Changes to intrinsic weather forecast uncertainty in one scenario of extreme future climate

Abstract

Future extreme climate scenarios may harbour flows that have notably different intrinsic uncertainty from those of the present day. Here, ensemble perturbation analysis is used to explore this possibility for tropospheric flow on weather time‐scales for a climate scenario from the National Center for Atmospheric Research (NCAR) Community Climate Modeling System version 4 (CCSM4). Statistically significant changes are found in terms of basic uncertainty metrics including ensemble‐average perturbation total energy, ensemble variance, and the growth rate of perturbation total energy and ensemble variance. Strong seasonal, geographic, and vertical variation in the changes are observed. For the winter season, the poles and the jet levels are regions that exhibit notable increases in ensemble uncertainty under the extreme scenario. These increases are accompanied in some cases by substantially larger peak growth rates. For the summer season, there are prominent increases in uncertainty under the extreme scenario in terms of moist metrics. Meanwhile, there are noteworthy reductions in uncertainty for many synoptic variables. These changes in uncertainty properties are accompanied by significant changes in measures of both baroclinic and convective flow stability. Collectively, the results suggest the changes in intrinsic uncertainty under the extreme climate scenario have a fairly nuanced character. Since the CCSM4 exhibits one of the lowest climate sensitivities of available climate models, the results define a conservative estimate of the changes to intrinsic uncertainty that might be expected from extreme climate change.