Spatial Gradients in Convective Parameters and Their Influence on Storm Dynamics and Structure

Previous idealized modeling work has shown that small changes in updraft slope and width can have large effects on updraft velocities; in turn, updraft slope and width can be related to environmental characteristics, including convective available potential energy (CAPE) and shear. To extend these prior idealized works we first present a climatology of the relevant environmental characteristics based on 12-km North American Model (NAM) reanalysis, focusing on the Great Plains of the United States, during May, which has a high probability of hail-producing and tornadic thunderstorms. This climatology is used to calculate spatial gradients (zonal and meridional), providing the mean change in CAPE and shear per kilometer in any direction. Calculating these gradients will reveal the mesoscale variability of the environmental parameters most critical for deep convective clouds, and this variability will be used to simulate convective storms in geographically similar but environmentally different environments.

To determine the impact of natural environmental variability on convective storm dynamics, we first use Oklahoma City as a basis, retrieving the sounding from the climatology and performing an idealized simulation initiated with a thermal bubble. We then extract the soundings from adjacent grid boxes in the zonal and meridional directions and perform the same idealized simulations except that each point has three simulations: change in thermodynamics, change in shear, and change in both thermodynamics and shear; these simulations will allow us to determine how updraft velocity is expected to change for storms forming at different locations within a given region.

The key result is a deterministic relation between distance and updraft velocity. This relationship has two important implications. First, it provides an estimate of the potential variability in convective updraft strength across a given area, which is critical because numerical weather prediction models are plagued by issues related to the timing and location of convective initiation. The relationship allows for a more statistical approach to understanding the potential strength of convection on a given day and over a certain region. Second, the derived relationship can be used to determine the required change in other fields, e.g., aerosol loading, over identical distances such that the natural environmental thermodynamic and shear variability effects on updrafts are overcome by these secondary factors.