Turbulence and gravity waves in a dynamically unbalanced upper-level jet-frontal system

High-resolution aircraft measurements and simulations from numerical models are analyzed for a clear-air turbulence event associated with an intense upper-level jet/frontal system. Cross spectral, wavelet, structure function and Stokes parameter analyses performed with the 25-Hz in-situ aircraft data are used to investigate the relationship between gravity waves and turbulence. Mesoscale dynamics are analyzed with the 20-km hydrostatic Rapid Update Cycle (RUC) model, and a nested 1-km simulation with the nonhydrostatic Clark-Hall (CH) cloud-scale model.

Inertia-gravity waves were generated within a region of unbalanced frontogenesis in the vicinity of a complex tropopause fold above the jet core. Turbulent Kinetic Energy fields forecast by the models displayed a strongly banded appearance associated with these mesoscale gravity waves. Smaller-scale gravity wave packets (horizontal wavelengths of 1–20 km) within the mesoscale wave field perturbed the background wind shear and stability, promoting the development of bands of reduced Richardson number conducive to the generation of turbulence.

The wavelet analysis revealed that brief episodes of high turbulent energy were closely associated with gravity wave occurrences. Structure function analysis provided evidence that turbulence was most strongly forced at a horizontal scale of 700 m. By combining cross-spectral and continuous wavelet transformation, we were able to extract the temporal and spatial characteristics of the waves. Upon further introduction of these results into a Stokes parameter analysis for partially polarized internal waves, we demonstrated that gravity waves possess distinctive polarization and coherency signatures allowing clear separation of waves from turbulence. Our analysis indicates that the turbulence production is closely related to an enhancement in both the polarization and coherency above a threshold value in the Cartesian wind components, and that turbulent surges are accompanied by a tendency for an abrupt reduction of the polarization and shifting of the horizontal wave vector. The implications of this research are that unbalanced upper-level jets may generate a broad spectrum of gravity waves, which at the shorter wavelengths, develop a tendency to steepen and break, resulting in surges of turbulent bursts that are readily identifiable using spectral, wavelet, and structure function techniques.