A Comparison of Bulk Shear and Cumulative Shear as a Predictor for Convective Mode

Supercell thunderstorms, while constituting a minority of all thunderstorms, are responsible for a large majority of significant hail, wind and tornado reports. Almost all tornadoes rated F3 or greater are associated with supercells. Forecasting when and where supercells are most likely to develop is critical for warning forecasters. Numerous researchers have noted that a minimal amount of shear is necessary to produce persistent rotating updrafts. Initially, bulk shear, the vector difference between the wind at the surface and at some level above the surface, usually 6 km, was used. With the availability of gridded data sets and computing power, cumulative shear (i.e. the length of the hodograph) could also be calculated. Other researchers have shown that helicity can also be used to determine the likelihood of supercells. To calculate the helicity the total shear in a layer, rather than bulk shear, must be known. Further, for two different layers with the same bulk shear, the total shear (and hence helicity) can be much larger when there is a more curved hodograph.

Knowing that both bulk shear and helicity appeared to have utility in determining the convective mode, does the calculation of cumulative shear provide more information and predictability than bulk shear? Over 40 severe supercell thunderstorms and 40 severe non-supercells storms were identified between 2001 and 2006 on the Sioux Falls, South Dakota (KFSD) radar covering southeast South Dakota, southwest Minnesota, northeast Nebraska, and northwest Iowa. Because no observed proximity soundings exist in the vicinity of KFSD, the 32 km North American Regional Reanalysis data was used to extract proximity soundings. Locations and times of each storm was found and a proximity sounding composed from the grid point closest to the storm and within 3 h of storm initiation. For each sounding, winds were interpolated to 500 m intervals from the surface to 10 km. Both bulk shear and total shear were calculated in layers from 0 to 1 km up to 0 to 10 km. For storms that initiated above the boundary layer, an effective shear was also calculated using the technique described by Thompson (2004). Mean values and standard deviations were compared to determine if there was statistically significant difference between the two samples and, if so, which method of calculating shear provides the best guidance for predicting storm mode.