Wind Shear


Storm Relative Helicity (SRH) - Storm-relative helicity (SRH) is a representative of streamwise vorticity within the storm inflow layer, and is proportional to the area swept out between the hodograph and the storm motion. Davies-Jones et al. (1990) first tested SRH as a tornado forecast tool. Work by Rasmussen and Blanchard (1998), Thompson et al. (2003), and Thompson et al. (2007) has confirmed the utility of SRH to discriminate diagnostically between tornadic and nontornadic supercells. The initial versions of SRH focused on the 0-3 km layer as a representation of the low-level air mass that feeds a thunderstorm updraft. Later work by Rasmussen (2003) and Thompson et al. (2003) has identified the importance of the 0-1 km layer in discriminating between tornadic and nontornadic supercells. Most recently, the "effective inflow layer" technique has been developed by Thompson et al. (2007) as a more realistic estimate of the storm inflow.

Bulk Shear - The bulk wind difference over a layer, calculated by vector subtraction. Bulk shear through 0-6 km AGL layer discriminates strongly between supercell and nonsupercell thunderstorm environments. The transition from nonsupercell to supercell thunderstorms occurs as the 0-6 km bulk wind difference increases from roughly 25 kt to 40 kt, with larger values favoring supercells. Research also suggests that increasing values in the 0-6km layer correlate with increasing tornado potential.

Storm Relative Wind - Mean storm-relative winds through a layer, minus the magnitude of the difference between an assumed right supercell motion and the environmental winds at the same level. The profile of storm-relative winds can be used to infer supercell precipitation distribution (e.g., "classic" versus "heavy precipitation") or tornado potential. Please refer to Rasmussen and Straka (1998), Thompson et al. (2003), and Markowski et al. (2003) for additional information.

BRN Shear - The denominator of the bulk Richardson number, known as the BRN shear term. The BRN shear term is the square of the bulk vector difference between the 0-500 m AGL mean wind and the 5500-6000 m AGL mean wind (both pressure weighted), multiplied by one half. Higher values are generally associated with an increasing risk of supercell storms.