A comprehensive survey of buoyancy and shear parameters for California tornadoes: 1951-2011

Abstract

The buoyancy and shear environments for 391 tornado events in California from 1951-2011 were evaluated on the basis of an analyses of proximity soundings and hodographs. The events were stratified into F0, FI, and F2/F3 categories or bins. The 0-1 km above ground level (AGL) Absolute Value Shear (ABVSHR), which is similar to positive shear [as defined in Lipari (2000)], was found to be a statistically significant discriminator between the bins, with increasing values of ABVSHR associated with the higher intensity tornadoes. Approximately 65% of the tornado events exhibited low level and deep layer shear values in the ranges consistent with mesocyclone formation in the parent thunderstorm. Shear values for F0 and FI tornado events occurring before 1980 suggest that there was a systematic misrating of the tornadoes, with most probably assigned too high an F-rating. Case studies of two of the more damaging tornadic events (associated with the Sunnyvale F2 and Los Angeles F2) early in the California tornado record showed that both occurred in shear environments favoring tornadic supercells. Based upon an evaluation of documentary evidence, the author recommends that the Sunnyvale event should be re-rated to F3 and three additional tornadoes occurring on the same day should be added to the database. The Los Angeles tornado event was probably accurately rated.