An Ensemble-Based Examination of Extratropical Cyclone Characteristics in Future Climates

As the primary drivers of synoptic-scale mid-latitude atmospheric variability, extratropical cyclones (ETCs) constitute an important influence on the day-to-day weather in many of the most populated regions of the planet. In connecting with the climate system on multiple length scales, the impacts of ETCs in a future climate are difficult to specify. While some facets of a future climate, such as a weaker equator-to-pole temperature gradient, may weaken ETCs, other factors, such as an increase in atmospheric moisture content, may act to strengthen ETCs. Understanding these competing influences on ETC strength is crucial to projecting the impacts of climate change; however, separating their effects is complicated by incomplete observational coverage and the nonlinear response of the system to environmental perturbations. Instead, our selected approach of utilizing numerical simulations can be used to systematically explore the response of ETCs to changes in different environmental factors.

In this presentation, an ensemble sensitivity analysis of ETCs is conducted with an idealized version of the Weather Research and Forecasting model. Using a channel model with full moist physics parameterizations, we examine the response of an idealized ETC to changes in environmental control variables such as temperature, moisture content, and baroclinicity. Simulations are performed at convection-permitting resolutions in order to explicitly include the effect of moist processes rather than relying on a convective parameterization. Results from a set of univariate sensitivity tests are presented, exploring the response of the ETC to changes in a single variable. A diverse set of output metrics (e.g., sea level pressure, average precipitation rates, eddy kinetic energy, and latent heat release) are examined in order to quantify the response of the system. These metrics exhibit greatly varying responses in magnitude for a consistent change in the control variables. Furthermore, the response of some ETC strength metrics to increases in bulk temperature and water vapor content are non-monotonic, indicating that optimal environments exist for ETC development. Finally, we present results of bivariate and multivariate sensitivity tests that more closely simulate future climate conditions by varying two or more control variables simultaneously. In examining this more complex response to changes in a future climate, we explore potential feedbacks within the climate system and provide a physically based assessment of potential future ETC impacts. The result is a set of sensitivity analyses that span a wide range of future climate scenarios while also offering insight into the possible dynamical changes of future ETCs.