Radar Observations of Mesovortices Associated with the Second 30 June–1 July 2014 Midwestern Derecho

A pair of intense, derecho-producing quasi-linear convective systems (QLCSs) impacted Iowa, northern Illinois, northern Indiana, and southern Lower Michigan from the late morning hours of 30 June through the pre-dawn hours of 1 July 2014. The first QLCS was a classic progressive derecho that produced widespread wind damage across Iowa and northern Illinois, along with a few tornadoes. As this QLCS weakened across northern Indiana and southern Lower Michigan, a secondary QLCS formed across central and eastern Iowa, in the wake of the first QLCS. This secondary QLCS produced 29 confirmed tornado tracks and numerous instances of 80-100 MPH straight-line wind damage. Many of these mesovortices occurred within close proximity of the Chicago (KLOT) and Northern Indiana (KIWX) Weather Surveillance Radar-88 Doppler (WSR-88D) sites and the terminal Doppler weather radar (TDWR) located at Chicago-Midway International Airport (TMDW). The close proximity of the mesovortices to these radar sites afforded numerous unique radar observations of mesovortex behavior and interaction, including:

1. A binary (Fujiwhara) interaction between two mesovortices, sampled with 1-minute TDWR resolution;

2. A 66 m/s Doppler velocity detected by the KLOT radar in a QLCS tornado circulation within 8 km of the radar site at approximately 195 m AGL;

3. The intensification, growth, and splitting of a large mesovortex into two subvortices along a thermal boundary and subsequent prolific tornado occurrence;

4. The evolution of mesovortex behavior from 2-5 simultaneously-ongoing, larger (generally ~3-10 km diameter) mesovortices in northeastern Illinois and northwestern Indiana to up to 12-15 simultaneously-ongoing, smaller (generally ~1-3 km diameter) mesovortices across north-central Indiana;

5. A row of tightly-spaced mesovortices, each associated with a tornado debris signature (TDS), and merging of the TDSs into a larger cloud of tornado debris;

6. Three satellite mesovortices rotating around a larger parent mesovortex; and

7. Mesovortices forming within a convective band immediately ahead of the primary QLCS.

In this presentation, we showcase observations of these behaviors. We place these observations in the context of past work completed on radar observations of QLCS mesovortices and the dynamics of mesovortex generation. Research and operational questions related to these observations are addressed.