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 for this relationship posits that steeper temperature lapse rates entail more environmental CAPE, which in turn leads to stronger updrafts and enhanced tilting and stretching of vorticity. This process seems relatively straightforward. However, adiabatic environments are also notable in that they prohibit gravity wave propagation, and in that they provide no resistance to downward parcel displacements. In stable environments, gravity waves quickly disperse convective heating to the far field through propagation. In the absence of such gravity waves, less efficacious dispersion by advection occurs, and a greater fraction of the latent heating remains in the convective column. This can produce comparatively strong subsidence in the immediate near-cloud environment, especially considering the minimal resistance provided by the ambient static stability.

This study assessed the response of the flow field to differing environmental lapse rates when the convective heating rate was controlled (held fixed). When the low-levels are neutrally stratified, heating-induced subsidence is anchored near the cloud edge and is substantially stronger than in a stably stratified layer. This is partly due to the absence of heating dispersion by gravity waves, and partly due to the weaker resistance to downward parcel displacements. This enhanced heating-induced subsidence can bring high angular momentum air to the surface without the need for evaporative cooling, which may in turn favor tornadogenesis. Additional experiments reveal that when the lateral gradient in heating is sharpened, or the initial vortex is strengthened, the resulting subsidence and surface vorticity are further enhanced.