P2.4 FINE-SCALE OUTFLOW STRUCTURE OF THE 10 JULY 2001 GREELEY, CO CONVECTIVE WIND EVENT

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 (Storm Data, NCDC). Minor flooding followed the wind damage. 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 of 44 m/s just 50 m AGL as the outflow passed over Greeley. The study of thunderstorm outflow dynamics and morphology is ultimately important for understanding regions of potentially damaging surface winds and dangerous wind shears of importance to the aviation community. A better understanding of boundary instabilities will also likely improve our understanding of moist convective forcing (Lee et al., 2000). A companion numerical modeling paper (P6.7) is included in this volume that investigates fine-scale outflow instabilities in an idealized environment. A concerted focus over the next several years will be on the investigation of the fine-scale structures of boundaries with projects like IHOP and studies using fixed-base research radars like the Colorado State University (CSU) CHILL (positioned in the usually boundary-rich region of northeast Colorado). The severe outflow event featured here allowed for a rare opportunity to obtain high-resolution radar data of the three-dimensional structure of a strong outflow. Of particular interest is the documentation of lobe and cleft instability (LCI, Simpson 1972) located in the gravitationally unstable region near the outflow nose where less-dense air is overrun by the outflow. We hypothesize that LCI may be associated with large horizontal and vertical wind shears (of concern to aviation) and may induce localized wind maxima associated with initial wind damage in severe outflows. The relationship between LCI and horizontal shearing instability (manifest in gustnadoes) is under current investigation by the authors. LCI has received very little attention in the atmospheric sciences community likely due to the difficulty in studying phenomenon on this _______________________ Corresponding author address: Dr. Bruce D. Lee Department of Earth Sciences Meteorology Univ. of Northern Colorado, Greeley, CO 80639 email: bdlee@unco.edu scale (~200 400 m). Kelvin Helmholtz instability (KHI) was also present atop this outflow boundary and is shown in the radar analysis of section 3. Unlike the wealth of observational and modeling documentation of KHI associated with outflows, there are very few references to observed lobe and cleft instability. While most people with an interest in meteorology have seen pictures or video of dust-filled outflows (often called haboobs, see Simpson 1997) with highly perturbed, three-dimensional leading edge structure, most probably did not realize they were looking at a specific gravitational instability. The atmospheric observational documentation of LCI is quite meager. Mueller and Carbone's (1987) dual-Doppler study of a Colorado outflow revealed a possible pattern of superimposed LCI at the outflow leading edge. Intrieri et al. (1990) employed Doppler lidar to identify lobe and cleft structure on the leading edge of a Colorado outflow boundary. Ohno and Suzuki (1993) identified small high wind cores of a few hundred meters in scale length along the leading edge of the sea breeze that were suggestive of LCI.