ABSTRACT
VOLLMER, DAVID RUSSELL. The Interaction of Jet/Front Systems and Mountain Waves: Implications for Lower Stratospheric Aviation Turbulence. (Under the direction of S. Pal Arya and Michael L. Kaplan.)
The role of jet streaks and their associated upper-level structures (fronts, troughs, thermal fields, etc.) in enhancing orographically-induced aviation turbulence near and above the tropopause is investigated. The primary hypothesis for this research suggests that there is an optimal configuration for the positioning of upper-level circulations leading to vertically confluent flow and differential thermal advection, forming an intense inversion. Such a configuration may be associated with vertically-intersecting ageostrophic jet circulations or trough-induced differential vertical motions leading to cold air undercutting a warm layer aloft, and compression of the warm layer in the presence of jet-induced shear. This structure is then perturbed by mountain waves, leading to a downscale cascade of kinetic energy, eventually leading to potential aviation turbulence. Two cases of clear-air turbulence (CAT) are examined using mesoscale numerical simulations. The first case involved a DC-8 attempting to cross the Colorado Front Range when it encountered extreme CAT resulting in loss of part of one wing and an engine. In this case the superposition of two distinct jet features was hypothesized to have established an unusually strong tropopause which allowed strong buoyancy-driven motions to enhance the horizontal shear and turbulent eddies, eventually leading to the turbulent downburst hypothesized to have played a role in damaging the aircraft. The second study used data from the Terrain-Induced Rotor Experiment (T-REX) and examined a turbulent wave-breaking event recorded by a research aircraft in the lower stratosphere. A different jet regime was found in this case, with a strong upstream trough and decreasing cyclonic curvature with height above the tropopause and a strong lower stratospheric inversion. The vertical variation of static stability in the lower stratosphere was found to create a favorable environment for amplification and breaking of the mountain wave in this case. Lastly, a series of two-dimensional idealized experiments were conducted to further examine the role of static stability and jet structure in developing the potential for stratospheric turbulence over complex terrain. Vertical static stability gradients in the lower stratosphere were found to play a significant role in the potential for aviation turbulence, as were the angle of incidence of the winds and the strength of the flow.
