Seminar coordinator: Prof. Jeff Trapp (jtrapp at illinois.edu)
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David Plummer, Graduate Student, DAS, University of Illinois
Room 112 of the Transportation Building
Department of Atmospheric Sciences
The role of cloud-top convection and precipitation banding within cold-season midlatitude cyclones is examined here. High resolution remote sensing and in-situ observations of cyclones and these embedded structures were made during the 2009-10 Profiling Of Winter Storms (PLOWS) field campaign, using instrumentation aboard the National Center for Atmospheric Research C-130 aircraft. Vertical profiles within the cyclones’ comma-head regions and horizontal passes across these regions provided detailed observations from multiple perspectives. The primary datasets used here include in-situ measurements of hydrometeor size, shape and phase obtained using a variety of optical array and forward-scattering cloud probes, and profiles of structural characteristics obtained using the University of Wyoming Cloud Radar and Lidar.
Cellular convection with approximate horizontal scales of a kilometer was consistently present at cloud top throughout the PLOWS cyclones, with the strongest vertical motions confined to cloud top. Supercooled water was commonly observed at cloud top at temperatures as low as -25 °C in association with this convection. The convective cells generated fall streaks of ice particles, which merged in some instances into larger-scale precipitation bands embedded within the cyclones’ warm-frontal and wraparound regions. Altitude-averaged in situ measurements (particle size, concentration, mass content) for several PLOWS cases were broadly consistent with generation near cloud top and growth by aggregation and diffusion with depth in cloud, with larger particle concentrations evident within precipitation bands. The impacts of these findings on the understanding of convective generating cells, precipitation banding, and the overall precipitation structure of midlatitude cyclones will be discussed.
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