VARIABILITY OF STORM-RELATIVE HELICITY DURING VORTEX

In this paper, storm-relative helicity (SRH) and low-level vertical sh ear of the horizontal wind fields were investigated on the mesoscale a nd stormscale in regions where tornadoes occurred for four case studie s using data collected during the Verification of the Origin of Rotati on in Tornadoes Experiment. A primary finding was that SRH was highly variable in bath time and space in all of the cases, suggesting that t his parameter might be difficult to use to predict which storms might become tornadic given the available National Weather Service upper-air wind data. Second, it was also found that the shear between the lowes t mean 500-m wind and the 6-km wind was fairly uniform over vast regio ns in all of the four cases studied; thus, this parameter provided lit tle guidance other than that there was possibly enough shear to suppor t supercells. It was contended that forecasters will need to monitor l ow-level features, such as boundaries or wind accelerations, which mig ht augment streamwise vorticity ingested into storms. Finally, it was suggested that one reason why one storm might produce a tornado while a nearby one does not might be due to the large variations in SRH on v ery small spatial and temporal scales. In other words, only those stor ms that move into regions, small or large, with sufficient SRH might p roduce tornadoes.