The interaction of gravity waves with tornadoes and mesocyclones: theories and observations

Several cases of gravity wave interactions with pre-existing mesocyclones and tornadoes have been observed over the southeastern United States. It appears that these wave interactions alter the vorticity and rotational velocity of the mesocyclones, sometimes significantly. In one exceptional case near Birmingham, Alabama on 8 April 1998, a tornado's intensity increased rapidly upon interaction with a gravity wave.

In this study, we will first examine dynamical hypotheses of these inferred interactions. We propose that gravity waves may enhance the vertical vorticity within a mesocyclone or tornado through two processes. First, convergence ahead of the gravity wave ridge may induce vertical stretching and a subsequent increase in vorticity. Second, horizontal wind perturbations within the gravity wave may locally increase the vertical wind shear/helicity, providing the potential for enhancement of vorticity through the vertical tilting process.

Doppler radar data for cases in which gravity waves passed directly over WSR-88D sites will be examined, including the 8 April 1998 Birmingham tornado case, in addition to some non-mesocyclone-related gravity wave events in northern Alabama/Mississippi. Rapidly-changing horizontal wind and convergence profiles are determined by implementing a VAD analysis over a limited (5-10 km) radius circle. These analyses will allow assessment and quantification of wave-induced convergence/divergence as well as perturbations in the wind profile and subsequent changes in vertical wind shear associated with wave passage. When available, data from 915- and 404-MHz wind profilers will also be utilized in this study.

These results will then be applied to examine the dynamical interactions of gravity waves with the 8 April 1998 Birmingham tornado, and with a 22 January 1999 mesocyclone over northwest Alabama, which intensified quickly upon interaction with a gravity wave train. We plan to utilize 1) Doppler velocity PPI images, 2) single-Doppler wind syntheses (performed along lines normal to wave propagation vectors), and, if possible, 3) synthetic-dual Doppler analyses. This will allow us to readily assess the contributions to vorticity increases in the tornado/mesocyclone cases from convergence/stretching ahead of the gravity wave ridge, and from enhanced vertical shear due to wind perturbations within the wave. Assessment of the relative contribution of each process is important, since the former is not theoretically dependent on the relative orientation of the wave propagation to storm motion, while the latter is dependent.

Finally, additional cases of suspected enhancement of mesocyclones due to gravity wave interactions will be briefly presented. These cases will provide additional opportunities for analysis of this potentially important process.