Extended Abstract (576K)P2.4
Fine-scale outflow structure of the 10 July 2001 Greeley, CO convective wind event
Bruce D. Lee, University of Northern Colorado, Greeley, CO; and C. A. Finley
On the evening of 10 July 2001, an intense macroburst and associated outflow moved through portions of southern Weld County, Colorado causing considerable damage. The area near Greeley experienced the most damage with power outages to about 10,000 homes and businesses and approximately 20-30 % of the area's crops damaged. Total property and crop losses were estimated at 3 million dollars. Minor flooding followed the wind damage.
This convective wind event occurred during a project the authors were undertaking with the CSU CHILL radar facility to better understand instabilities resident along and near the leading edge of lower tropospheric convergence lines. The project was carried out in northeast Colorado within an approximate 60 km radius of Greeley. Investigating the fine-scale leading edge structure of outflow boundaries was one facet of this research, so outflows passing within 15 km of the CHILL site were of particular interest. The underlying motivation for this project involved the generally inadequate observations of outflows and other convergence boundaries at close range. Generally speaking, the along-line resolution in documented observational studies of thunderstorm outflows have been inadequate to describe fine-scale features such as lobe and cleft instability (Simpson 1969, 1972).
Fortunately, the outflow core made a direct passage over the CHILL radar site, allowing for the collection of high resolution data as the outflow propagated both inbound and outbound within 15 km of the radar. Analysis of the radar data revealed winds exceeding 42 m/s just 50 m AGL as the outflow passed over Greeley. At such close range, Kelvin-Helmholtz instability (Simpson 1969; Simpson and Britter 1980) atop the outflow was a prominent feature in RHI scans. Another feature observed in the RHI scans within about 7.5 km of the radar is the "nose" of the outflow, a feature only rarely captured for outflow boundaries (usually by tower observations, Goff, 1976; Mueller and Carbone, 1987). Of much interest, a coherent signature of lobe and cleft instability became apparent as the outflow boundary approached within 10 km of CHILL. The atmospheric sciences observational literature has only scant documentation of lobe and cleft structure due likely to resolution limitations associated with these instabilities that may have an average length scale of only several hundred meters. We will present radar data and analysis documenting these instabilities and outflow structure. Further, ongoing analysis addressing potential links between these instabilities and high near-surface wind cores and strong horizontal/vertical wind shear regions will be presented.
Poster Session 2, Convectively Driven High Wind Events
Monday, 12 August 2002, 3:00 PM-4:30 PM
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