Investigating the Vertical Structure of Updraft Helicity in Convection-Allowing Models

Updraft helicity is defined as the vertical integral of the product of vertical velocity and vertical vorticity between two levels and is commonly used as a proxy for mid-level rotation in simulated supercells. Previous studies have shown that updraft helicity has skill in predicting severe weather occurrence when used a surrogate for severe weather reports. Updraft helicity is most commonly calculated between 2-km and 5-km above ground level (AGL). This layer was chosen during the initial development of the updraft helicity diagnostic, and little further analysis has been published regarding the layer over which updraft helicity is calculated. Layers closer to the surface (0km-3km AGL) have shown utility in forecasting tornadoes, particularly at short lead times. To examine the best layer over which to calculate updraft helicity, the vertical profiles of vertical velocity and vertical vorticity will be examined. This will be done for two cases, 15 February 2016 and 11 May 2014, which represent southeastern cool season convection and plains spring convection, respectively. For each case, the 0000 UTC NSSL-WRF configuration was rerun with 3-km horizontal grid spacing. Time-height profiles of vertical velocity and vertical vorticity as well as vertical cross sections through the model-simulated storm will be examined. The sensitivity to horizontal and vertical model resolution will also be explored.