Observations of Wall Cloud Formation in Supercell Thunderstorms

This study presents an integrated analysis of dual-Doppler radar, cloud photogrammetry, surface mesonet, and sounding data to examine the formation of two well-defined wall clouds on 05 and 11 June 2009, and a weaker wall cloud on 26 May 2010 observed during the Verification of the Origins of Tornadoes Experiment (VORTEX2) field experiment. In order to determine the origin of the wall cloud air mass, 3-dimensional backwards trajectories were computed using trilinear interpolation. Ground level thermodynamic data were obtained from a series of Mobile Mesonets deployed in and around the storms. This data was then time adjusted to enlarge the spacial coverage and reduce any time discontinuities. Dual-Doppler retrieved pressure perturbations were also calculated to determine if induced lower pressure within the mesocyclone would partially explain a lowering of cloud base.

Results for all three cases showed that the air comprising the wall cloud originated from three regions of the storm; the forward flank, rear flank, and inflow regions. The wall cloud lowering was produced primarily by rain-cooled air in the forward region being ingested into the low-level updraft. Parcels that originated in the rear-flank region descended into the wall cloud. Estimation of wall cloud base height based on mobile mesonet data in the forward flank of the storm was consistent with photogrammetric estimates. Some of the lowering within the 5 June 2009 storm was created by the pressure deficit associated with strong low-level rotation centered on the wall cloud. The results of this study will also be compared to historical modeling results of wall cloud formation.