11.3 The structure and role of generating cells and precipitation bands in cold-season midlatitude cyclones

Thursday, 8 August 2013: 11:00 AM
Multnomah (DoubleTree by Hilton Portland)
David M. Plummer, University of Illinois at Urbana-Champaign, Urbana, IL; and R. M. Rauber, G. M. McFarquhar, B. F. Jewett, and D. Leon

This presentation examines the role of cloud-top generating cells as a fundamental component of the precipitation structure of cold-season midlatitude cyclones, including their contribution to the formation of precipitation banding. The data used here were obtained during the Profiling of Winter Storms (PLOWS) field campaign using instrumentation aboard the National Center for Atmospheric Research/National Science Foundation C-130 aircraft. The primary observational data sets are high-resolution profiles of structural characteristics obtained using the University of Wyoming Cloud Radar and measurements of hydrometeor properties obtained using a variety of in situ cloud probes. These are supplemented with the results of simplified advective model simulations relating deformational flow to the evolution of atmospheric moisture and hydrometeor concentration fields for several cyclones sampled during PLOWS.

Cloud radar measurements revealed the nearly-ubiquitous presence of kilometer-scale convection at cloud top throughout the PLOWS cyclones' comma-head regions. Fall streaks of ice particles generated by this convection broadly coalesced to form deeper stratiform regions below, merging in some cases into enhanced linear features. The strongest vertical motions were confined to cloud top, with convection commonly producing supercooled water at temperatures as low as -25 °C. Altitude-averaged in situ measurements (particle size, concentration, and mass content) were largely consistent with particle generation in the cloud-top convection. Growth by diffusion and aggregation dominated in ice-phase conditions deeper in cloud, with accretion also occurring in mixed-phase environments. Enhanced particle concentrations were consistently apparent within the principal merging fall streaks. These observations are combined with the results of the advective simulations to evaluate the hypothesis that deformational flow is sufficient to organize convectively-generated fall streaks into embedded linear bands. Together, these findings are used to advance the understanding of the structure of convective generating cells and precipitation banding and the fundamental processes controlling their development.

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