The Role of Boundary-Parallel Vertical Shear in Convection Initiation

The boundary-normal component of the vertical wind shear vector has been of interest in studies of convection initiation (CI) and density current dynamics. The boundary-normal shear component is considered in RKW Theory for determining the relationship between environmental shear and density current horizontal vorticity that controls the erectness of ascent at outflow gust fronts. Additionally, the boundary-normal component is considered in flow-force balance which determines the depth and propagation speed of density currents. However, the boundary-parallel component of the vertical wind shear has not been explored thoroughly. We hypothesize that changes in the boundary-parallel shear will alter density current propagation speed through downward transport of v-momentum and temperature mixing.

In this study, idealized simulations were conducted to test the sensitivity of density current evolution and CI to changes in the boundary-parallel component of the vertical wind shear. Both 2D and 3D simulations were conducted. The 2D simulations aid in isolating the major components associated with changes in boundary-parallel shear, while 3D simulations aid in considering complex processes such as the formation and impacts of vortices along the leading edge of the density current. Preliminary analysis reveals sensitivity of propagation speed and vertical velocity to the boundary-parallel vertical shear environment. The analysis also revealed sensitivity to the intensity of the density current, suggesting a relationship between boundary-parallel shear and density current intensity. Since vertical velocity alone is an incomplete measure of the likelihood of CI, passive fluid tracer motion and location are analyzed as a CI proxy.