Impact of lapse rates upon low-level rotation in idealized storms

A brief survey reveals numerous examples of tornadoes that have occurred in environments with adiabatic, or nearly-adiabatic, lapse rates. One reasonable hypothesis

is that steeper temperature lapse rates entail more environmental CAPE, greater updraft buoyancy, a larger vertical gradient in updraft speed above the LFC, and therefore greater tilting and stretching of vorticity. However, another notable property of adiabatic environments is that they prohibit gravity wave propagation. In stabler environments, gravity waves quickly disperse convective heating to the far field through propagation. In the absence of such gravity waves, advective mechanisms are needed to disperse the convective heating. Because these mechanisms are slower (at least initially), convective latent heating remains trapped in the convective column for comparatively long periods of time, which can quasi-statically lower the surface pressure and enhance the low-level convergence of vertical vorticity.

This study seeks specifically to address the role of the environmental lapse rate upon heat dissipation from a storm, and upon the corresponding low-level vertical vorticity. Accordingly, the environmental lapse rates are varied in idealized simulations while the convective heating rate is held constant (removing the effect of varying CAPE). Using an axisymmetric version of the Bryan Cloud Model 1 (CM1), a Rankine vortex is initialized with three different vertical depths, and using six different environmental temperature profiles, including some with adiabatic layers of differing depths, and some with temperature inversions. It is found that, under constant heating, the largest values of vertical vorticity at the surface occur in environments with adiabatic lapse rates, even if the adiabatic layer is shallow or is topped by a temperature inversion. The governing dynamics and possible implications to tornadogenesis will be discussed in the conference preprint and presentation.