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Steven E. Koch and Paul B. Dorian

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.

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Steven E. Koch, Robert E. Golus, and Paul B. Dorian

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 (pu*′) 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.

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Paul B. Dorian, Steven E. Koch, and William C. Skillman

Abstract

A study of three years of GOES satellite imagery has been conducted to determine whether synthesis of the imagery with surface diagnostic analyses may prove useful for predicting the precise location and time of formation of squall lines generated by a particular type of frontal circulation transverse to surface cold fronts. Existence of this circulation is inferred from the development of a thin Line of shallow Convection clouds (LC) along the front simultaneously with that of a mesoscale (<100 km wide) Clear Zone (CZ) immediately behind the front and at the leading edge of a large area of stratus clouds. The observations suggest that a thermally direct circulation transverse to the surface cold front generated the line convection and clear zone (in the upward and downward branches of the circulation, respectively) in all 15 cases which met the strict criteria for an LC/CZ.

Squall lines were observed to form from the LC in 10 of the 15 cases examined, and nearly always within 90 min following the time when the CZ reached its maximum width. In addition, initial cumulonimbus development always occurred within 100 km of the diagnosed frontogenesis center at the LC. Therefore, this study suggests that both the timing and location of such squall lines should be predictable with very high accuracy. It is also shown that thermodynamic instability was insufficient for the formation of deep convection in the five non-thunderstorm cases.

Our results also strongly support the hypothesis of Koch (1984) that this mesoscale circulation was generated by differential sensible heating acting in conjunction with geostrophic deformation effects. The contrast of cloudy skies behind the front (prior to CZ formation) with nearly clear skies ahead of the front is largely responsible for creation of the differential heating pattern. This suggests that forecasters should watch for such cloud patterns near cold fronts.

Synoptic climatological conditions favoring the occurrence of this relatively rare phenomenon are also identified. The LC/CZ appears during the afternoon almost solely over the Great Plains states during spring and autumn. The line convection was found in all but one case to be parallel to, and either along or on the cyclonic side of, a prefrontal 850 mb jet. Although the LC/CZ is usually found on the anticyclonic side of upper-level jet streaks, it does not seem to prefer any particular jet quadrant. Diagnosis of the Sawyer-Eliassen equation for one case suggested that the mesoscale circulation was linked to a thermally direct circulation cell associated with the upper-level frontal zone.

The information provided in this paper should be valuable to the operational forecaster concerned with having some guidance about specific mesoscale trigger mechanisms for squall lines. This phenomenon can be isolated with conventional surface and satellite data in real time to provide accurate and timely forecasts of the formation of squall line activity.

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Steven E. Koch, Paul B. Dorian, R. Ferrare, S. H. Melfi, William C. Skillman, and D. Whiteman

Abstract

Detailed moisture observations from a ground-based Raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented. A synthesis of the lidar data with conventional meteorological data, in conjunction with theoretical calculations and comparison to laboratory studies, leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore. The disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site. Bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current. Analysis suggests that the bore was generated by the collapse of the gravity current into a stable, nocturnal inversion layer, and subsequently propagated along this wave guide at nearly twice the speed of the gravity current.

The Raman lidar provided detailed measurements of the vertical structure of the bore and its parent generation mechanism. A mean bore depth of 1.9 km is revealed by the lidar, whereas a depth of 2.2 km is predicted from hydraulic theory. Observed and calculated bore speeds were also found to agree reasonably well with one another (∼ ±20%). Comparison of these observations with those of internal bores generated by thunderstorms in other studies reveals that this bore was exceedingly strong, being responsible for nearly tripling the height of a surface-based inversion that had existed ahead of the bore and dramatically increasing the depth of the moist layer due to strong vertical mixing. Subsequent appearance of the relatively shallow gravity current underneath this mixed region resulted in the occurrence of an elevated mixed layer, as confirmed with the special radiosonde measurements.

A synthesis of the lidar and radiosonde observations indicates that bore-induced parcel displacements attenuated rapidly at the same height as the level of strongest wave trapping predicted from the theory of Crook. This trapping mechanism, which is due to the existence of a low-level jet, results in a long-lived bore, and seems to he a common phenomenon in the environment of thunderstorm-generated bores and solitary waves. Despite the weakening of a capping inversion by this strong and persistent bore, analysis indicates that the 30-min averaged lifting of 0.7 m s−1 was confined to a too shallow layer near the surface to trigger deep convection, and could only produce scattered low clouds as deduced from the lidar measurements.

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Steven E. Koch, F. Einaudi, Paul B. Dorian, Stephen Lang, and Gerald M. Heymsfield

Abstract

This paper summarizes the results of a detailed study from the Cooperative Convective Precipitation Experiment (CCOPE) of the vertical structure of mesoscale gravity waves that disturbed a sizable part of the troposphere and that played a significant role in the generation of a mesoscale convective complex. These bimodal waves displayed periods of 148 (50) min, wavelengths of 135 (60) km, and phase speeds of 15.2 (19.8) m s−1. A comparison is made between wave-induced pressure perturbation fields derived from triple-Doppler wind fields within regions of essentially nonconvective precipitation, pressure perturbation fields obtained by bandpass filtering of surface mesonetwork data, and the vertical structure of the pressure eigenfunctions as predicted from a linear stability analysis. It is believed that this represents the first such application of the Doppler radar pressure retrieval technique to the study of gravity waves. In addition, an analysis of the potential for shear instability was performed on all of the special CCOPE soundings taken on this day to determine the representativeness of the chosen soundings for the theoretical analysis and the likelihood that a wave maintenance mechanism endured throughout the 33-h wave event.

The analysis of the potential for shear instability and the eigenfunctions both indicate that the bimodal waves were able to efficiently extract energy from the mean flow near several closely spaced critical levels in the 4.0– 6.5-km layer to maintain their coherence for many wave cycles. This result serves as the explanation for the observed ability of the waves to organize precipitation into long convective bands whose axes were along and just ahead of the wave crests. The eigenvalue analysis predicts unstable modes that are hydrostatic, nondispersive, ducted gravity waves characterized by half of a vertical wavelength contained between the ground and the lowest critical level (at z = 4 km). Eigenfunctions of pressure and other variables all display negligible tilt below 2.3–3.3 km, above which a sudden reversal in phase occurs.

The vertical structure of the Doppler-derived fields associated with one of these gravity waves agrees in terms of the following respects with the eigenfunction predictions and/or the surface mesoanalyses: (a) the vertical wavelength, horizontal structure, and amplitude of the perturbation horizontal wind and pressure fields, and (b) the in-phase covariance between the pressure and horizontal wind fields at levels below 2.5 km. On the other hand, the theory predicted a much more abrupt vertical transition in phase in the pressure fields and weaker amplitudes aloft than were evident in the Doppler analyses. In addition, the size of the multiple-Doppler analysis domain was too small to capture an entire horizontal wavelength of the 135-km-scale gravity wave, which made direct comparisons difficult. Furthermore, the linear theory predicts much smaller amplitudes and somewhat longer horizontal wavelengths for the vertical motions characterizing both wave modes than those seen in the Doppler winds, which likely also contain nonwave effects. These discrepancies are largely due to the combined effects of weak convection, turbulence, and data sampling problems. Despite these drawbacks, the findings from this and other recent studies using Doppler radars and ground-based radiometers suggest that remote sensing of mesoscale gravity waves that occupy a significant fraction of the troposphere should be exploited further.

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