Processes governing the spatial and temporal variability of precipitation within the "comma head" sector of extratropical cyclones remain poorly understood. This sector of baroclinic storm systems is often the focus of hazardous winter weather including heavy snowfall, blizzards and ice storms that markedly impact transportation and other human activities. This investigative team seeks to improve our understanding of precipitation substructures within this zone by addressing outstanding questions arising from the Profiling of Winter Storms (PLOWS) field campaign, which was carried out during the winters of 2008-09 and 2009-10. Observations collected during PLOWS included extensive in-situ microphysical data gathered by the NSF/NCAR C-130 aircraft, high-resolution remote sampling of precipitation structures by the University of Wyoming Cloud Radar and Lidar carried aboard the C-130, complementary views from ground-based radars and profiling systems, and special serial rawinsonde launches. Additional insights will be gained through high-resolution simulations suitable for comparison with a wide range of observed precipitation substructures.
Planned investigations will center on: (1) The nature and source of instability creating cloud top generating cells, including determination of updraft magnitudes, origins and role supercooled water in the generation and growth of ice particles near cloud top, ice particle concentrations within generating cells and associated precipitation plumes, and processes leading to the rather ubiquitous generating-cell structures observed during PLOWS; (2) the means by which potential instability is generated within zones characterized by deep upright elevated convection on the warmer side of comma-head regions, the relationship between the "dry-slot" upper-tropospheric airstream moving over warm-frontal surfaces, and determination of the role of synoptic-scale vertical motions accompanying frontogenesis in triggering release of this instability; (3) the origins of linear precipitation bands and their potential creation by synoptic-scale deformation acting upon descending ice particle plumes issued by elevated generating cells, as well as differing ice particle characteristics within and outside these bands; (4) the relationship of polarization radar signatures to measured in situ microphysical properties, and in particular determination of whether supercooled water or its effects (e.g., rimed particles) can be dependably detected via remote sensing; and (5) the nature of stratiform- vs. convective-cloud region flows with attention to fine scale wave features, frontal interfaces, their effects on microphysical processes, and their relationship to isentropic surfaces, shearing instability, and low-level fronts in the production of locally-enhanced precipitation rates.
The intellectual merit of this effort rests on full exploitation of the unique and extensive PLOWS project dataset to address longstanding questions concerning the nature and origins of precipitation within winter cyclones in more complete and definitive ways than has heretofore been possible. Broader impacts of this research will include a carefully designed mix of undergraduate and graduate education and testing of improved operational strategies for remote sensing of winter weather systems hinging on emerging observational assets including NOAA's newly-upgraded dual-polarization network of WSR-88D radars. Anticipated longer-term impacts include improved ability of numerical models to simulate precipitation substructures in winter storms with direct application to forecasts benefiting the public, as well as recruitment of new scientists qualified to undertake atmospheric field research.
