Analysis of turbulence generation by gravity waves within an upper-Tropospheric front

Instrumented aircraft and wind profiling radar measurements have established the propensity for clear air turbulence (CAT) to arise from Kelvin-Helmholtz instability in thin sheets of large vertical wind shear above and below the upper-tropospheric jet. The horizontal scale of the overturning eddies responsible for CAT experienced by an aircraft is ~100-300 m. On the other hand, operational model guidance for forecasting CAT consists of a statistical combination of various algorithms applied to forecast fields from the 20-km Rapid Update Cycle model. Although each algorithm has its origin in the application of the shearing instability idea, none has a rigorous basis in turbulence theory; rather, it is assumed that the model is useful for identifying resolvable, larger-scale features that are conducive to the formation of turbulent eddies.

An experiment involving the NOAA Gulfstream-IV aircraft was conducted in the winter of 2001 over the Pacific Ocean to test the performance of various RUC model predictors of turbulence and to improve understanding of CAT generation processes. The aircraft released dropsondes at ~40-km horizontal resolution and collected in situ 25-Hz meteorological and 1-Hz ozone data along four flight levels perpendicular to and to the cyclonic exit side of an intense upper-tropospheric jet on 17-18 February. Spectral and wavelet analysis were performed upon the 25-Hz data to uncover relationships between the gravity waves and turbulence. Experimental versions of the RUC model were generated in this data-sparse region, where the RUC is not normally run. In addition, a very high (1-km) resolution nested model (the Clark-Hall model) was spawned from a COAMPS simulation to investigate the gravity wave development in greater detail.

Analysis of the observational and model data showed that moderate-or-greater (MOG) turbulence occurred in association with a wide spectrum of upward propagating gravity waves, with horizontal wavelengths between 6 and 260 km, within a region of diagnosed imbalance immediately above the jet core and a deep tropopause fold. The observed MOG turbulence regions were well predicted by the RUC turbulence prediction algorithms, which displayed a strongly banded behavior parallel to the upper-level front. The Clark-Hall simulation showed that the inertia-gravity waves perturbed the background wind shear and stability, promoting the development of bands of reduced Richardson number, which were conducive to Kelvin-Hemholtz instability. Consistent with the fine-scale model results, the wavelet analysis revealed that the times of occurrence of the strongest gravity waves and the appearance of episodes of high turbulence energy were highly related. Rapid fluctuations in ozone correlated well with potential temperature fluctuations and the occurrence of turbulent patches derived from the aircraft data just above the jet core, but they did not do so at higher flight levels. These results suggest the existence at higher levels of “fossil turbulence” from earlier events and that ozone cannot be used as a replacement for more direct measures of turbulence.