Lake-effect snowstorms are a key source of wintertime precipitation and high-impact weather over the Great Lakes region. These storms typically evince one of two contrasting morphologies: Wind-parallel roll circulations in which elongated precipitation features are preferentially oriented along the prevailing low-level flow (and accompanying vertical wind shear), versus long lake-axis parallel (LLAP) precipitation bands that are typically more intense with a preferred mid-lake location. LLAP-type storms are more common over the Eastern Great Lakes (viz. Erie and Ontario), but have been studied far less than wind-parallel bands. This exploratory project will field a newly polarized single "Doppler on Wheels" (DOW) mobile radar to observe LLAP-type snowbands. These data, which will be collected during a five-week period during winter 2010-11, will be complemented by mobile rawinsonde-derived thermodynamic profiles obtained both within and external to these bands.
The intellectual merit of this effort centers on improved specification of the mesoscale morphology, evolution and precipitation mechanisms within intense LLAP-type lake effect snowbands. In view of this first-ever deployment of a polarized DOW in a cool-season setting, attention will be directed to a critical evaluation of polarization quantities and hydrometeor classification output from this recently upgraded platform.
Broader impacts will accrue through support of research at a primarily undergraduate academic institution and via direct involvement of a group of undergraduate students in both the collection and analysis of this exploratory dataset. Results will be disseminated via conference presentations and formal publications.
The purpose of this project is to obtain and analyze high temporal and spatial resolution radar data on lake-effect snow bands that form parallel to the long axes of the Great Lakes. These long-lake axis parallel (LLAP) bands have the potential to produce several feet of snow over only a few days as recently occurred near Buffalo, NY, on November 17-20, 2014. The snowfall gradients associated with these events can be similarly extreme, as storm total snowfall amounts can vary by feet over less than 10 miles. To improve our understanding and better forecast these often high-impact snow events, a Doppler on Wheels mobile weather radar gathered data on seven separate lake-effect snow events in Upstate New York between December 2010 and February 2011. These data were collected at much finer spatial and temporal scales than allowed by traditional National Weather Service Doppler radars, and revealed a multitude of features that had been previously unobserved in lake-effect snow bands. The first radar image depicts convective cells (which, like the band, form owing to rising unstable air that has been heated and moistened by the warm lake surface) on the south side of the snowband that are much smaller than the band itself. The band measures about 20 miles across, while cells are less than 5 miles wide. The use of a dual-polarization radar allowed for inferences to be made about the shape of the precipitation particles. This, along with the collection of surface precipitation data made it clear that snow pellets (similar in appearance to bits of Styrofoam) were more common in the smaller cells, while larger aggregate snowflakes were the dominant precipitation type within the band. We believe that this difference in snow crystal types can be attributed to the stronger updrafts (currents of rising air) lofting more supercooled liquid water in the cells than in the band, leading to more riming of particles within the cells than the band (riming is crucial to the formation of snow pellets). Detection of these different snow types is also important to the forecasting process because these different snow particles accumulate at different rates. The radar data also reveal the presence of a myriad of vortices within the snowbands, ranging in size from a mile or so across to as large as 6 miles across. The largest vortex measured was about 6 miles in diameter; winds on the north side of this vortex were nearly calm, while winds on the south side ranged from 35-40 mph. Winds of this magnitude when combined with moderate or heavy snowfall can lead to near-blizzard conditions. It is suspected that some of the smaller, but more intense vortices that were observed over the lake may also be associated with waterspouts. We were surprised by the frequency at which these small vortices were observed; over 1100 radar observations of 137 separate vortices were made during this project, although most such vortices were not associated with waterspouts. It is surmised that a mechanism known as horizontal shear instability was the source of these smaller vortices. To demonstrate this, hold your hands vertically so they touch, place a pen between them, and move your hands in opposite directions; the pen will begin to rotate. Other phenomena typically observed only in thunderstorms were also seen in these high resolution data, including intense updrafts, horizontal vortices, anvil clouds, and sharp wind shift lines. We are confident that these phenomena are not new to the atmosphere, but have long existed within these snowbands but are only now being observed for the first time owing to this high resolution data set (see Steiger et al. 2013, Monthly Weather Review, for a more complete description). The project also trained around a dozen undergraduate students in data collection. At the University of Illinois, two students were trained in data analysis techniques and went on to give oral presentations and write conference papers for the 2012 Annual Meeting of the American Meteorological Society prior to graduation. These students later earned masters degrees and are now working as professional meteorologists. These manuscripts were later refined and further analysis was performed by a graduate student. Many of the findings in this project, including the precipitation distrubtion and the presence of vortices within lake-effect systems formed the basis of the much larger Ontario Winter Lake-effect Systems (OWLeS) Project that took place during December 2013 and January 2014. This project included multiple Doppler radars, an instrumented aircraft, and produced over two dozen data sets for analysis.
