Impacts of Increasing Low-Level Shear on Supercells during the Early Evening Transition*

Abstract

The dynamical response of simulated supercells to temporally increasing lower-tropospheric vertical wind shear is investigated using idealized simulations. These simulations are based upon observed soundings from two cases that underwent an early evening transition during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Mature supercells were simulated in observed afternoon environments with moderate vertical wind shear and then compared to simulated supercells experiencing observed evening increases in lower-tropospheric shear. The primary effect of the increase in low-level shear is to establish larger values of vertical vorticity at lower altitudes in the storm’s updraft. In turn, this leads to a nonlinear increase in the updraft strength due to the enhanced dynamic pressure minimum associated with larger vorticity in the storm’s mesocyclone. This is particularly important at low levels, where it increases the storm's ability to lift cool surface air (including outflow). Trajectories launched in developing vortices show that, despite comparable buoyant accelerations, parcels experience greater vertical velocity and stretching of vertical vorticity due to increased dynamic accelerations when the low-level shear is increased. Thus, even as low-level stability gradually increases in the early evening, the supercells’ low-level updraft intensity and surface vorticity production can increase. These results are consistent with climatological observations of a supercell’s likelihood of tornadogenesis during the early evening hours.

Budget Analyses of a Record‐Breaking Rainfall Event in the Coastal Metropolitan City of Guangzhou, China

“…Numerically, , and can be diagnosed by solving the three‐dimensional Poisson equations – with appropriate boundary conditions (Coffer & Parker, ; Wang et al, ). Then the nonlinear dynamic perturbation pressure can be obtained as the residual .…”

“…Then the nonlinear dynamic perturbation pressure can be obtained as the residual . We used the same boundary conditions and method as in Coffer and Parker () to solve these Poisson equations. Please refer to Coffer and Parker () for the detailed description.…”

“…We used the same boundary conditions and method as in Coffer and Parker () to solve these Poisson equations. Please refer to Coffer and Parker () for the detailed description. Therefore, the total dynamic acceleration (hereafter ACCD) is induced by the linear ( , hereafter LD_VPPGF) and nonlinear dynamic ( , hereafter NLD_VPPGF) pressure perturbation gradient force.…”

“…From (8), relatively low perturbation pressure is found on the downshear side of a localized updraft, and high perturbation pressure on the upshear side. (5)-(7) with appropriate boundary conditions (Coffer & Parker, 2015;Wang et al, 2016). Then the nonlinear dynamic perturbation pressure p ′ DNL can be obtained as the residual p ′ DNL ¼ p ′ D −p ′ DL .…”

Investigating the impact of atmospheric stability on thunderstorm outflow winds and turbulence

“…This counter-intuitive result should be studied in detail. In fact, the impact of increased stability on tornadogenesis has been investigated recently; the climatologies of tornadic events have suggested a peak in the occurrence frequency of tornadoes in the early evening (Coffer and Parker, 2015). In the present study, we will probe further into how stability impacts downburst winds and into such implications on the wind energy and wind engineering communities.…”