Vortex Structure and Evolution within Bow Echoes. Part I: Single-Doppler and Damage Analysis of the 29 June 1998 Derecho

Abstract

Single-Doppler radar along with damage observations are examined to investigate the structural evolution of vortices observed within the 29 June 1998 derecho event that propagated through southeastern Iowa into central and eastern Illinois. A total of 13 meso-?-scale vortices observed primarily at low levels (0?3 km AGL) along the leading edge of the convective system were detected by the Weather Surveillance Radar-1988 Doppler (WSR-88D) radars at Davenport, Iowa, and Lincoln, Illinois. All but one of the vortices formed after the system evolved into a bow echo. Ten of the vortices formed north of the apex while three formed south of the apex. Seven of the vortices produced tornadoes that created F0?F1 surface damage. None of the vortices exhibited appreciable upscale growth. Careful analysis of the radar data suggests that it may be possible to discern between the tornadic and nontornadic vortices. The tornadic vortices tended to be stronger, longer-lived, and deeper than their nontornadic counterparts. The forecasting implications of these findings are discussed. Single-Doppler radar observations documenting the evolution of midlevel (3?7 km AGL) ?bookend? vortices associated with two embedded bow echoes are also presented. The first pair of midlevel vortices formed approximately 20 min after the time that the larger-scale convective system began its transition into a bow echo, had a lifetime of about 30 min, and was observed north of the primary bow apex. A second embedded bow echo formed approximately 20 min after the first, again north of the primary bow apex. The cyclonic member of this second embedded bow echo grew upscale and eventually became the dominant northern line-end vortex of the convective system. There appears to be no significant relationship or interaction between the low-level and midlevel vortices observed with this case.