Production and Magnitude of Elevated Instability in the Comma-head of Continental Winter Cyclones

Understanding processes in the comma-head of winter cyclones is important due to the high impact weather in this region. Improved knowledge will allow better interpretation of observations and improved simulation of cold season extratropical cyclones. This work illustrates how the convergence of air from different sources with differing thermodynamic properties produces elevated instability within the comma-head region. Previous work has shown that elevated convection in this region contains vertical velocities in excess of 5 m s^-1, with the convection typically found on the poleward side of the comma-head, emerging into the dry slot aloft. This study utilizes 3 km resolution, cyclone scale simulations from the Weather Research and Forecasting model (WRF) to quantify the elevated instability present in the comma head. Simulations of two cyclones from December 8-9, 2009 and February 1-2, 2011 that produced lightning within the comma head are used to illustrate the distribution and magnitude of elevated CAPE with respect to ice in the comma-head region. The coincidence of modeled potential instability with convective updrafts in cloud radar measurements from the Profiling of Winter Storms (PLOWS) field campaign is shown. Finally, trajectory analysis is used to show the interaction of the moist southern airstream and the dry western airstreams that produce elevated instability, as well as the kinematic and thermodynamic evolution of these airstreams.