storm-relative environmental helicity
From Glossary of Meteorology
(Abbreviated SREH.) A measure of the streamwise vorticity within the inflow environment of a convective storm.
It is calculated by multiplying the storm-relative inflow velocity vector by the streamwise vorticity and integrating this quantity over the inflow depth. Geometrically, the storm-relative environmental helicity is represented by the area on a hodograph swept out by the storm-relative wind vectors between specified levels (typically the surface and 3 km to represent the primary storm inflow). It is thought to be a measure of the tendency of a supercell to rotate.
