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

Thirteen case studies of mesoscale wave disturbances (characterized by either a singular wave of depression or wave packets with periods of 1–4 h, horizontal wavelengths of 50–500 km, and surface pressure perturbation amplitudes of 0.2–7.0 mb) are reviewed to isolate common synoptic features for these cases and to shed light on possible energy sources for the waves. A strong thermal inversion in the lower troposphere (north of a frontal boundary) and a jet streak propagating toward a ridge axis in the upper troposphere are commonly observed in all the cases. In general, the area of wave activity is bounded by the jet axis to the west or northwest, a surface front to the southeast, an inflection axis (between the trough and ridge axes) to the southwest and the ridge axis to the northeast.

The conditions specified by Lindzen and Tung as being necessary to form a wave duct, which include the existence of the lower-tropospheric inversion, seem to be met in many of these cases. This suggests that a ducting mechanism contributes to the long duration of these wave events by preventing the vertical propagation of wave energy.

Questions are raised concerning the role of either convection or shear instability as source mechanisms for the generation of these mesoscale wave disturbances. The observed development of the waves within the exit region of a jet streak propagating toward an upper-level ridge axis is shown to be consistent with the hypothesis that the actual energy source needed to initiate and sustain thew wave events may be related to a geostrophic adjustment process associated with upper-tropospheric jet streaks.

Abstract

Synoptic and special mesoscale observations taken during the Cooperative Convective Precipitation Experiment (CCOPE) are used to describe the multiscale environment of a gravity wave event, understand the wave-environment interactions that led to the development of severe thunderstorms, and asses possible wave-generation mechanisms. The storms formed sequentially as a packet of gravity waves propagated across a stationary thunderstorm outflow boundary. Convection developed most rapidly in that part of the mesonetwork in which existed the combination of relatively high parcel buoyant energy, weak restraining inversion, strong storm downdraft potential, and substantial vertical wind shear (associated with a mesoscale jet streak).

Synoptic-scale analysis reveals that the waves were excited north of a stationary front and within the right exit region of the jet streak as it approached a stationary ridge in the 300 mb height field. Strong indications of unbalanced flow were diagnosed within the gravity wave source region. Hence, it is suggested that the propagation of the jet streak toward the ridge resulted in the shedding of a gravity-inertia wave packet in a association with a geostrophic adjustment process, which in turn triggered severe thunderstorms along the preexisting outflow boundary.

A shear instability analysis conducted upon a representative CCOPE sounding shows that the vertical shear associated with the jet also could have served as a wave energy source, since a wave critical level was found at which the calculated Richardson number fell to a value Ri∼¼. Additional analyses indicate that the observed waves were nondispersive and hydrostatic and that vertical energy propagation was impeded by a wave duct associated with the presence of the critical level and lower-tropospheric static stability. The highly coherent nature of the waves, which persisted for many horizontal wavelengths, is explained by this ducting mechanism.

These results would seem to point to both geostrophic adjustment and shear instability as plausible wave source mechanisms. It is conjectured that the observed waves were generated by geostrophic adjustment processes, additional energy was supplied through interaction with the critical level, and their coherence maintained through the ducting mechanism.

Abstract

One of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that primarily impacted the southeastern United States, including two quasi-linear convective systems (hereinafter QLCS1 and QLCS2) that preceded the notorious outbreak of long-track, violent tornadoes spawned by numerous supercells on the afternoon of 27 April. The ∼36-h period encompassing these three episodes was part of a longer multiday outbreak that occurred ahead of a slowly moving upper-level trough over the Rocky Mountains. Here in Part I, we detail how the environment evolved to support this extended outbreak, with particular attention given to the three successive systems that each exhibited a different morphology and severity. The amplifying upper-level trough and attendant jet streak resulted from a Rossby wave–breaking event that yielded a complex tropopause structure and supported three prominent shortwave troughs that sequentially moved into the south-central United States. QLCS1 formed ahead of the second shortwave and was accompanied by rapid flow modifications, including considerable low-level jet (LLJ) intensification. The third shortwave moved into the lee of the Rockies early on 27 April to yield destabilization behind QLCS1 and support the formation of QLCS2, which was followed by further LLJ intensification and helped to establish favorable deep-layer shear profiles over the warm sector. The afternoon supercell outbreak commenced following the movement of this shortwave into the Mississippi Valley, which was attended by a deep tropopause fold, cold front aloft, and dryline that promoted two prominent bands of tornadic supercells over the Southeast.

Abstract

One of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that culminated with the development of numerous long-track, violent tornadoes over the southeastern United States during the afternoon of 27 April. This notorious afternoon supercell outbreak was preceded by two quasi-linear convective systems (hereinafter QLCS1 and QLCS2), the first of which was an anomalously severe nocturnal system that rapidly grew upscale during the previous evening. Here in Part II, we use a series of RUC 1-h forecasts and output from convection-permitting WRF-ARW simulations configured both with and without latent heat release to investigate how environmental modifications and upscale feedbacks produced by the two QLCSs contributed to the evolution and exceptional severity of this multiepisode outbreak. QLCS1 was primarily responsible for amplifying the large-scale flow pattern, inducing two upper-level jet streaks, and promoting secondary surface cyclogenesis downstream from the primary baroclinic system. Upper-level divergence markedly increased after QLCS1 developed, which yielded strong isallobaric forcing that rapidly strengthened the low-level jet (LLJ) and vertical wind shear over the warm sector and contributed to the system’s upscale growth and notable severity. Moreover, QLCS2 modified the mesoscale environment prior to the supercell outbreak by promoting the downstream formation of a pronounced upper-level jet streak, altering the midlevel jet structure, and furthering the development of a highly ageostrophic LLJ over the Southeast. Collectively, the flow modifications produced by both QLCSs contributed to the notably favorable shear profiles present during the afternoon supercell outbreak.

Abstract

An automated near-real-time system for the surface analysis of gravity waves and other mesoscale phenomena is developed, tested, and applied to several cases. Five-minute observations from the Automated Surface Observing System (ASOS) network provide the primary source of data for the mesoanalysis system. ASOS time series data are downloaded, subjected to considerable quality control, bandpass filtered, and objectively analyzed using a time-to-space conversion (TSC) adaptation of the traditional Barnes scheme. The resultant analyses, which can resolve features in the ASOS network with wavelengths as short as 150 km and at 15-min intervals, are made available as animated contoured fields.

Even though this mesoanalysis system was designed primarily for gravity wave detection, it is capable of resolving other kinds of mesoscale phenomena and allowing the analyst to monitor their changing structure. The effectiveness of the system is demonstrated with two recent events selected from several cases that have been analyzed. The first case consisted of a gravity wave train that propagated through the Ohio River valley and produced multiple precipitation bands. The second event involved a complex family of mesohighs and wake lows associated with a convective system over the southeastern United States. Variations in the surface wind field and precipitation distribution are related to the mesoscale pressure field in both cases.

The ability of this mesoanalysis system to monitor mesoscale phenomena resides in the successful application of TSC principles to high temporal resolution surface data. Although the TSC assumption may not be strictly valid in more complex situations, for many applications this mesoanalysis system offers critical information needed for making accurate nowcasts, with the caveat that the means by which ASOS 5-min data are made available can be improved.

Abstract

This paper presents the results of a very detailed investigation into the effects of preexisting gravity waves upon convective systems, as well as the feedback effects of convection of varying intensity upon the waves. The analysis is based on the synthesis of synoptic surface and barograph data with high-resolution surface mesonetwork, radar, and satellite data collected during a gravity wave event described by Koch and Golus in Part I of this series of papers. Use is also made of the synoptic barograph data and satellite imagery to trace the waves beyond the mesonetwork and thus determine their apparent source region just upstream of the mesonetwork.

It is shown that two of the gravity waves modulated convection within a weak squall line as they propagated across the line. The other six waves remained closely linked with convective systems that they appeared to trigger. However, it is shown that the waves were not excited by convection. Furthermore, the waves retained their signatures in the surface mesonetwork fields in the presence of rainshowers. Two episodes of strongest gravity wave activity are identified, each of which consisted of a packet of four wave troughs and ridges displaying wavelengths of ∼150 km. A Mesoscale Convective Complex (MCC) forms rapidly from very strong or severe thunderstorms apparently triggered by the individual members of the second wave packet. It is suggested that the large size and long duration of this complex were due in part to the periodic renewal and organization provided by this wave packet.

Strong convection appears to substantially affect the gravity waves locally by augmenting the wave amplitude, reducing its wavelength, distorting the wave shape, altering the wave phase velocity, and greatly weakening the in-phase covariance between the perturbation wind and pressure (p′u*′) fields. These convective effects upon the gravity waves are explained in terms of hydrostatic and nonhydrostatic pressure forces and gust front processes associated with thunderstorms. Despite the implication from these findings of the loss or obscuration of the original wave signal, the gravity wave signal remained intact just outside of the active storm cores and the entire wave-storm system exhibited outstanding spatial coherence over hundreds of kilometers.

The observations are also compared to the predictions from wave-CISK theory. Although qualitative agreement is found, quantitative comparisons give rather unimpressive agreement, due in large measure to simplifications inherent to the theory.