Effective storm-relative helicity in supercell thunderstorm environments

Since its introduction in the late 1980s early 1990s, storm-relative helicity (e.g., Davies-Jones et al. 1990) has received widespread acceptance within the operational forecasting community as a supercell and tornado forecast parameter. Predictive estimates of storm-relative helicity have relied on various storm motion algorithms (most recently Bunkers et al. 2000) and approximations of the storm "inflow layer" depth (typically the lowest 1-3 km above ground level). In an attempt to refine the estimates of the storm inflow layer, the depth of the layer is constrained by the vertical profiles of temperatures and moisture. Specifically, it is assumed that only lifted parcels associated with substantial buoyancy will sustain a supercell updraft, whereas parcels associated with little buoyancy or large convective inhibition will ultimately result in storm demise. Several values of CAPE and convective inhibition (CIN), ranging from 100 to 500 J kg^-1, were tested as potential thresholds for determining storm inflow depth for the "effective" storm-relative helicity. The tests were performed by beginning at the ground level in the sounding and searching upward for the first lifted parcel to satisfy the CAPE and CIN constraints, and this level was designated the "effective base". Continuing upward from the effective base, each level in a sounding was examined until either of the CAPE or CIN constraints were violated, and this level was designated the "effective top". The vertical distance between these two levels defines the effective storm inflow layer.

The RUC model close proximity sounding sample described in Thompson et al. (2003) has been expanded to include storm cases from 2003 and 2004, increasing the sample size to nearly 1000 soundings. In that sounding sample, the depth of the inflow layer varies linearly with the threshold choices such that the deepest layers correspond to the least stringent CAPE and CIN thresholds (e.g., CAPE > 100 J kg^-1 and CIN > -500 J kg^-1) and the shallowest inflow layers correspond to the most stringent thresholds (e.g., CAPE > 500 J kg^-1 and CIN < 100 J kg^-1 ). However, the ability to discriminate between significantly tornadic, weakly tornadic, and nontornadic supercells is not sensitive to the specific threshold choices tested. For CAPE > 250 J kg^-1 and CIN > -250 J kg^-1 , the effective storm inflow layers typically range from 1 to 3 km above ground level, though there is substantial variability from case to case. In comparing storm-relative helicity calculations for the effective inflow layers and the fixed 0-1 km and 0-3 km layers, the effective helicity discriminates more clearly between the classes of surface-based supercells. The effective inflow layer also allows calculation of a more meaningful storm-relative helicity for elevated thunderstorms. A comparison of effective helicity between elevated and nontornadic surface-based supercells reveals little difference between these storm groups.