Increases in Wind Shear and Helicity During the AET Period

The afternoon-to-evening transition (AET) period is broadly defined as the time period when the net radiative heat flux at the surface changes from downward to upward, causing low-level cooling (e.g., Angevine 2008; Busse and Knupp 2012).  This low-level cooling causes stabilization and reduces vertical mixing in the boundary layer (BL), and therefore a reduction in vertical momentum flux.  During the daytime BL, the effects of the frictional force, that tends to slow winds down and turn them across the isobars toward low pressure, is spread vertically to some extent by vertical momentum flux.  However, during the AET this flux decreases (e.g., Wingo and Knupp 2014), and the effects of the frictional force decrease at the top of the daytime BL and increase near the surface.

Therefore, as the influence of the frictional force is no longer “carried” upward away from low-levels, frictional influence increases at low-levels.  The low-level winds decelerate and back, flowing towards low pressure.  However, near the top of the daytime BL, frictional influence decreases with time and parcels become more geostrophic, veering to become more parallel with the isobars.  As near-surface winds decelerate and winds above the surface accelerate, speed shear increases at low-levels that composed the daytime BL, typically 0-3 km AGL.  Also, since near-surface winds back and winds above the surface veer, directional shear increases.  These factors increase low-level bulk wind shear and helicity during the AET.

Increases in low-level wind shear during the AET, and their potential effects on severe storms, have been examined by several authors (e.g., Markowski et al. 1998; Mead and Thompson 2011; Coffer and Parker 2015).  MT11 found that storm-relative helicity may increase by more than 100 m² s^-2 during sunset.  CP15 showed that the increase in low-level shear increased vertical velocities and vertical vorticity in supercell storms during VORTEX-2. 

The purpose of this paper is to determine, through a combination of computer modeling and high-resolution measurements, the changes in vertical momentum flux and flux divergence, changes in the wind field, and subsequent changes in wind shear and helicity during the AET period.  Instrumentation including 915 MHz wind profilers, mini-sodars with range gates of 10 m, and a Doppler wind lidar, are used to directly and indirectly measure momentum flux and flux divergence.  The 915 MHz profiler, along with sodar data and VAD wind profile data from NEXRAD radars, is used to confirm the climatology of increasing low-level wind shear and helicity during the AET period.  The case of 28 April 2014, when supercell storms that had been morphing into a QLCS prior to the AET, then re-generated into tornadic supercells during the AET, will be examined.