The Effect of Surface Roughness on Storm-Relative Helicity

Wind shear and helicity, especially at the lowest levels (below 500 m AGL), are being shown in the literature as a critical factor in tornadogenesis (e.g., Coffer et al. 2019). Therefore, any factors that cause spatial variability in low-level wind shear outside of the usual synoptic or meso-alpha scale phenomena that dictate shear (surface cyclones and isallobaric winds, low-level jets, thermal advection, fronts, etc.) should be examined. Surface heterogeneities in roughness length (z₀) that affect low-level wind shear and storm-relative helicity (SRH) may be critical in determining the likelihood of tornadogenesis in mesocyclones, especially in weaker mesocyclones like the ones typically found in QLCSs.

The boundary layer is, by definition in many fluid mechanics fields, the layer of flow affected by frictional forces. In boundary layer meteorology, friction causes the flow to slow and back near the surface. This effect decreases with height, producing wind shear. The magnitude of the wind shear due to friction is proportional to z₀. So, in theory, areas with larger surface roughness should have more slowed and backed surface-layer flow, and given the same flow at the top of the boundary layer, more helicity. High-resolution rapid refresh (HRRR) model results show this increase in shear and helicity due to higher z₀ relative to a nearby area with lower z₀.

During a VORTEX-SE deployment on December 11, 2021, 915 MHz wind profilers were positioned adjacent to surface stations providing 10 m winds at two locations only about 10 km apart: one at the Severe Weather Institute Radar and Lightning Laboratories on the University of Alabama in Huntsville campus, where the roughness is fairly high due to urban surroundings, and the other at the Huntsville International Airport (KHSV), where the roughness is quite low, especially during southerly flow. The differences in 0-500 m and 0-1 km AGL wind shear and storm-relative helicity will be examined.