Thunderstorm outflow winds as measured by the TTUKA mobile doppler radars and sticknet with applications to wind engineering
Gunter, William Scott
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For most regions away from hurricane-prone coastlines, thunderstorms produce the majority of extreme winds and wind-induced damage. While certain extreme wind - producing phenomena have been thoroughly investigated in numerical models, little full-scale data exist to validate the conclusions of these studies. Using instruments that collect high-resolution wind measurements, Project SCOUT (Severe Convective OUtflow in Thunderstorms) was designed to address this dearth in observations by seeking out extreme wind producing thunderstorms and collecting valuable wind profile and surface measurements within these events. An initial nomadic field campaign (Spring 2011) and continued local data collection efforts have yielded over 15 events, including an extreme event that produced many severe wind reports and substantial wind damage across West Texas. In addition to thunderstorm events, several datasets were collected at the National Wind Institute field site in close proximity to an instrumented 200 m tower to validate the novel technique used to retrieve dual-Doppler wind profiles. This research will include an overview of the validation efforts, a comparison of the wind profiles from several events collected during Project SCOUT and an in depth case study of data collected in the extreme wind event on 5 June 2013. The extensive validation efforts revealed good agreement between the dual-Doppler wind speed and direction profiles above 50 m. Below this level, a lag in scatterer deceleration as compared to the deceleration of the wind contributed to an observed 2.2 m s-1 overestimation of the mean dual-Doppler wind speed near the surface in environments with liquid hydrometers. Despite wind speeds being slightly underestimated, radar-derived turbulence statistics also compared well with anemometer measurements. Using the validated methodology, dual-Doppler wind speed and direction profiles acquired in three thunderstorm outflow events are compared. These events in particular exhibited substantial variation not only among themselves, but also in the evolution of the instantaneous dual-Doppler wind speed and direction profiles of individual events. Much of the variation was attributed to the different structure and maturity of each event. Dual-Doppler profiles were combined with TTU StickNet measurements in a case study of a damaging wind event to reveal a low-level wind maximum during the time of peak surface winds. As with the wind profiles of this event, the driving meteorology significantly affected the wind time histories and evolution of the surface turbulence parameters. While the majority of StickNet towers experienced a peak wind gust in association with the main cold pool, several towers recorded a peak wind gust with the passage of a small circulation. The variation of the wind direction in association with these features (the gust front, the circulation, and the main cold pool) allowed for the separation of the outflow into different regions. Turbulence parameters varied between the different regions but were typically maximized near the gust front. The region of the outflow associated with the peak mean wind speed demonstrated larger scales of turbulence and reduced turbulence intensity.