New quantification of hodograph shape in nocturnal tornadic environments and its application to forecasting

Hodographs associated with tornadoes are typically described as straight, curved, or a hybrid of these two extremes. Kis and Straka's (2010) sample identified a bent low-level hodograph shape in proximity of the majority of nocturnal tornadoes. This shape is similar to the “sickle” shape noted by Miller (2006) and “kinked” shape noted by Esterheld and Guiliano (2008) that were associated with particularly destructive tornadoes. This hodograph shape indicates strong speed shear below the bend and dominance of directional shear above the bend. Low-level hodograph bends in Kis and Straka's sample were typically located above nocturnal boundary layer inversions and associated with strong low-level jets. All of the bends occurred below 1.2 km above ground level (AGL), and the majority of those in proximity of significant (F2–F5) nocturnal tornadoes occurred roughly at or below 500 m AGL.

Bent low-level hodographs in proximity of significant nocturnal tornadoes were associated with 0–1 km AGL storm relative environmental helicity (SREH) that was on average nearly 2.5-times greater than SREH with curved low-level hodographs. This large difference is especially surprising when considered from a streamwise vorticity perspective, since storm motion in the center of a curved low-level hodograph would contain more streamwise vorticity than would storm motion in the center of a bent low-level hodograph. In order to examine the disparity in SREH, so-called critical angles that appear in the dot product in SREH integrands were computed for each bent low-level hodograph in the sample. The angle between the storm motion vector and the 5 m AGL inflow vector is referred to as the inflow critical angle, and the angle between the storm motion vector and the average velocity vector below the bend height is referred to as the average critical angle. An inflow critical angle of 90-degrees would indicate purely streamwise storm-relative inflow, and an average critical angle of 90-degrees would indicate a large fraction of purely streamwise flow beneath the low-level hodograph bend height. As expected, the majority of critical angles were much less than 90-degrees. Eighty-five (60) percent of the average critical angles associated with significant (weak) nocturnal tornadoes deviated from the purely streamwise situation by more than 20-degrees; eighty-seven (56) percent of the inflow critical angles associated with significant (weak) nocturnal tornadoes deviated from the purely streamwise situation by more than 20-degrees. Nocturnal tornadoes associated with bent low-level hodographs did, however, have 5 m AGL storm relative inflow that was nearly two-times faster than the 5 m AGL inflow with curved hodographs, and this may explain the SREH disparity.

To make better use of critical angles computed by this technique, the storm motion vectors of ordinary storms in proximity of the sampled tornadic storms should additionally be analyzed so that the critical angles of tornadic and ordinary storms associated with bent low-level hodographs can be compared.