Dynamically-Downscaled Estimates of Favorable Convective Environments and Storms Over the Upper Midwestern U.S.

As anthropogenic forcing continues to warm the planet, there is still uncertainty on the implications this may have on hazardous convective weather (HCW) events such as damaging wind, hail, and tornadoes in the future. Recent research has focused mainly on storm-related proxies in regions of high HCW activity such as the southern and Great Plains of the United States (e.g. Gensini et al. 2015, Hoogewind et al. 2017), while climate change impacts will also affect areas such as the highly populated region surrounding the Great Lakes. This study makes use of the NCAR CESM CMIP5 bias-corrected 1 degree output downscaled using the Weather Research and Forecasting (WRF) model (version 3.7) to an outer domain with a 15km grid for environments, down to a nested domain with 3km grid. The outer domain includes the northeastern United States, the upper Midwest, and southeast Canada, while the nested domain is located over Michigan’s lower peninsula and its surrounding Great Lakes. These runs were calculated for two 15-year periods, 1991-2005 for the historical record, and a future period 2085-2099 using a highly warming RCP 8.5 emissions scenario.

Changes in the distributional characteristics of environmental parameters commonly associated with severe weather between the two 15-year periods were used to explore how the frequency/intensity of HCW may change in an anthropogenically forced climate. Given the peak frequency of these events over the region occurs between May and August, changes were explored on both a monthly and seasonal basis. To focus on changes when convection was more likely to occur, environments were only included in the analysis if a model simulated storm was present nearby in the domain. Unlike earlier studies which have focused on the produce of Convective Available Potential Energy (CAPE) and 0-6km deep layer shear, other parameters typically used to characterize favorable severe thunderstorm conditions were also included, such as lapse rates, lifted condensation level (LCL), helicity and composite parameters such as the Significant Tornado Parameter and the Supercell Composite Parameter.

Large increases in CAPE are found between the two 15-year periods, with the largest increase occurring at the 90^thpercentile. While changes in helicity are less robust and more variable throughout the domain, the frequency of model simulated storms exceeding an updraft helicity threshold of 60 m²s^-2and a composite reflectivity threshold of 45 dBZ increased by more than 35%. Evaluations of model performance also suggest that the high-resolution model is emulating the effects of the Great Lakes on environmental parameters with skill, allowing for further assessment of the role that these large bodies of water could play in moderating or enhancing climate impacts on HCW events and environments. Through the use of high-resolution dynamically downscaled climate data and thorough analysis of numerous HCW proxies and environmental parameters, this presentation will provide an in-depth look into the future of HCW in an anthropogenically altered climate.