A High-Resolution Aerial Survey and Radar Analysis of Quasi-Linear Convective System Surface Vortex Damage Paths from 31 August 2014

Abstract

n the evening of 31 August 2014, a powerful quasi-linear convective system (QLCS) impacted much of Iowa. In the weeks following the event, the entire path of the QLCS was imaged at ~1-m resolution using aerial photography through the National Agriculture Imagery Program. The predominantly flat, mature agricultural land cover of central Iowa provided an excellent medium on which to document wind phenomena of varying scales. The high-resolution aerial data, in combination with recent spatial, temporal, and polarimetric upgrades to the Weather Surveillance Radar-1988 Doppler (WSR-88D) network, offer an extraordinary glimpse into the quantity, evolution, and scale of surface vortices generated throughout the entire lifespan of this QLCS. One hundred eleven damage tracks associated with these vortices were cataloged along the storm?s 350-km path, ranging in length from 130 m to nearly 18 km. This study classified 35 of these circulations as tornadoes using a series of tests that weighed track characteristics and radar data. Unusual features, such as a likely tornado merger and multiple instances of tornadoes occluding behind the leading edge of the QLCS surface cold pool, are examined. Possible genesis mechanisms and National Weather Service operational implications are also discussed. A new, behavioral-based approach for identifying a tornadic debris signature (TDS) is presented that may be better suited for QLCS tornadoes. Twelve TDSs were cataloged on 31 August 2014 using this methodology at ranges up to 90 km from the Des Moines, Iowa, WSR-88D.