Abstract

No abstract available.

Abstract

A line of severe thunderstorms is observed in Satellite imagery to develop explosively from a narrow line of shallow convection at the most rapidly intensifying part of a surface cold front. Concurrent evaporation of the leading edge of a large area of stratus and stratocumulus clouds behind the front results in the appearance of a mesoscale clear zone adjoining the line convection feature. The clear zone enlarges to its maximum width of 65 km less than an hour prior to the genesis of the frontal squall line.

These observations suggest the possibility that a transverse circulation about the front generated the line convection and clear zone (in the upward and downward branches of the circulation, respectively), and ultimately the squall line. Analysis of the synoptic surface data indicates the likely presence of a thermally direct frontogenetic circulation at the leading edge of the clear zone. The implied frontogenetic process exhibits a rapid e-folding time of ∼3 h, corresponding to the development time of the clear zone.

The transverse circulation implied by the observations cannot be explained solely on the basis of geostrophic deformation acting upon the cross-frontal horizontal temperature gradient field, since the observed circulation is characterized by spatial and temporal scales much smaller than those predicted by semigeostrophic theory. The observed scales can be explained by considering a superposition of the cross-frontal variation in surface sensible heat flux upon the deformation field. The resulting transverse circulation is shown to be capable of producing vertical motions strong enough to generate the clear zone and squall line. The possible relevance of other mesoscale processes as explanations for these satellite-observed features is also examined.

Abstract

This paper presents the mesoscale part of a two-part evaluation of thirty forecasts produced by a mesoscalenumerical weather prediction model (MASS 2.0). The general approach taken to evaluate the mesoscale predictivecapabilities of the model is to utilize observed patterns of convection as “verification” of the unfiltered forecastfields. More specifically, these fields are combined into convective predictor fields, the loci of which are thenrelated at two hourly intervals to the loci of strong mesoscale convective systems (MCSs) identifiable in nationalradar summary plots and GOES satellite imagery. Results show that the genesis of 48% of the 149 observed MCSs could be accurately (−+3 h/250 kin) relatedto coherent predictor fields with a very low false alarm rate of 13%. Convection “underforecasts” (or “misses”)were related in 67% of the instances to systematic forecast errors at the synoptic scale, many of which arediscussed in detail in Part I. This suggests that a necessary, but insufficient, condition for accurate forecasts ofmesoscale phenomena is accurate initialization and temporal integration of the larger-scale circulation patterns. Four cases are selected from the sample as demonstrations of the degree of coherent, detailed informationprevalent in the model forecasts of vertical motion and convective instability fields in a variety of convectivesituations. Examples of model “forecasts” of intense convective storm clusters, a severe squall line triggeredalong a dryline, orographically induced hailstorms, and sea breeze thunderstorms are provided. It is concludedthat the model can be used to gain insight into mesoscale convective processes in situations where synopticscale forecast errors have minimal impact.

Abstract

A structured methodology for detecting the presence of split cold fronts in an operational forecast environment is developed and applied to a case in which a split front passed over a region of cold air damming in the southeastern United States. A real-time mesoscale model and various products from the WSR-88D—including the velocity–azimuth display wind profile (VWP) and hodograph products, plus a thermal advection retrieval scheme applied to the VWP data—are used to study this split front and an associated convective rainband that occurred on 19 December 1995.

Wet-bulb temperature and vertical motion forecasts at 700 hPa from the model revealed the arc-shaped split front 300–500 km ahead of the surface cold front. As this midtropospheric front passed across the surface warm front and entered the cold air damming region, model vertical cross-section analyses showed that it created a deep elevated layer of potential instability. Furthermore, an ageostrophic transverse circulation associated with the split front provided the lifting mechanism for releasing this instability as deep convection. Analysis of the absolute geostrophic momentum field provided greater understanding of the structure of the split front and a deep tropospheric frontal system to its west that connected with the surface cold front.

An “S–inverted S” pattern in the zero isodop on WSR-88D radial velocity displays indicative of wind backing above wind veering suggested the presence of the split front in the observations (as did the hodographs). Detection of the passage of the split front could be discerned from temporal changes in the vertical profile of the winds, namely by the appearance of midlevel backing of the winds in VWP time–height displays. Because of the subtlety of this backing and the need to be more quantitative, a temperature advection retrieval scheme using VWP data was developed. The complex evolving structure of the split front was revealed with this technique. Results from this retrieval method were judged to be meteorologically meaningful, to exhibit excellent time–space continuity, and to compare reasonably well with the frontal structures evident in the mesoscale model forecasts. The thermal advection scheme can easily be made to function in operations, as long as there is real-time access to level II radar data.

Abstract

This paper is the first in a series of three papers concerning a gravity wave event that occurred over the north-central United States on 11–12 July 1981. The event is analyzed with superb detail resulting from the availability of digitized radar, surface mesonetwork, and other special data from the Cooperative Convective Precipitation Experiment (CCOPE) in Montana. The subject matter of this paper consists of 1) a statistical determination of the wave characteristics, 2) a demonstration that the observed phenomena display a nature consistent with that of gravity waves, and 3) a discussion of the principles and limitations of statistical methods for detecting and tracking mesoscale gravity waves.

Two distinct wave episodes of ∼8 h duration within a longer (33 h) period of wave activity are studied in detail. Both episodes contain strongly coherent, bimodal wave activity. The primary (secondary) wave mode isolated from autospectral and perturbation map analyses displays mean periods of 2.5 (0.9) h and mean horizontal wavelengths of 160 (70) km. The horizontal phase velocities are essentially identical for the two wave modes. Cross-spectral analyses confirm the impression that the wavefronts are not truly planar, but rather are arc- or comma-shaped in appearance.

Perturbation pressure (p′) and wave-normal wind (u*′) are found to be in phase with one another. The importance of this finding is that it strongly supports the interpretation of the wave signals as gravity waves, a conclusion that rests upon the availability of the mesonet wind data. The observation that rainbands were positioned immediately ahead of the wave crests in those situations where the waves did not propagate through the rainbands also agrees with gravity wave theory. Consistency checks between the observed values of p′, u*′, and the wave phase velocity are made using the impedance relationship to further substantiate the gravity wave interpretation of these data. The certainty of these interrelationships between the pressure, wind, and precipitation fields is the direct consequence of statistically analyzing data with unprecedented detail compared to previous case studies of mesoscale gravity waves.

Abstract

In an effort to better understand mesoscale gravity waves in winter storms in the central United States—their frequency of occurrence, wave characteristics, the general conditions under which they occur, and their effects upon the weather—mesoscale surface and rawinsonde data as well as radar and satellite imagery collected during the Storm-scale Operational and Research Meteorology–Fronts and Experimental System Test are analyzed. In addition, factors affecting the ability of objective surface map analysis to properly represent the waves are investigated.

Thirteen coherent pressure pulse events with amplitudes of 0.2–4.0 mb and periods of 1–6 h were identified in the surface pressure data during the 6 weeks of the project, involving 34% of the total hours investigated. A variety of wave types occurred, including wavelets, wave trains, and singular waves. The three largest amplitude events were analyzed in detail using autospectral analysis and a Barnes time-to-space conversion objective analysis of bandpass-filtered mesonet data. All three events displayed high perturbation pressure–wind covariances ( p′u*′ ), consistent with a gravity wave explanation for the disturbances (u* is the wind component in the direction of wave propagation). The p′u*′ values were closely related to the strength of the wave amplitudes. The waves found in these events displayed mean phase velocities of 19.9–27.9 m s⁻¹, wavelengths of 200–260 km, and periods of 2.3–3.5 h.

Wave crests appeared to be closely aligned with associated rainbands throughout their lifetimes, suggesting that a codependency existed. Some of the waves were evident before the rainbands formed, indicating that the precipitation developed in response to the waves, though this was not true for all of the waves. Values of p′u*′ decreased during the development stage of deep convection, but high covariance between the pressure and wind fields redeveloped as the thunderstorms and incipient gravity wave matured into a stable, coupled mesoscale convective system.

Three of the four wave events displaying the largest amplitudes occurred primarily on the cool side of a stationary front in an environment in which a jet streak was approaching an inflection axis in a diffluent height field downstream from an upper-level trough. The waves also extended some distance into the warm sector in the presence of a statically stable lower troposphere, suggesting wave ducting was operative. The results indicate that this conceptual model for the wave environment should prove useful as a tool for forecasting the most significant mesoscale gravity wave events.

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

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.