Ulf Hansson

21st century changes in the European climate: uncertainties derived from an ensemble of regional climate model simulations

Seasonal mean temperature, precipitation and wind speed over Europe are analysed in an ensemble of 16 regional climate model (RCM) simulations for 1961–2100. The RCM takes boundary conditions from seven global climate models (GCMs) under four emission scenarios. One GCM was run three times under one emission scenario differing only in initial conditions. The ensemble is used to; (i) evaluate the simulated climate for 1961–1990, (ii) assess future climate change and (iii) illustrate uncertainties in future climate change related to natural variability, boundary conditions and emissions. Biases in the 1961–1990 period are strongly related to errors in the large-scale circulation in the GCMs. Significant temperature increases are seen for all of Europe already in the next decades. Precipitation increases in northern and decreases in southern Europe with a zone in between where the sign of change is uncertain. Wind speed decreases in many areas with exceptions in the northern seas and in parts of the Mediterranean in summer. Uncertainty largely depends on choice of GCM and their representation of changes in the large-scale circulation. The uncertainty related to forcing is most important by the end of the century while natural variability sometimes dominates the uncertainty in the nearest few decades.

Evaluation and future projections of temperature, precipitation and wind extremes over Europe in an ensemble of regional climate simulations

Temperature, precipitation and wind extremes over Europe are examined in an ensemble of RCA3 regional climate model simulations driven by six different global climate models (ECHAM5, CCSM3, HadCM3, CNRM, BCM and IPSL) under the SRES A1B emission scenario. The extremes are expressed in terms of the 20-yr return values of annual temperature and wind extremes and seasonal precipitation extremes. The ensemble shows reduction of recurrence time of warm extremes from 20 yr in 1961–1990 (CTL) to 1–2 yr over southern Europe and to 5 yr over Scandinavia in 2071–2100 (SCN) while cold extremes, defined for CTL, almost disappear in the future. The recurrence time of intense precipitation reduces from 20 yr in CTL to 6–10 yr in SCN over northern and central Europe in summer and even more to 2–4 yr in Scandinavia in winter. The projected changes in wind extremes have a large spread among the six simulations with a disperse tendency (1–2 m s−1) of strengthening north of 45◦N and weakening south of it which is sensitive to the number of simulations in the ensemble. Changes in temperature extremes are more robust compared to those in precipitation extremes while there is less confidence on changes in wind extremes.

The Rossby Centre Regional Atmospheric Climate Model Part I: Model Climatology and Performance for the Present Climate over Europe

The Rossby Centre Atmospheric Regional Climate Model (RCA2) is described and simulation results, for the present climate over Europe, are evaluated against available observations. Systematic biases in the models mean climate and climate variability are documented and key parameterization weaknesses identified. The quality of near-surface parameters is investigated in some detail, particularly temperature, precipitation, the surface energy budget and cloud cover. The model simulates the recent, observed climate and variability with a high degree of realism. Compensating errors in the components of the surface radiation budget are highlighted and the fundamental causes of these biases are traced to the relevant aspects of the cloud, precipitation and radiation parameterizations. The model has a tendency to precipitate too frequently at small rates, this has a direct impact on the simulation of cloud-radiation interaction and surface temperatures. Great care must be taken in the use of observations to evaluate high resolution RCMs, when they are forced by analyzed boundary conditions. This is particularly true with respect to precipitation and cloudiness, where observational uncertainty is often larger than the RCM bias.