Cause and Characteristics of Changes in Mesoscale Convective Systems within a Convection-Permitting Regional Climate Model

Abstract Changes in the frequency and intensity of mesoscale convective systems (MCSs) are assessed using convection-permitting regional climate model simulations. We present a novel classification method that relates MCSs to the magnitude of their large-scale forcing environments to better understand the changing nature of MCSs in a possible future climate scenario. Overall, the annual frequency, intensity, and amount of precipitation associated with MCSs are projected to increase for broad portions of the eastern conterminous United States (CONUS). Furthermore, changes in the characteristics of MCSs show larger, longer-lived, and faster MCSs particularly over the Midwest and Southeast. Seasonal examination of this response reveals a robust intensification of March–May (MAM) MCSs. The higher frequency of MAM MCSs is found to occur in weakly forced synoptic environments. Increased rainfall amounts are explained by an intensification of MCSs due to changing thermodynamics and enhanced lower-tropospheric moisture transport into the central CONUS by the North Atlantic subtropical high. Differences in midlatitude storm tracks are favorable toward the enhanced development of MCSs associated with strong baroclinic forcing within the Midwest and northern plains but are only realized in the presence of strong changes in the lower-tropospheric moisture transport. Overall, these results suggest a shift in the behavior of MAM MCSs that more closely corresponds to the behavior of June–August (JJA) MCSs in the historical climate. The increased occurrence of MCSs in weakly forced environments, particularly in MAM, deserves further investigation. Significance Statement Mesoscale convective systems (MCSs) make large contributions to the annual precipitation in the central United States, but we have a limited understanding as to how they may change in a warmer climate. Using a sophisticated tracking algorithm in conjunction with output from a high-resolution regional climate model, we find that MCSs will become more intense overall and occur more frequently in spring in the eastern United States. This frequency increase is strongly supported by a greater occurrence of MCSs in environments lacking support from large-scale weather systems. Instead, simulated changes in persistent large-scale atmospheric features facilitate the climatological transport of warmer and moister air into the eastern United States in spring, supporting an enhanced MCS response despite statistically insignificant changes in the frequency of individual weather systems.