Three-Dimensional Numerical Modeling of Convection Produced by Interacting Thunderstorm Outflows. Part II: Variations in Vertical Wind Shear

Abstract

In this second paper in a series on outflow interactions, we use the three-dimensional model described in Part I to examine the effects of vertical wind shear variations on cloud development along intersecting thunderstorm outflow boundaries. Three wind shear profiles are used in this study: shear only above cloud base, shear only below cloud base, and shear both above and below cloud base. As in Part I, the shear is unidirectional and is oriented perpendicular to the line containing the two initial outflow-producing clouds (which are spaced 16 km apart). Using the environmental thermodynamic structure from the control simulation in Part I, we vary the shear magnitude in each profile and examine the properties of cloud development in the region where the two outflows collide (the outflow collision line or CL). The model results show that the intensity and the time interval between successive cell updraft maxima of the first two clouds along the CL (both of which are triggered by the outflow collision) are controlled by the strength of the vertical wind shear. In strong shears, the upshear member of this pair of clouds has a head start in development and becomes the stronger cell of the two. The timing difference between these two clouds is a few minutes. In weaker shears, the two clouds grow at nearly the same rate, and therefore have similar intensities and a smaller timing difference. The presence of wind shear in the boundary layer is found to enhance the updrafts of these two cells in all cases. The strength of the third and subsequent clouds which form along the CL is related to the speed at which the gust front moves away from the developing cells. The larger this separation speed, the more quickly the gust front-induced convergence is removed from the clouds, and thus the weaker they are. The third and subsequent cells along the CL are found to be more intense when shear is present in the cloud-bearing layer. The factors governing the timing difference of the third and successive cells to form along the outflow's leading edge are not clear at this time.