Turbulence and Gravity Waves within an Upper-Level Front

Abstract

High-resolution dropwindsonde and in-flight measurements collected by a research aircraft during the Severe Clear-Air Turbulence Colliding with Aircraft Traffic (SCATCAT) experiment and simulations from numerical models are analyzed for a clear-air turbulence event associated with an intense upper-level jet/frontal system. Spectral, wavelet, and structure function analyses performed with the 25-Hz in situ 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.Turbulence occurred in association with a wide spectrum of upward propagating gravity waves above the jet core. Inertia-gravity waves were generated within a region of unbalanced frontogenesis in the vicinity of a complex tropopause fold. Turbulent kinetic energy fields forecast by the RUC and CH models displayed a strongly banded appearance associated with these mesoscale gravity waves (horizontal wavelengths of ϳ120-216 km). 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.Fluctuations in ozone measured by the aircraft correlated highly with potential temperature fluctuations and the occurrence of turbulent patches at altitudes just above the jet core, but not at higher flight levels, even though the ozone fluctuations were much larger aloft. These results suggest the existence of remnant "fossil turbulence" from earlier events at higher levels, and that ozone cannot be used as a substitute for more direct measures of turbulence. The findings here do suggest that automated turbulence forecasting algorithms should include some reliable measure of gravity wave activity.

Aircraft measurements of gravity waves in the upper troposphere and lower stratosphere during the START08 field experiment

“…This phenomenon has been identified repeatedly in both observational studies (Uccellini and Koch, 1987;Schneider, 1990;Fritts and Nastrom, 1992;Ramamurthy et al, 1993;Bosart et al, 1998;Koppel et al, 2000;Rauber et al, 2001; and numerical investigations of the observed cases (Powers and Reed, 1993;Pokrandt et al, 1996;Kaplan et al, 1997;Zhang and Koch, 2000;Zhang et al, 2001Zhang et al, , 2003Koch et al, 2001Koch et al, , 2005Lane et al, 2004). In addition, idealized simulations of dry baroclinic jet-front systems in a high-resolution mesoscale model have been performed to investigate the generation of mesoscale gravity waves (Zhang, 2004), the sensitivity of mesoscale gravity waves to the baroclinicity of jet-front systems (Wang and Zhang, 2007), and the source of gravity waves with multiple horizontal scales (Lin and Zhang, 2008).…”

“…Also, strong horizontal and vertical shear in the layer and the discontinuity in static stability at the tropopause provide a favorable environment to reflect, capture, break and dissipate gravity waves generated in the lower troposphere, such as those produced by surface fronts (Plougonven and Snyder, 2007). Gravity wave breaking and wave-induced turbulence (e.g., Koch et al, 2005) can contribute significantly to mixing of trace gases in the ExUTLS, thereby affecting chemical composition (Vaughan and Worthington, 2000). Also, convectively generated gravity waves may extend the impact of moist convection far above cloud tops through wave-induced mixing and transport (Lane et al, 2004).…”

Reinterpreting aircraft measurements in anisotropic scaling turbulence

“…The scaling behaviour, the wind shears and jet stream characteristics on such targeted flights were indistinguishable from those on the three ferry flights HonoluluAnchorage-Long Beach-Honolulu. More detailed information can be found in Hovde et al (2009), in Ray et al (2004 and in Koch et al (2005). The Winter Storms 2004 data analysed here involved 10 aircraft flights over a roughly 2 week period over the northern Pacific each dropping 20-30 drop sondes.…”

Mountain waves modulate the water vapor distribution in the UTLS

“…The transport of trace gas species may be reversible or irreversible, depending on processes occurring on different scales. Irreversible mixing is promoted by turbulence induced, for example, by nonlinear wave interaction, wave breaking, or dissipation Whiteway et al, 2003;Koch et al, 2005;Lane and Sharman, 2006). Danielsen et al (1991) showed that waves with large horizontal wavelengths (∼ 36-270 km) and enhanced vertical amplitudes are significant carriers of energy, momentum and trace species.…”