The dynamic and microphysical processes that govern the spatial and temporal variability of precipitation within extratropical cyclones remain poorly understood. Variability in the location, type, and intensity of precipitation is often determined by precipitation banding and/or embedded convection, particularly in the northwest and warm frontal quadrant in cyclones where frontal structures and associated frontal circulations are modified by deformation flow. The Principal Investigators will execute a comprehensive field campaign and numerical modeling study that will address outstanding scientific questions targeted at improving understanding of precipitation substructures in the northwest and warm frontal quadrants of continental extratropical cyclones. In the course of this study the following questions will be addressed: 1) What are the predominant spatial patterns of organized precipitation substructures, such as bands and generating cells and how do they evolve? 2) How do frontal scale systems above and within the boundary layer such as warm fronts, trowals, cold fronts aloft, and occluded fronts relate to these precipitation substructures? 3) What are the thermodynamic and kinematic structures of these frontal systems including the distribution of moisture and vertical motion? 4) What instabilities and types of mesoscale forcing control the generation and evolution of precipitation substructures? 5) How do microphysical processes vary between the different precipitation substructures and what are the consequences? 6) Is instability triggered in ice-saturated ascent critical in some of these instances and is it through the release of the latent heat of deposition that instabilities can persist?
The Principal Investigators will obtain and analyze detailed, high resolution observations of precipitation substructures using three mobile ground-based observing systems, the University of Alabama at Huntsville Mobile Integrated Profiling System, the Mobile Alabama X-band dual polarization radar, and the NSF/National Center for Atmospheric Research (NCAR) Mobile GPS Advanced Upper Air Sounding System, along with the NSF/NCAR C-130 Aircraft equipped with microphysical probes and the Wyoming Cloud Doppler Radar and Cloud Lidar. They will also simulate the precipitation substructures using the Weather Research and Forecasting Model at high horizontal and vertical resolution. The observations will provide the basis for the development of testable hypotheses that can be addressed systematically in the modeling studies. The field campaign (termed Profiling Of Winter Storms, or PlOWS) will have an education component, involving students from 9 universities with atmospheric science departments.
Intellectual merit: The combined dynamical/microphysical observational strategy in conjunction with high resolution numerical modeling, will provide the basis for answering key scientific questions in a more complete and definitive way than heretofore possible. The new observational capabilities that will be applied in this research will provide fundamentally new information about the structure, dynamic and physical properties of these storms on scales never before observed.
Broader impacts: Nationwide, nearly 7000 deaths, 600,000 injuries, and 1.4 million accidents per year are due to adverse road weather, mostly during winter, and costs associated with a single blizzard can range from $0.1M to $3.0B. The research will provide new insight concerning remote sensing of winter weather systems that can translate directly into better operational interpretation and observation strategies of winter weather mesoscale features. The research will contribute to a fundamental understanding of the relationship between the microphysical properties of clouds in winter cyclones and radar and lidar sensing of those properties. The modeling studies will determine if modeled precipitation substructures are consistent with observed features in winter storm systems and provide a better understanding of processes responsible for the occurrence of those substructures. Thus the findings will have direct application to forecasting. The education component, involving students from nine universities with atmospheric science departments, will contribute to undergraduate and graduate education and recruitment of new scientists into atmospheric field research.
Winter weather poses a significant drain on the national economy. Nationwide, nearly 7000 deaths, 600,000 injuries, and 1.4 million accidents per year are due to adverse road weather, mostly during winter. This is but one facet of the impact of winter weather on the transportation and power industries as well as schools and businesses. The costs associated with a single blizzard in a populated region can range from $0.1M to $3.0B. Improvement of forecasts of winter precipitation depends on obtaining a greater understanding of the structure and dynamics of winter cyclones, the large weather systems that bring most hazardous winter weather to the United States. The research conducted under this NSF grant directly targeted this problem. The Universities of Illinois, Alabama-Huntsville, Missouri, and Wyoming, using an instrumented C-130 research aircraft, special ground-based radar systems, and other equipment, conducted a two-year field campaign called the Profiling of Winter Storms (PLOWS) project in the winters of 2008-9 and 2009-10 designed to address outstanding scientific questions about winter storm structure, with the goal of improving our understanding of snowstorms and blizzards in the northwest and warm frontal quadrants of extratropical cyclones that occur over the continental United States. The field campaign was designed to provide new insight concerning radar measurements of winter weather systems that can translate directly into better operational interpretation and observation strategies of winter weather. The education components of the project involved students from nine Midwestern universities with atmospheric science departments, and allowed over 400 students from these universities to tour the research facilities and hear lectures from the project scientists about the measurements and the research. The data analyses performed under this grant have led to several discoveries about winter storm structure. The most important of these is the ubiquitous presence of convection within these storms, particularly near cloud top and in the southern quadrant of the cyclone’s comma head (the northern part of a cyclone most commonly containing severe winter conditions), and the role of the dry airstream wrapping into a cyclone from the southwest in triggering this convection. Our research showed that the dry air associated with a cyclone’s "dry slot" frequently intrudes over air from the Gulf of Mexico creating two zones of precipitation within the comma head, a northern zone characterized by deep stratiform clouds and topped by convective cloud top "generating cells", and a southern zone marked by deeper elevated convection occurring aloft over cold air at the surface. We found that lightning and so-called "thundersnow", when it occurs, appears to originate from this elevated convection within the southern zone. An outcome of the projects was that we were able to unambiguously determine the structure of the convection, the nature of the instability that caused it to occur, the microphysical processes by which precipitation forms in the convection, and how the dry airstream creates the instability. A key outcome of the research was the new understanding the research provided concerning the importance of cloud-top convective "generating cells" in the generation of snowfall across the storms. The cloud-top convective cells are called generating cells because they generate the "seed" ice crystals that grow to become the snow that falls to the ground in the lower part of the storm. Our field and modeling research provided a clear understanding of the role of the dry airstream in creating the instability to trigger the generating cells and the role of cloud-top radiative cooling in maintaining the strength of generating cells over time. Our field studies allowed us to determine the microphysical structure (ice particle concentrations and supercooled water contents) of the generating cells, the vertical velocities that occur within the cells, and clearly document that rapid ice particle growth that occurs within the cells. Another key outcome of the research was a new fundamental understanding of frontal structure within the northwest quadrant of the winter cyclones. The fronts across the northwest quadrant of the storm were found to be atypical of fronts commonly observed in other sectors of extratropical cyclones. The front bounding the Gulf and Canadian airmasses, although quite distinct in terms of precipitation distribution, wind, and moisture, was marked by almost no horizontal thermal contrast. The higher altitude front bounding Gulf and dry Pacific airmasses had characteristics of a warm front, but with stable Gulf air below and less-stable Pacific air aloft. Additional research has elucidated the influence of atmospheric gravity waves and their impacts on radar signatures, precipitation intensity and microphysics, and the relationship of polarization radar signatures to the shapes, sizes and concentrations of ice particles comprising snowfall. The research has also provided some of the first information available on the frontal structure and precipitation processes occurring within "Alberta Clipper" type cyclones.
