Abstract

Spectral and structure function analyses of horizontal velocity fields observed in the upper troposphere and lower stratosphere during the Severe Clear Air Turbulence Collides with Air Traffic (SCATCAT) field program, conducted over the Pacific, were carried out in an effort to identify the scale interactions of turbulence and small-scale gravity waves. Because of the intermittent nature of turbulence, these analyses were conducted by clearly separating out the cases when turbulence did or did not occur in the data. In the presence of turbulence, transitional power spectra from k ⁻² to k ^−5/3 were found to be associated with gravity waves and turbulence, respectively. The second-order structure function analysis was able to translate these spectral slopes into r and r ^2/3 scaling, consistent with the Monin and Yaglom conversion law, in physical space, which presented clearer pictures of scale interactions between turbulence and gravity waves. The third-order structure function analysis indicated the existence of a narrow region of inverse energy cascade from the scales of turbulence up to the gravity waves scales. This inverse energy cascade region was linked to the occurrence of Kelvin–Helmholtz instability and other wave-amplifying mechanisms, which were conjectured to lead to the breaking of small-scale gravity waves and the ensuing generation of turbulence. The multifractal analyses revealed further scale breaks between gravity waves and turbulence. The roughness and intermittent properties were also calculated for turbulence and gravity waves, respectively. Based on these properties, turbulence and gravity waves in a bifractal parameter space were mapped. In this way, their physical and statistical attributes were clearly manifested and understood.

Abstract

A study is made of the 8 June 1974 Oklahoma dryline and tornado outbreak case, using data synthesis 1) to fit existing concepts on dryline structure and behavior to this case, and 2) to identify processes contributing to moisture convergence along the dryline. The dryline undergoes a major transformation in structure (from sloped to slopeless) during the day, as implied from mesoscale (10–100 km) and subsynoptic scale (100–1000 km) analysis of virtual potential temperature fields. Mesoscale examination of dryline movement reveals the presence of wavelike perturbations which propagate along the dryline, irregardless of its slope, and contribute more to its eastward progression than does the downward slope of the terrain.

All but one of 22 tornadoes reported in Oklahoma on this date were associated with thunderstorms that formed within a subsynoptic moisture convergence region at the dryline in central Oklahoma. Results indicate a downward transport of southwesterly momentum through a well-developed mixed-layer west of the dryline and isallobaric effects at the dryline contributed to the buildup of convergence.

Abstract

Data from the National Severe Storms Laboratory surface mesonetwork are objectively analyzed to give insight into processes that contributed to the development of three tornadic mesoconvective systems near the 8 June 1974 Oklahoma dryline. Storm cells constituting each of the systems form over recurring zones of convergence within 20 km of the dryline. Different mechanisms appear to force the individual convergence zones.

Storms of the first system appear simultaneously only after the establishment of a pressure trough just cast of a zone of convergence 15 km east of the dryline. The convergence zone intensifies and progresses eastward with the storms; meanwhile, a second convergence zone appears at the dryline in response to apparent storm-induced pressure systems trailing the storms. The fact that deep convection did not occur over the second zone is attributed to static stabilization caused by mesoscale unsaturated downdrafts in the upper troposphere. Storms of the second system develop in a consecutive manner over a third set of convergence anomalies that originally appeared at the dryline and subsequently propagated northeastward. These propagating disturbances have gravity wave characteristics. Formation of the third system, a solid squall line, is related to a frontogenetic circulation about a progressing cold front as it encountered the abundant moisture present at the stalled dryline.

It is concluded that precursor conditions to severe convective occurrences can be determined from surface mesoscale analysis and, moreover, provide considerable insight into mechanisms that produce low-level convergence.

Abstract

This case study addresses the issue of gravity current and bore development at surface cold fronts, and the role of these phenomena in the generation of severe frontal convection. The event investigated occurred on 27 April 1991 during the Cooperative Oklahoma Profiler Studies 1991 field project. The development of a bore from a gravity current–like structure along a cold front, the subsequent propagation of the bore ahead of the front on a low-level inversion, and the process of severe thunderstorm development along the front are revealed by a dense network of remote sensing and other special observations. Evidence for the gravity current and bore is strengthened by comparisons made between the synthesized observations and theory.

The bore developed after a nocturnal inversion, which acted as a waveguide, had become established. The bore and gravity current were both evident as “fine lines” in the radar reflectivity displays. A microscale envelope of enhanced water vapor with an embedded roll cloud, a strong vertical circulation, and a low-level microscale“jetlet” were associated with the bore. A pronounced “feeder flow” was present behind the gravity current, in association with a second vertical circulation, which was more elevated than the one associated with the bore. The jetlet provided an efficient wave-trapping mechanism for the bore, due to the combined effects of wind curvature on the Scorer parameter profile and mass convergence enhancement by the low-level shear.

Effects of the bore and gravity current passage on the atmosphere were assessed by applying parcel displacement profiles derived from wind profiler analysis to an observed prebore sounding, and then to a computed postbore sounding. These calculations suggest that the strong bore-induced lifting was insufficient to trigger the storms; rather, it was the dual lifting provided by the bore and the gravity current that made it possible for low-level parcels to reach their level of free convection. These results confirm other recent findings that indicate that even though bores generated by gravity currents can produce strong lifting, this may be insufficient to trigger deep convection whenever the lifting is confined to too shallow a layer and/or is of insufficient duration.

Abstract

The effects of sensible heating and momentum mixing on the low-level structure and dynamics of a two-dimensional cold front are studied with a hydrostatic primitive equation model. Effects of inhomogeneous heating arising from a contrast in low-level cloud cover across the front are emphasized. The relative importance of grid resolution and the choice of method for parameterizing planetary boundary layer (PBL) processes in the model are also examined. Frontal updraft dynamics are studied in terms of the following inquiries: (a) the relative importance of turbulent momentum transport, differential sensible heating, and the reduction in static stability in the heated region ahead of the front; (b) the nature of the interaction between the adiabatic, semigeostrophic frontal circulation and the thermally forced circulation; and (c) possible roles played by dry symmetric instability and density current dynamics. The terms in the frontogenesis and divergence budget equations are computed to determine the relative roles played by the various physical and dynamical processes in generating the frontal secondary circulation system.

A strong, narrow updraft jet forms in the presence of uniform sensible beating across the front. Although the greatest impact on frontogenesis occurs as a response to the reduction in static stability resulting from uniform sensible heating, additional forcing results from the nonlinear interaction between the adiabatic frontal circulation and the thermally forced circulation arising from a cross-front gradient in heating (due to the introduction of an overcast low cloud deck behind the front). The relative importance of inhomogeneous heating, however, increases with the grid resolution and the use of a multilevel treatment in place of bulk mixed-layer PBL models.

Numerical experiments reveal that symmetric instability does not create the updraft jet, despite the development of negative potential vorticity ahead of the surface cold front. Highly unbalanced dynamics and a density current-like “feeder flow” behind the cold front are strongly indicated in the presence of sensible heating effects. Budget analyses show that the frontogenetical effect of sensible heating is only indirectly important through its strengthening of the confluence (convergence) field. The nonlinear and unbalanced ageostrophic vorticity terms in the divergence budget equation exert the strongest controls on the development of the updraft jet when sensible heating is nonuniform.

These results suggest that differential cloud cover across cold fronts may promote the development of frontal squall lines. Nonhydrostatic models that include explicit prognostic equations for microphysics and use improved parameterization of boundary layer fluxes in the presence of clouds are needed to more fully address this possibility.

Abstract

Numerical simulations of a gravity current in an environment characterized by complex stratification and vertical wind shear have been performed using a nonhydrostatic, two-dimensional, dry, primitive-equation model. Data from one of the most complete documentations to date of gravity waves associated with a gravity current, presented in an earlier study, are used both to prescribe the gravity current's environment and for validation of the simulated gravity current and its associated gravity waves. These comparisons indicate that the gravity current observed by a Doppler wind profiler and sodars was well simulated in terms of depth, density contrast, and propagation speed and that the model produced a variety of gravity waves similar in many ways to these observed.

Because uncertainties remained concerning the gravity wave generation mechanisms derived from the observations (e.g., wavelengths were not observed), the validated simulations are used to test these tentative hypotheses. The simulations confirm that trapped lee-type gravity waves formed in response to flow over the head of the gravity current and that Kelvin-Helmholtz (KH) waves were created because of shear atop the cold air within the gravity current. The 2.8-km wavelength of the simulated KH waves agrees with the 2- to 3-km wavelength inferred from the observations. However, the 6.4-km wavelength of the simulated lee-type waves is significantly shorter than the 12.5-km wavelength inferred from the observational data, even though wave periods (20-23 minutes) are nearly identical. Sensitivity tests indicate that the curvature in the wind profile associated with the low-level opposing inflow and an elevated isothermal layer worked together to support the development of the trapped lee-type waves. The model produces a deep vertically propagating wave above the gravity current head that was not present in the observations. As deduced in the earlier study, sensitivity tests indicate that the prefrontal, near-surface stable layer was too shallow to support the generation of a bore; that is, conditions were supercritical. Synthesis of detailed observations and numerical simulations of these mesoscale phenomena thus offers the broadest examination possible of the complex physical processes.

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 ∼120–216 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.