A 2XCO2 climate change scenario over Europe generated using a limited area model nested in a general circulation model 2. Climate change scenario

In this paper we discuss a regional climate change scenario over Europe induced by doubling of carbon dioxide concentration (what we call the 2XCO2 experiment) as simulated with a limited area model (LAM) nested in a general circulation model (GCM). The models used are a 4.5×7.5 latitude×longitude resolution (R15) version of the comrnunity climate model (CCM) of the National Center for Atmospheric Research (NCAR) and a 70‐km resolution version of the NCAR/Pennsylvania State University mesoscale model (MM4). Overall, the model produces a 2XCO2 warming over Europe in all seasons, which is in the range of 1.5°–7°. The response of the large scale circulation patterns, and therefore precipitation, over various European subregions varies from season to season. For example, the mid‐latitude jet stream over Europe is weaker in the perturbed climate run than in the control run in January and October, while it is stronger in July and April. As a consequence, simulated 2XCO2 changes in regional precipitation amounts also vary for different seasons, e.g., from −20% of present‐day values over the Alpine region in October to +177% over the western Mediterranean in July. Because of its finer representation of European topography and coastlines the nested MM4 produces a temperature and precipitation change scenario which shows finer spatial detail than that of the driving CCM. The 2XCO2‐induced average temperature modifications predicted by the two models differ locally by up to a few degrees, and predicted precipitation changes differ in some instances not only in magnitude but also in sign. Similarly, the nested MM4 predicts changes in snow depth and other components of the surface hydrology, such as surface runoff and soil water drainage, which show a great deal of sub‐GCM grid‐scale detail. These results emphasize the inadequacy of simply interpolating coarse resolution GCM output to obtain estimates of local changes in surface climatic variables for impact assessments.