A closer look at damaging surface winds associated with mesoscale gravity waves

Observations show that mesoscale gravity waves of significant amplitude (> 3 hPa) occur frequently in the atmosphere. These waves often produce significant wind perturbations. When the low-level background wind field is oriented in the opposite direction of, or approximately normal to, the maximum perturbation winds, large changes in wind direction (> 90 degrees) may occur. However, when the background wind field is in the same direction as the wind perturbations, high surface winds may result. Sometimes, these winds are in excess of 25 m s-1 and cause wind damage.

In this paper, we plan to examine several mesoscale gravity waves with associated large wind perturbations in detail. We will utilize cross-sections of horizontal winds based on Doppler radar data, WSR-88D VAD wind profiles, and constant-radii VAD analysis to examine the structure of the wind fields in gravity waves. Surface observations of wind and pressure will also be integrated into the analysis. Interestingly, cross-sections often show that the highest winds within a mesoscale gravity wave are concentrated very close to the surface, in the lowest 1 km AGL. Two case studies of damaging winds due to mesoscale gravity waves in Alabama will be examined.

The classical impedance relation (e.g., Gossard and Munk 1954; Gossard and Hooke 1975) relates the wind perturbation to the pressure perturbation and the intrinsic speed of a gravity wave. This relation may be derived by integrating the linearized horizontal equation of motion over a sinusoidal disturbance in pressure. However, this relation may be improved in some situations by including non-linear terms in the momentum equation. This non-linear impedance relation will be developed, and the results compared to observed wind perturbations at low-levels in mesoscale gravity waves.