Doppler Radar Observations of the Interactions of Gravity Waves with Mesocyclones

Several cases of mesocyclone intensification associated with the interaction of the mesocyclone with an apparent gravity wave have been observed over the eastern United States. Theory and numerical simulations (Coleman and Knupp 2006) show that at least two processes may play a role in this interaction. The vorticity of a pre-existing mesocyclone may be enhanced through stretching due to convergence ahead of the wave ridge. Also, the gravity wave wind perturbations may enhance vertical wind shear, which may then be tilted into the vertical by a storm updraft further enhancing the mesocyclone.

Radar data is being used extensively in this study. The gravity waves in the case studies typically appear as fine lines on radar, presumably caused by the vertical motion in the wave either lifting parcels to their LCL (lifting condensation level), or lifting insects to radar level near the wave ridge. The fine lines in each case will be examined to determine whether they are gravity waves. First of all, the speed of motion of the fine line will be compared to that predicted by the duct theory of Lindzen and Tung (1976). Vertical cross-sections of reflectivity and velocity will also be created. For a gravity wave, the velocity in the direction of wave propagation may vary substantially from the wave speed, whereas for density currents the velocity should be near or slightly greater than the movement. Also, in Doppler radar velocity cross-sections of gravity waves, we have found that the wind shear is significant, so shear profiles obtained from radar cross sections in the fine line may also be used to indicate a gravity wave. In cases where the fine line passes a surface station allowing for measurement of the pressure perturbation, the impedance relation for gravity waves (e.g., Gossard and Munk 1954) and the speed equation for density currents (Seitter 1986) may be used to determine whether the fine line is a gravity wave or a density current.

Extensive radar analysis is also being done at UAH to analyze in greater detail the mesoscale flows within gravity waves. Radar cross-sections are being produced, and software has been developed on-site which synthesizes horizontal winds along the direction of wave propagation, in addition to convergence and vertical motion profiles. On-site velocity azimuth display (VAD) programs are also being developed in order to examine the convergence and vertical motion within waves. The information gathered may be compared to the numerically simulated convergence and wind shear fields in our current gravity wave-mesocyclone interaction model.