Comparison of future climate change over North America simulated by two regional models

Climate versions of the Pennsylvania State University/National Center for Atmospheric Research (NCAR) MM5 and of the International Centre for Theoretical Physics/NCAR RegCM2 are intercompared with two 10-year simulations over North America for present (1990-1999) and close to doubled CO₂ (2090-2099), driven by the same general circulation model forcing from a 100-year transient run of the NCAR Climate System Model. Some model options and modules are identical in MM5 and RegCM2, including radiation and the entire land surface model. MM5 and RegCM2 agree in their general patterns of surface temperatures. Details related to mesoscale effects are well represented in both models. Precipitation is more consistent in MM5 and RegCM2 in winter than in summer. Differences in temperature and precipitation between MM5 and RegCM2 are small in winter due to the strong large-scale forcing but are large in summer because of the increasingly important contribution of local processes. Despite the general agreement between MM5 and RegCM2, significant differences exist over some areas due to the regional models' internal atmospheric physics and dynamics. The magnitude of these differences is large enough in some areas to cause serious uncertainties in any potential assessment of societal impacts of regional climate change.