Calculation of Storm-Relative Helicity and CAPE


Storm Relative Helicity (SReH)

The SReH calculated by SCAN is an integrated total value for the atmospheric layer from the surface to 3 km AGL. Proportional to both streamwise vorticity and storm-relative winds , SReH over the depth, h, of the storm inflow is defined as:

where vector V is the environmental wind vector, C is the storm's translation velocity, k x dV/dz is the horizontal vorticity, and vector k is the unit vector in the vertical. In SCAN, h is taken as 3 km as in most studies. The V(z) wind profile is obtained from the LAPS model data. Unlike most other studies, which use an averaged storm motion for SReH calculation, SCAN uses radar detected storm cell motion (direction and speed obtained from Composite Reflectivity radar product) for each individual storm cell as the vector C in the formula to calculate SReH.

Convective Available Potential Energy (CAPE)

The CAPE is a net positive energy. The LAPS model data provide hourly values of "positive buoyant energy" and "negative buoyant energy". For each detected storm cell, its CAPE value is determined by the subtraction of any negative energy from the positive energy value at the LAPS model grid point which is closest to the storm cell.