individual small scale events across a bigger area. Coarser horizontal and vertical resolution may also degrade the representation of some physical processes. To study the impact of horizontal and vertical model resolution on the representation of extreme precipitation events, we analysed simulations with the ECHAM5 AGCM at different horizontal (T213, T159, T106, T63, T42, T31) and vertical resolutions (L31 for T213 to T42, and L19 for T63 to T31) using the same transient present day (1982-2009) boundary forcing (sea surface temperature and sea ice concentration). For each season and grid box, parameters of a stationary generalised extreme value (GEV) distribution were estimated and 20 season return values were derived as a measure of extreme precipitation. All results are compared to the simulation with the highest resolution, T213L31. To disentangle the effect of averaging from the scale dependent representation of physical processes, the high resolution T213 was averaged to the grids of the coarser resolutions for comparison on equal spatial scales. As expected, the return values decrease with coarser resolution. However, the scale dependency changes with region and season. Strongly decreasing return values were found between T106 and T63 covering an almost entire zonal band, which is particularly pronounced in regions of deep convection. It is worthwhile to note, however, that even at the most crude T31L19 resolution, the model was still capable of representing extreme precipitation over the northern hemisphere in winter reasonably well. These precipitation events are mainly caused by large frontal systems. We found high vertical resolution to also be very important for a better simulation of extreme precipitation, as a coarser vertical resolution yields an equatorwards shift of the intertropical convergence zone (ITCZ). The representation of mean precipitation was found to be practically independent of horizontal model resolution

Description: Numerical experiments with the ECHAM5 atmospheric general circulation model have been performed in order to simulate the influence of changes in the ocean surface temperature (OST) and sea ice concentration (SIC) on climate characteristics in regions of Eurasia. The sensitivity of winter and summer climates to OST and SIC variations in 1998-2006 has been investigated and compared to those in 1968-1976. These two intervals correspond to the maximum and minimum of the Atlantic Long-Period Oscillation (ALO) index. Apart from the experiments on changes in the OST and SIC global fields, the experiments on OST anomalies only in the North Atlantic and SIC anomalies in the Arctic for the specified periods have been analyzed. It is established that temperature variations in Western Europe are explained by OST and SIC variations fairly well, whereas the warmings in Eastern Europe and Western Siberia, according to model experiments, are substantially (by a factor of 2-3) smaller than according to observational data. Winter changes in the temperature regime in continental regions are controlled mainly by atmospheric circulation anomalies. The model, on the whole, reproduces the empirical structure of changes in the winter field of surface pressure, in particular, the pressure decrease in the Caspian region; however, it substantially (approximately by three times) underestimates the range of changes. Summer temperature variations in the model are characterized by a higher statistical significance than winter ones. The analysis of the sensitivity of the climate in Western Europe to SIC variations alone in the Arctic is an important result of the experiments performed. It is established that the SIC decrease and a strong warming over the Barents Sea in the winter period leads to a cooling over vast regions of the northern part of Eurasia and increases the probability of anomalously cold January months by two times and more (for regions in Western Siberia). This effect is caused by the formation of the increased-pressure region with a center over the southern boundary of the Barents Sea during the SIC decrease and an anomalous advection of cold air masses from the northeast. This result indicates that, to estimate the ALO actions (as well as other long-scale climatic variability modes) on the climate of Eurasia, it is basically important to take into account (or correctly reproduce) Arctic sea ice changes in experiments with climatic models.