The LaGrange, WY Tornado during VORTEX II. Part II: Photogrammetric Analysis of the Tornado Combined with Dual-Doppler Radar Data

This study presents analyses of the 5 June 2009 Lagrange, WY tornado where dual-Doppler data has been superimposed on photographs of the wall cloud and tornado using photogrammetric techniques. The primary objective of this study is to better understand the relationship between the radar-derived characteristics of the mesocyclone and tornado with the visual appearance of the wall cloud and tornado. Both dual-Doppler and photo data were collected concurrently for about 40 minutes. This time period begins approximately ten minutes before tornado genesis and covers much of the tornado life cycle thereafter. This study represents the first time that ground-based mobile dual-Doppler radar data of a tornadic supercell has been combined with visual data of the wall cloud and tornado using photogrammetric techniques.

It will be shown that the visual appearance (vertical extent and diameter) of the funnel does not correlate well with tornado strength inferred by examining time height diagrams of maximum vertical vorticity and circulation. Evolution of the swirl ratio and retrieved pressure field will be presented in an attempt to understand why the visual appearance of the funnel was nearly ten minutes after the time of tornado genesis as determined by Doppler radar. Analyses will also be shown illustrating the relationship between the radius of maximum wind with the funnel cloud width.

The low-level wind field in the tornado corner flow region will also be compared to the funnel appearance. It appears that the funnel diameter narrows substantially in the lowest levels where radial inflow is observed. The funnel widens abruptly above the inflow where weak radial outflow was observed.

Finally, a vortex-line analysis will be shown at a time just before tornado genesis when a well-defined cyclonic and anticyclonic cloud base lowering was observed. Vortex lines will be plotted through the visual vortex couplet in an attempt to confirm a recent hypothesis that the vortex pair is created by the upward tilting of baroclinic-generated vorticity.