A spectral transform dynamical core option within the Community Atmosphere Model (CAM4)

An ensemble of simulations covering the present day observational period using forced sea surface temperatures and prescribed sea-ice extent is configured with an 85 truncation resolution spectral transform dynamical core (T85) within the Community Atmosphere Model (CAM), version 4 and is evaluated relative to observed and model derived data sets and the one degree finite volume (FV) dynamical core. The spectral option provides a well-known base within the climate model community to assess climate behavior and statistics, and its relative computational efficiency for smaller computing platforms allows it to be extended to perform high-resolution climate length simulations. Overall, the quality of the T85 ensemble is similar to FV. Analyzing specific features of the T85 simulations show notable improvements to the representation of wintertime Arctic sea level pressure and summer precipitation over the Western Indian subcontinent. The mean and spatial patterns of the land surface temperature trends over the AMIP period are generally well simulated with the T85 ensemble relative to observations, however the model is not able to capture the extent nor magnitude of changes in temperature extremes over the boreal summer, where the changes are most dramatic. Biases in the wintertime Arctic surface temperature and annual mean surface stress fields persist with T85 as with the CAM3 version of T85, as compared to FV. An experiment to identify the source of differences between dycores has revealed that the longwave cloud forcing is sensitive to the choice of dycore, which has implications for tuning strategies of the physics parameter settings.