Improved Understanding of Changes in Convective Available Potential Energy and Links to the Large-scale Circulation

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The build-up of a thunderstorm, from scattered popcorn clouds to a dark towering storm cloud, is a common sight on a summer afternoon. The vigorous growth of such clouds is fueled by the convective available potential energy (CAPE) of the atmosphere. Formally CAPE is the amount of work that would be done by the buoyancy force on a saturated plume of air rising through the portion of the atmosphere in which it is lighter than the surrounding air, assuming that no ambient air mixes into the rising plume. Weather forecasters routinely calculate CAPE from atmospheric temperature soundings and use it to predict the likelihood of severe convective storms.Computer simulations of greenhouse-gas induced climate change commonly show large increases of CAPE with global mean temperature, a result which has raised concerns that thunderstorms may become more common or intense as a consequence of climate change. But the reasons why CAPE should increase with global temperature are not clear, and the lack of a theory for the dependence of CAPE on temperature limits confidence in model results.Under previous funding the PI's group developed a simple model which explains the dependence of CAPE on temperature. But the theory assumes that the atmosphere is in a state of radiative-convective equilibrium, a state which approximates the condition of the atmosphere over warm tropical oceans. The theory is compelling as a starting point but cannot be directly applied to understand CAPE change over land or at higher latitudes. Work under this award thus seeks a more general understanding of the relationship between CAPE and global climate, including the effects of large-scale atmospheric circulation. The research is conducted through examination of climate model simulations produced for the Coupled Model Intercomparison Project, combined with experiments using a cloud resolving model on a limited domain to test hypotheses. The role of atmospheric circulation is assessed through calculation and analysis of moist mean available potential energy (MAPE), the maximum amount of kinetic energy that can be reversibly produced from the mean state of the atmosphere by transforming to a lower energy reference state. The MAPE analysis seeks to relate the mean state of the atmosphere in middle and high latitudes to its potential to generate CAPE through large-scale circulations. The impact of land surface conditions on CAPE is another focus of the research, as the strong diurnal cycle of temperature and moisture plays a key role in the development of convection over land. Further work considers the impact of changes in CAPE on the Walker circulation, a large-scale overturning circulation between the western and eastern Pacific.The work has societal as well as scientific value given the damaging effects of convective storms, including hail, lightning, tornados, and flash floods, along with indications that their intensity or frequency of occurrence may increase due to climate change. Results with practical implications are shared with interested parties through workshops and other venues, and research results are incorporated into classroom teaching and other educational activities. In addition, the project provides support and training to a graduate student, thereby providing for the future work force in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.