Analysis of vertical velocities and elevated instability in the comma-head of continental winter cyclones

The vertical motion and physical structure of elevated convection and generating cells within the comma heads of three continental winter cyclones are investigated using the Wyoming W-band Cloud Radar mounted on the NSF/NCAR C-130, supplemented by analyses from the Rapid Update Cycle model and WSR-88D data. The cyclones followed three distinct archetypical tracks and were typical of those producing winter weather in the Midwestern United States. In two of the cyclones, dry air in the middle and upper troposphere behind the Pacific cold front intruded over moist Gulf of Mexico air at lower altitudes within the comma head, separating the comma head into two zones. Elevated convection in the southern zone extended from the cold frontal surface to the tropopause. The stronger convective updrafts ranged from 2 to 7 m s-1 and downdrafts from -2 to -6 m s-1. The horizontal scale of the convective cells was ~5 km. The poleward zone of the comma head was characterized by deep stratiform clouds topped by cloud top generating cells that reached the tropopause. Updrafts and downdrafts within the generating cells ranged from 1-2 m s-1, with the horizontal scale of the cells ~1-2 km. Precipitation on the poleward side of the comma head conformed to a seeder-feeder process, the generating cells seeding the stratiform cloud, which was forced by synoptic scale ascent. In one case, shallow clouds behind the cyclone’s cold front were also topped by cloud top generating cells, with vertical motions ranging from 1 2 m s-1. The development and distribution of potential instability in the elevated convective region of one of these cyclones is examined using a Weather Research and Forecasting (WRF) model simulation. The strong 8-9 December 2009 cyclone is simulated with a large outer domain and convection-allowing nest to simulate the convective region of the cyclone. The distribution of Most Unstable Convective Available Potential Energy (MUCAPE) is presented, with MUCAPE values up to 93 J kg-1 produced in the simulation. The region with positive MUCAPE was based from 2-4 km altitude, located above an elevated frontal boundary as seen in the observations. Backwards trajectories were calculated in the convective region to show how potential instability formed. These trajectories showed that the potentially unstable layer consisted of five distinct layers, and that the air in the lowest layer, the source air for convective cells, originated near Baja California at low elevation, 5000 km away from the source region for air at the top of the potentially unstable layer, which originated in the Arctic at high altitude. Almost all of the trajectories in the potentially unstable region originated over the Pacific coast of Mexico, the Pacific Ocean, or the arctic regions of Canada. Notably absent in the potentially unstable layer was air originating over the Gulf of Mexico. Over the length of the trajectories, air consistently underwent radiational cooling, and was also affected by orographic forcing as it passed over mountains, mixed, and interacted with clouds and precipitation. Notably, no trajectory moved on isentropic surfaces. The constant changes in thermodynamic properties along trajectories also showed that it is the arrangement of airmasses in the comma-head that is responsible for the formation of potential instability, and not the initial thermodynamic properties of the air that eventually arrives at the comma head