Resolution Dependence of Future Tropical Cyclone Projections of CAM5.1 in the US CLIVAR Hurricane Working Group Idealized Configurations

The four idealized configurations of the U.S. CLIVAR Hurricane Working Group are integrated using the global Community Atmospheric Model version 5.1 at two different horizontal resolutions, approximately 100 and 25km. The publicly released 0.9 degrees x 1.3 degrees configuration is a poor predictor of the sign of the 0.23 degrees x 0.31 degrees model configuration's change in the total number of tropical storms in a warmer climate. However, it does predict the sign of the higher-resolution configuration's change in the number of intense tropical cyclones in a warmer climate. In the 0.23 degrees x 0.31 degrees model configuration, both increased CO2 concentrations and elevated sea surface temperature (SST) independently lower the number of weak tropical storms and shorten their average duration. Conversely, increased SST causes more intense tropical cyclones and lengthens their average duration, resulting in a greater number of intense tropical cyclone days globally. Increased SST also increased maximum tropical storm instantaneous precipitation rates across all storm intensities. It was found that while a measure of maximum potential intensity based on climatological mean quantities adequately predicts the 0.23 degrees x 0.31 degrees model's forced response in its most intense simulated tropical cyclones, a related measure of cyclogenesis potential fails to predict the model's actual cyclogenesis response to warmer SSTs. These analyses lead to two broader conclusions: 1) Projections of future tropical storm activity obtained by a direct tracking of tropical storms simulated by coarse-resolution climate models must be interpreted with caution. 2) Projections of future tropical cyclogenesis obtained from metrics of model behavior that are based solely on changes in long-term climatological fields and tuned to historical records must also be interpreted with caution.