Investigating the Structure of Updraft Helicity in an Idealized Supercell Simulation

Updraft helicity (UH) is a commonly used diagnostic output from convection-allowing models (CAMs) for forecasting severe convective weather. It is calculated by taking the vertical integral of the product of vertical velocity and vertical vorticity over a fixed layer, commonly 2-5km and 0-3km AGL. Until recently, this calculation was only done in operational CAMs when both vertical velocity and vertical vorticity are positive (i.e., cyclonically rotating updrafts) throughout the integrated layer. Now operational CAMs also output negative UH for anticyclonically rotating updrafts over these fixed layers. This study examines the spatial and temporal variations in the product of vertical velocity and vertical vorticity in an idealized simulation and shows that there are both positive and negative contributions to the product of vertical velocity and vertical vorticity outside of the commonly used layers. Based on the spatial and temporal distribution of the product of vertical velocity and vertical vorticity, this study also investigates methods for calculating UH over variable layers, including anticyclonic rotation within updrafts. For the variable layer calculations, the depth of the layer will also be explored as a potentially useful output for diagnosing severe weather potential.