The Influence of Horizontal Variations in Vertical Shear and Low-Level Moisture on Numerically Simulated Convective Storms

Severe storms are typically simulated assuming either an idealized, horizontally homogeneous environment or an observed inhomogeneous environment. These represent opposite ends of the spectrum, and both have limitations with regard to our understanding of severe storms. Conclusions drawn from the former are difficult to generalize because real storms often move through environments that exhibit considerable spatial variation. Real data experiments, on the other hand, include these variations but are so inherently complex that meaningful conclusions about basic storm responses to any one factor in the environment are difficult to construct.

In this study, which builds on a previous work shown at the 19th Conference on Severe Local Storms, horizontal variations in vertical shear and low-level moisture are included in an idealized manner so that their influence can readily be diagnosed. Simulations are performed using the Advanced Regional Prediction System (ARPS) with significant modification to accommodate the inhomogeneous, but idealized, environmental fields. The need for accurate boundary conditions is a particular challenge. The solutions created to solve these problems will be presented.

Inhomogeneous environments that remain steady are optimal for modeling purposes but are difficult to devise. However, we will present several that retain a good degree of realism and are scientifically interesting. In particular, we show results in which the low-level moisture varies meridionally in an environment with westerly shear. The resultant preferred development of the storm system toward the higher moisture region and the differences between high and low shear cases is examined.

Simulations in which the vertical shear varies spatially are also presented. In the most interesting case, an idealized environment is constructed such that a multicell storm system moves into a region of much stronger shear over its lifetime and evolves into a bow echo. The adaptation of storm structure to the increase in shear is described.

In all of the results to be presented, simulations in the inhomogeneous domain are compared to control simulations of storms in homogeneous environments using soundings taken from different locations in the inhomogeneous domain. Both multicell and supercell storms are found to respond to changes in their environment. The response differs depending on the type of storm. The response of key storm attributes, such as updraft strength and mid and low level vorticity, to this change will be presented.