THE DYNAMICAL SIMULATION OF THE NCAR-COMMUNITY-CLIMATE-MODEL VERSION-3 (CCM3)

The dynamical simulation of the standard configuration of the latest v ersion of the National Center for Atmospheric Research (NCAR) Communit y Climate Model(CCM3) is examined, including the seasonal variation of its mean state and its intraseasonal and interannual variability. A 1 5-yr integration in which the model is forced with observed monthly va rying sea surface temperatures (SSTs) since 1979 is compared to coexis ting observations. Results show that the most serious systematic error s in previous NCAR CCM versions have either been eliminated or substan tially reduced. At sea level, CCM3 reproduces the basic observed patte rns of the pressure field very well. Simulated surface pressures are h igher than observed over the subtropics, however, an error consistent with an easterly bias in the simulated trade winds and low-latitude su rface wind stress. Amplitude errors and phase shifts of the subpolar l ow pressure centers over both hemispheres during winter produce the la rgest regional errors, which are on the order of 5 mb. In the upper tr oposphere, both the amplitude and location of the major circulation ce nters are very well captured by the model, in agreement with relativel y small regional biases in the simulated winds. Errors in the zonal wi nd component at 200 mb are most notable between 40 degrees and 50 degr ees lat of both hemispheres, where the modeled westerlies are stronger than observed especially over the Southern Hemisphere during winter. A similar to 50% reduction in the magnitude of the zonally averaged we sterly bias in the equatorial upper troposphere that plagued previous CCM versions can be attributed to a significantly improved tropical hy drologic cycle and reduced Walker circulation. Over middle latitudes, the CCM3 realistically depicts the main storm tracks, although the tra nsient kinetic energy is generally underestimated, especially over the summer hemispheres. Over lower latitudes, the model simulates tropica l intraseasonal oscillations with marked seasonality in their occurren ce. Typical periodicities, however, are near 20-30 days, which are sho rter than observed, and the simulated amplitudes are weaker than in bo th observations and previous versions of the model. The simulated resp onse to interannual variations in tropical SSTs is also realistic in C CM3. A simulated index of the Southern Oscillation agrees well with th e observed, and the model captures the overall structure and magnitude of observed shifts in tropical and subtropical convergence zones and monthly rainfall anomalies associated with the tropical SST changes.