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Dynamic and Thermodynamic Forcing of Cloud-top Generating Cells in Midlatitude Winter Cyclones

Event Type
Department of Atmospheric Sciences
Room 114 of the Transportation Building
Apr 23, 2014   3:00 pm  
Jason Keeler, Graduate Student, Department of Atmospheric Sciences, University of Illinois
Shirley Palmisano

Cloud-top precipitation generating cells (GCs) are responsible for the production of ice particles in the comma-head and warm-frontal regions of midlatitude winter cyclones. The ice particles then grow and aggregate as they fall through a deep stratiform cloud layer, and can lead to heavy precipitation at the surface. GCs have bases above stable frontal layers and tops at the tropopause. Observed GC dimensions during the Profiling of Winter Storms (PLOWS) field campaign were 0.75 - 1.5 km wide and 1.5 - 2.0 km deep, with maximum vertical velocities of approximately +/- 3 m s-1. Cloud-top GCs are ubiquitous in observations from PLOWS and the Front Range Orographic Storms (FROST) projects. This suggests that atmospheric conditions that favor the development and maintenance of GCs should be as ubiquitous.  Two such conditions include a moist-dry interface at cloud top that favors potential instability, and longwave radiative cooling. Since GCs form in the vicinity of frontal layers, vertical wind shear should also be present.

This research investigates the sensitivity of GC kinematics to stability, radiative processes, and vertical wind shear in idealized Weather Research and Forecasting (WRF) model simulations. A sounding from a case study WRF simulation of the 14-15 February 2010 cyclone provided input data for the idealized simulations capable of simulating GCs at very high horizontal resolution (100 m). The wealth of in situ and cloud radar data from the PLOWS campaign present a unique opportunity to examine the idealized GCs in the context of their real-world counterparts. Idealized simulations with conditions spanning the potential instability, longwave and shortwave radiative process, and vertical wind shear phase spaces give a unique perspective on how these three factors influence GC dynamics. GCs had stronger updrafts and downdrafts with higher potential instability initial conditions, and persisted when longwave radiative processes were permitted by the model. An offset of longwave radiative cooling by shortwave warming weakened the simulated vertical velocities. GCs did not form under stable conditions with no radiative processes, except when strong shear was present. Potential implications on other areas of atmospheric research, and applications of results to forecasting and planning of future field campaigns will be discussed.

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