What can simulated convective storms tell us about thunderstorm behavior under climate change?

Isolated convective storms are responsible for numerous high-impact weather events. Their frequency and intensity under the present climate change has been the subject of much speculation, and this uncertainty is magnified when considering that these storms are sensitive to modest changes in their ambient environments. In this work we use an idealized, cloud-resolving model to study the role of environmental changes on storm behavior. This approach is useful since it allows direct comparison of storm morphology between one environment (e.g., the present) and another (e.g., the future). Our current understanding is that for much of the globe, atmospheric temperature and water vapor will increase, and, at least in the middle latitudes, tropospheric wind shear may decrease owing to a weakening of the zonal temperature gradient that maintains the polar jet stream. The vertically integrated effects of temperature and water vapor can be summarized by the convective available potential energy (CAPE), a measure of buoyant energy available to updrafts. These two parameters, CAPE and tropospheric wind shear, are known to exert significant control on storm behavior and are explored herein. Between simulations, either CAPE or shear or both are varied (by 200 J kg^-1 and 2 m s^-1, respectively), and storm properties in the various environments are then compared. In 112 experiment pairs with CAPE increased and shear decreased, 56% of storms produce stronger updrafts, with some that are stronger by as much as 20 m s^-1. However, the magnitude and sign of the change is strongly dependent on the starting values of CAPE and shear. For example, in simulations where CAPE is already >2000 J kg^-1 and shear is weak, such as the moist tropics, small changes to either parameter actually work to reduce updraft intensity, by 5 to 10 m/s. In environments where CAPE is very low (400 J kg^-1), addition of just 200 J kg^-1 of buoyancy can be the difference between storms that last 30-40 minutes, and storms that last over 2 hours and produce updrafts in excess of 30 m s^-1. When considering storm rotation, only 44% of storms produce more vorticity near the ground when CAPE is increased and shear decreased. As with updraft speeds, these results are also highly sensitive to starting values. Additional analyses will be presented, including impacts on total rainfall, surface wind speeds, and hail production. These results underscore the need for further study of convective storm predictability as climate continues to change and highlight the importance of climate change as a regional phenomenon with implications that go beyond global temperatures. It is hoped that this work will foster discussion on such regional trends, what their outcomes might be, and the vital role of convective storms in earth's climate system.