Observations of Wall Cloud Evolution and Structure in Supercell Thunderstorms during VORTEX2

This study examines wall cloud structure and evolution relative to the updraft, rear-flank downdraft, and low-level mesocyclone within the hook region of supercells. An integrated analysis of single- and dual-Doppler data, cloud photogrammetry, and sounding data are used to examine wall clouds in four supercell thunderstorms observed during the Verification of the Origins of Rotation in Tornadoes Experiment II (VORTEX2). One of the supercells produced an EF2 tornado while the others were non-tornadic.

Wall cloud visual appearance and evolution is noticeably different between the non-tornadic and tornadic supercells. Non-tornadic supercell wall clouds exhibit weak or no rotation and have a ragged appearance at cloud base. The wall clouds are located underneath the storm updraft in a region of weak or no precipitation. The ragged visual appearance and lack of rotation persists throughout the wall cloud life cycle. The tornadic wall cloud structure and evolution is more complex. During the time of wall cloud genesis, the visual appearance, strength of low-level rotation, and location relative to the updraft and precipitation are similar to the non-tornadic wall clouds. A short time later as the low-level mesocyclone intensifies, the wall cloud collocates with the low-level mesocyclone and becomes more laminar in appearance. This evolution is in response to the rear flank downdraft wrapping around the wall cloud cutting it off from the storm's primary updraft. While the visual appearance of the wall cloud does not noticeably change just before tornadogenesis, the spatial scale considerably contracts.

Four additional supercells that have well defined updraft bases but no visual wall clouds are also examined. Updraft base heights estimated from photogrammetry calculations and inflow soundings show that all of the air feeding the updraft is coming from the inflow; no rain-cooled air in the forward flank region is ingested into these supercells. Structural differences between supercells having well defined updraft bases with and without wall clouds will be presented.