17th Cyclone Workshop (2015)

Exploring the Sensitivity of Extratropical Cylcones in a Changing Climate

Presentation (PDF Format)

Abstract

With an influence across weather and climate, extratropical cyclones (ETCs) play a critical role in the Earth system, driving a majority of day-to-day weather in the midlatitudes as well as being a longer term mechanism for the poleward transport of energy. Their location at the climate/weather interface means that ETCs are not only tangible signs of a changing climate to much of the world’s population, but also influence the changing climate through their own structural changes. In addition, understanding ETC characteristics in a changing climate supports the development of successful climate adaptation and mitigation strategies. With warming, extratropical cyclone strength is modulated by two potentially opposing influences on ETC strength: a weaker equator-to-pole temperature gradient, decreasing baroclinicity and weakening the storm, against an increase in atmospheric moisture content, increasing latent heating and strengthening the storm. These competing effects encompass a vast range of length scales from the microphysical to the synoptic, highlighting a multitude of interscale links and feedbacks. Such complexity makes observational and theory based approaches to this question difficult.

Our work presents a numerical modelling approach to untangle these feedbacks. Using the Weather Research and Forecasting (WRF) model in an idealized mode, we test the sensitivity of an idealized ETC to a range of background environmental conditions. For consistency and simplicity, we identify relevant control variables for a changing climate, such as baroclinicity, bulk temperature, and moisture content. After determining reasonable ranges for these variables, we span the range with moist, full-physics WRF runs to test the sensitivity of the ETC to changes in each control variable. Effects on several output metrics, including sea level pressure, average precipitation rates, eddy kinetic energy, and latent heat release are plotted and measured for each suite of runs. We find that in the moist environment, adjusting bulk temperature results in non-monotonic changes in peak eddy kinetic energy, perhaps due to the increasing role of convection in warmer environments, especially in the warm sector of the cyclone.