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

No abstract available.

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

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

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

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

For the first time, an analysis has been made of the evolving vertical structure of a long-lived mesoscale gravity wave that exerted a strong influence upon the precipitation distribution across a large area. This paper describes this gravity wave system on 14 February 1992, which was observed using a combination of a surface mesonetwork, digital satellite and radar imagery, and several Doppler wind profilers. The resulting vertical structures are compared to the predictions of linear stability theory.

Since the signature of the gravity waves in the profiler vertical beam data was often complicated by the presence of precipitation, a kinematic method was developed for estimating the vertical air motions during these periods. The resultant time–height fields show vertical and horizontal winds that are consistent with a gravity wave conceptual model, the microbarograph traces, and the cloud and precipitation patterns. In the early stages of development, a strong vertically erect wave of depression was observed in southwestern Kansas. A few hours later, in central Kansas, a distinct discontinuity had developed at the 4-km level. This phase shift and the vertical motion profiles are both shown to be consistent with linear theory, as well as the notion that the critical level at 5.4 km acted as a nodal surface for a complex ducted wave mode.

Precipitation patterns were strongly affected by the waves. According to the profiler analysis, the sharp back edge to the associated rainband was provided by strong low-level subsidence ahead of the wave of depression. The waves and precipitation strengthened in a synergistic fashion—as strong convection developed along the wave, the wave of depression evolved into a wave train in which the leading wave crest eventually dominated over the initial wave of depression. The profiler results reveal the existence of the incipient wave (and other waves) at midlevels several hours before the surface mesonet stations detected the presence of the waves. Thus, an important and unexpected finding from the profiler analysis is that surface microbarograph detection of mesoscale gravity waves may be limited to those waves that primarily affect the lower troposphere.

Abstract

A mesoscale numerical model and detailed observations are used to investigate the generation and maintenance of a mesoscale gravity wave event observed in eastern Montana on 11 July 1981 during the Cooperative Convective Precipitation Experiment (CCOPE). It is shown that the interaction between an orographic density current and a mountain barrier leads to the generation of the gravity waves.

The simulation results suggest the following four-stage conceptual model. During stage I, shortly after sunset, the remnant up-branch of a thermally driven upslope flow east of the Rockies was driven back toward the mountain by the pressure gradient force associated with a cool pool over North Dakota. The nocturnal stable layer over eastern Montana was strengthened during passage of this density current. During the 1–2-h transition period of stage II, the advancing density current became blocked as it encountered the higher terrain. An isentropic ridge developed above the original warm lee trough due to strong adiabatic cooling caused by the sustained upward motion in the presence of orographic blocking. During stage III, an even stronger upward motion center formed to the east of the density current head updraft in response to an eastward horizontal pressure gradient force produced by the isentropic ridge. In stage IV, as the density current head collapsed and downward motion developed to the west of the original updraft in quadrature phase with the isentropic perturbation, a gravity wave was generated. This wave propagated eastward with the mean wind (opposite to the motion of the earlier density current) and was maintained by the strong wave duct established earlier by the density current. Thus, the mountain–plains circulation may at times generate mesoscale gravity waves (and deep convection) hours after diurnal heating has ended.