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An undular mesospheric bore event has been recorded over the western Himalayan region in O(1S) 557.7 nm airglow images on a clear and moonless night of 02 October 2018 using a multi-wavelength all-sky imager at Hanle, Leh Ladakh, India (32.77°N, 78.97°E). The bore has a prominent leading dark front followed by trailing waves and it propagates with a mean observed phase velocity of ~31 ± 5 m/s. It also shows a small-scale undulation and clockwise rotation in its phase front. In order to understand the evolution of the bore, vertical temperature profiles from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard TIMED satellite and HWM14 wind maps are used. SABER temperature shows a mesospheric inversion layer prior to the occurrence of the bore event which acted as a thermal duct layer to guide the propagation of the bore. The analyses also suggest that inhomogeneity in the duct depth at different parts of the bore’s horizontal extension could lead the small-scale undulations in the bore phase fronts. The same can also be responsible for the rotation of the bore. Additionally, the peak separation of the bore’s trailing waves suggests that large-scale gravity wave interaction with pre-existing thermal duct could be the potential source for the generation of the undular bore at mesospheric height. Furthermore, the present results indicate that neutral instabilities and weakening duct layer in the path of the bore propagation might have accelerated the faster dissipation of the bore’s energy and consequently suppress its long-distance horizontal propagation.

Over the last several decades, various analytical, numerical and experimental studies have reported that leading-depression N-type tsunamis could be more destructive than leading-elevation N-type tsunamis. However, in many analytical or experimental studies, the effects of shelf geometry and wave breaking under the geophysical scale, which are known to significantly affect tsunami evolution, have been ignored. Therefore, it is still unclear whether leading-depression tsunamis always result in more serious damage than leading-elevation tsunamis if earthquakes with the same magnitude occur at the same epicentre. Thus, this study investigated the runup and wave evolution characteristics of idealized leading-depression and leading-elevation tsunamis, particularly considering the effects of shelf geometry and wave breaking under the geophysical scale. Using a Boussinesq-type model for fully nonlinear, weakly dispersive, rotational and turbulent flow, the evolution of tsunami was simulated, and the energy and momentum of the tsunami were calculated. As a result, on steeply sloped bathymetry, the large momentum and potential energy induced during the rundown process of the leading-depression tsunami results in runup that was higher than that of the leading-elevation tsunami, which was consistent with the results of previous studies. Interestingly, the leading-depression tsunami resulted in a lower runup height on mild and long sloped bathymetry, in contrast to previous studies; this process originated from the energy dissipation caused by the wave breaking of the undular bore and the mass exchange occurring between the positive and negative waves.

This study documents the evolution of an impressive, largely undular bore triggered by an MCS-generated density current on 20 June 2015, observed as part of the Plains Elevated Convection at Night (PECAN) experiment. The University of Wyoming King Air with profiling nadir- and zenith-viewing lidars sampled the south-bound bore from the time the first bore wave emerged from the nocturnal convective cold pool and where updrafts over 10 m s−1 and turbulence in the wave’s wake were encountered, through the early dissipative stage in which the leading wave began to lose amplitude and speed. Through most of the bore’s life cycle, its second wave had a higher or equal amplitude relative to the leading wave. Striking roll clouds formed in wave crests and wave energy was detected to about 5 km AGL. The upstream environment indicates a negative Scorer parameter region due to flow reversal at midlevels, providing a wave trapping mechanism. The observed bore strength of 2.4–2.9 and speed of 15–16 m s−1 agree well with values predicted from hydraulic theory. Surface and profiling measurements collected later in the bore’s life cycle, just after sunrise, indicate a transition to a soliton.

The paper reports unique high-resolution observations of meteotsunami by a large array of oceanographic instruments deployed on the Atchafalaya Shelf (Louisiana, USA) in 2008 with the primary aim to study wave dissipation in muddy environments. The meteotsunami event on March 7, 2008, was caused by the passage of a cold front which was monitored by the NOAA NEXRAD radar. The observations of water surface elevations on the shelf show a highly detailed textbook picture of an undular bore (solibore) in the process of its disintegration into a train of solitons. The picture has a striking feature never reported before not only for the meteotsunamis but in other contexts of disintegration of a long-wave perturbation into a sequence of solitons as well—the persistent presence of a single soliton, well ahead of the solibore. Data analysis and simulations based on the celebrated variable-coefficient KdV (vKdV) equation first proposed by Ostrovsky and Pelinovsky (Izv Atmos Ocean Phys 11:37–41, 1975) explain the physics of this phenomenon and suggest that the formation of the lone soliton ahead of the solibore is very likely to be the result of the specific interplay of natural meteotsunami forcing and nearshore bathymetry. The analysis strongly suggests that the patterns of coexisting lone solitons and packets of cnoidal waves should be quite common for meteotsunamis. They were not observed before only because of the scarcity of high-resolution observations. The results highlight the effectiveness of the vKdV equation in providing understanding of the fundamental mechanisms of the complex natural phenomenon that would otherwise require computationally very expensive numerical models.

This study explores the mesoscale processes that led to the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Observations and model simulations revealed that when the prefrontal trough entered southeast Australia it resembled a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed in the Doppler wind field of a radar, parts of which were collocated with bands of reflectivity. Strong winds coincident with this band of reflectivity suggest the undular bore triggered convection that eventually led to the bore’s demise. An ensemble of high-resolution model simulations (with perturbed initial and boundary conditions) was used to understand the key processes affecting the undular bore and two prefrontal precipitation events. While no member of the ensemble reproduced the first prefrontal precipitation event, at least six members (20%) reproduced parts of the second prefrontal precipitation event. Despite the low precipitation predictability, analysis of the ensemble suggests the undular bore was both a predictable phenomenon and integral to the initiation and/or evolution of the two prefrontal precipitation events.

This study addresses the impact of nonlinear wave evolution on the resulting wave force values on a vertical wall. To this end, the problem of interaction between non-breaking water waves and a vertical wall over constant depth is investigated. The investigation is performed using a two-dimensional wave flume model which is based on the high-order spectral method. Wave generation is simulated at the flume entrance by means of the additional potential concept. This model enables to preserve full dispersivity. Therefore, the model enables to examine the role of nonlinear evolution in the formation of extreme wave force values on a vertical wall for a wide range of water depths. The results for the force exerted on a vertical wall are presented for shallow and deep water conditions. In shallow water, extreme wave force values occur due to the formation of an undular bore. In deep water, extreme wave forces have been obtained as a result of disintegration process of incident wave groups into envelope solitons. Multiple maximum force values have been detected for each of the highest run-up peaks. This phenomenon has been introduced in shallow water conditions and is extended here for deep water conditions.

Mesospheric bores have been observed and measured in the mesopause region near 100-km altitude, where they propagate horizontally along a duct of relatively strong density stratification. Here, a weakly nonlinear theory is developed for the description of these mesospheric bores. It extends previous theories by allowing internal gravity wave radiation from the duct into the surrounding stratified regions, which are formally assumed to be weakly stratified. The radiation away from the duct is expected to be important for bore energetics. The theory is compared with a numerical simulation of the full Navier–Stokes equations in the Boussinesq approximation. Two initial conditions are considered. The first is a solitary wave solution that would propagate without change of form if the region outside the duct were unstratified. The second is a sinusoid that evolves into an undular bore. The main conclusion is that, while solitary waves and undular bores decay by radiation from the duct, they can survive as significant structures over sufficiently long periods (~100 min) to be observable.

The paper presented a smoothed particle hydrodynamics (SPH) method to study the three-dimensional (3D) tidal bore scenarios. The SPH method is a mesh-free particle modeling technique that can track the large deformation of free surfaces in a straightforward and accurate way. Two benchmark cases of the tidal bore propagation were computed and compared with the experimental results. The first one is related to the undular and breaking bores in a regular open channel, and the second one considers the undular bore passing through the contraction of bridge piers. Physical laboratory experiments have also been carried out to validate the numerical investigations. The comparisons of both the free surface profile and velocity field demonstrated that the SPH technique could provide a very promising tool to simulate tidal bore phenomena in engineering practice. The work is the first to systematically explore the potentials of mesh-free SPH modeling approach in predicting the tidal bore features under 3D flow conditions.

A mesospheric bore event was observed using an OH all-sky airglow imager (ASAI) at Xinglong (40.2° N, 117.4° E), in northern China, on the night of 8–9 January 2011. Simultaneous observations by a Doppler meteor radar, a broadband sodium lidar, and TIMED/SABER OH intensity and temperature measurements are used to investigate the characteristics and environment of the bore propagation and the possible relations with the Na density perturbations. The bore propagated from northeast to southwest and divided the sky into bright and dark halves. The calculations show that the bore has an average phase velocity of 68 m s−1. The crests following the bore have a horizontal wavelength of ~ 22 km. These parameters are consistent with the hydraulic jump theory proposed by Dewan and Picard, as well as the previous bore reports. Simultaneous wind measurements from the Doppler meteor radar at Shisanling (40.3° N, 116.2° E) and temperature data from SABER on board the TIMED satellite are used to characterize the propagating environment of the bore. The result shows that a thermal-Doppler duct exists near the OH layer that supports the horizontal propagation of the bore. Simultaneous Na lidar observations at Yanqing (40.4° N, 116.0° E) suggest that there is a downward displacement of Na density during the passage of the mesospheric bore event.

The complexity of bora flow in the lee of Southern Velebit, Croatia, is investigated by means of high-resolution numerical simulations carried out with the US Naval Research Laboratory's (NRL) Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. The aim of this study is to identify reasons for the strong spatial variability of bora in the wider Zadar region and the uncharacteristically weak bora in the city of Zadar. The primary focus is put on the severe bora episode of 20 December 2004 during which a sodar (sonic detection and ranging) system was operated at Zemunik airport. Numerical results are verified against the available surface and upper-air observations. Upstream conditions governing dynamics of this particular bora event are characterized by the presence of a mean-state critical level at 3–4 km above mean sea-level (AMSL) defining the upstream bora layer. Low-level airflow over Zadar peninsula during this event displays marked temporal variability. During the morning, a highly three-dimensional and unsteady wake (i.e. region of reduced wind speed) is established over Zadar peninsula, downstream of a hydraulic jump that forms in the lee of, but not collocated with, highest portions of the Southern Velebit terrain. During its most extensive phase the wake scales the entire length of Zadar peninsula in the horizontal and extends up to the critical level in the vertical. A wake vortex develops as the flow reverses along the wake centreline. In the evening hours, an undular bore forms in the same location, with large-amplitude waves on top of it inducing boundary layer separation and rotors. Wake flow is affected both by changes in the upstream atmospheric structure and diurnal boundary-layer evolution. Sensitivity experiments show that the influence of Velebit's height is particularly strong, governing the onset and strength of bora. The terrain of Zadar peninsula exerts a strong influence on the characteristics of the developed bora flow, particularly when non-hydrostatic effects are significant. Boundary layer separation is particularly sensitive to downstream orography. Copyright © 2012 Royal Meteorological Society

This study uses data from a microwave profiling radiometer (MPR), along with 915-MHz wind profiler, Doppler radar, and surface data to quantify the kinematic and thermodynamic effects of two wave features, an undular bore and a soliton, on the nocturnal boundary layer (NBL) at high temporal resolution. Both wave features passed directly over the MPR and the wind profiler, allowing for detailed analyses. The effects of the wave features on the convective environment are examined, and convective initiation (CI) associated with the wave features is discussed. The undular bore was illustrated well in Doppler velocity data, and profiler measurements indicated that it produced four wavelengths of upward and downward motion. MPR-derived time–height sections of potential temperature and mixing ratio showed an increase in the depth of the stable boundary layer, along with a decrease in stability, partially associated with mixing of the NBL. The soliton produced a temporary decrease in the depth of the NBL, and also produced destabilization. Trajectory analyses were performed assuming the wave features were two-dimensional, allowing a time-to-space conversion of profiler data. Trajectory analyses, in addition to propagation speed, confirm that the wave features were indeed a bore and a soliton, and that there was vertical divergence in the NBL, likely associated with the decrease in static stability. MPR data were also used to produce time series of convective parameters, including CAPE, convective inhibition (CIN), and the level of free convection (LFC). The CIN was initially too large for free convection despite sufficient CAPE, but MPR data showed that the CIN decreased by more than 50% upon passage of the bore, and again with the soliton. The waves also decreased the LFC due to cooling above the NBL and slight warming near the surface in the bore. Both the reduction in CIN and the lowering of the LFC made convection more likely. Convective initiation occurred behind both wave features, and the vertical motion provided by the waves may have also aided in this CI.

The evolution of an undular bore and its associated wind shift, spawned by the passage of a shallow surface cold front over the Southern Great Plains of the United States, is examined using surface and remote sensing observations along with output from a high-resolution numerical model simulation. Observations show that a separation between the wind shift and thermodynamic properties of the front was induced by the formation of a bore over south-central Kansas around 0200 UTC 29 November 2006. By the time the front–bore complex passed through Lamont, Oklahoma, approximately 4 h later, the bore had reached its maximum intensity and its associated wind shift preceded the trailing baroclinic zone by 20 min. Within several hours the bore decayed and a cold frontal passage, characterized by a wind shift coincident with thermodynamic properties was observed at Okmulgee, Oklahoma. Thus, a substantial transformation in both the structural and dynamical characteristics of the bore as well as its relationship to the parent surface front occurred during a short period of time. The details of this evolution are examined using output from a finescale numerical simulation, performed using the Weather Research and Forecasting (WRF) model. Analysis of the output reveals that as the bore advanced southeastward it moved into a region with a weaker surface stable layer. Consequently, the wave duct that had supported its maintenance steadily weakened resulting in dissipation of the bore. This circumstance led to a merger of the surface temperature and moisture boundaries with the orphaned wind shift, resulting in the cold frontal passage observed at Okmulgee.

A visually impressive undular bore moved across much of Iowa on 2 October 2007, and video animations were captured by numerous Webcams. The bore was sampled very well by Doppler radar at close range, and also by the high-density mesoscale network of surface stations in place over Iowa and 1-min Automated Surface Observing System (ASOS) surface data at Des Moines, Iowa. Radar and surface observations are presented, along with a brief analysis of the structure of the bore.

Observations from the 2004 North American Monsoon Experiment (NAME) have been used to identify an atmospheric internal bore that occurred over the Gulf of California (GoC) on 31 July 2004. This bore disturbance was identified at Bahia Kino along the northwest coast of Mexico during the late evening of 31 July. It was hypothesized to have originated from the interaction of a gravity current from a large mesoscale convective system (MCS), which formed along the western slopes of the Sierra Madre Occidental during the afternoon, and a surface stable layer that developed from a sea-breeze circulation. It is suggested that the bore’s energy was trapped at low levels by an elevated stable layer. The vertical structure and undular nature of the bore was initially identified from 915-MHz wind profiler data at Bahia Kino. Results show a series of waves along the bore’s leading edge and turbulent mixing of air from above the stable layer to the surface on the downstream face of the leading undulation. The speed of the bore calculated from satellite imagery and surface observations (approximately 16.8 m s−1) compared favorably with the speed of a bore from hydraulic theory when a reliable estimate of the bore depth was used. A real-data simulation of the event was performed using the Weather Research and Forecasting model (WRF). Results show the model captured both the formation mechanism and structure of the bore, but it was produced too far south compared to observations, as the MCS also developed too far south. Model results indicated that while evidence of a trapping mechanism due to the stability of the atmosphere was present in the simulation, the conditions for trapping were modified by the passage of the bore allowing vertical propagation of wave energy. The bore led to increased moisture in the lowest levels of the atmosphere across the GoC, providing evidence of the possible importance of these features as moisture transport mechanisms in this region.

On the morning of 12 June 2002, a series of upper boundary layer (UBL) rapid drying and moistening events (RDEs and RMEs, respectively) occurred at the “Homestead” site of the International H2O Project (IHOP_2002). Over a period of 10 h, atmospheric water vapor in the UBL decreased or increased within a matter of minutes four separate times. High-temporal-resolution data of the RDEs and RMEs collected by numerous instruments deployed for this intensive observation period are presented. The results of an Advanced Regional Prediction System (ARPS) simulation of the weather conditions around the time period reproduced one of the two RDE–RME pairs with reasonably accurate amplitude and timing. Both the observational data and ARPS numerical model output indicate that the second RDE–RME pair resulted from the interaction between a dry air mass descending from the Rocky Mountains and a cold pool–internal undular bore couplet propagating over the Homestead site from a mesoscale convective complex to the north. The RDEs and RMEs, which were rarely observed during IHOP_2002, are believed to be an indirect indicator of such bores.

In a stably stratified atmosphere the propagation of small amplitude internal gravity waves is rather common but often not noticed. The appearance of a group of large amplitude, nonlinear gravity waves in the lower atmosphere is, in comparison, a special event. These waves with periods of several minutes, pressure amplitudes partly more than 1 hPa may cause periodic wind changes up to 10 m/s (speed) and more than 100 degrees (direction). They appear as a packet of a few amplitude-ordered single waves: as an internal undular bore or as an internal solitary wave group. Their occurrence in North Germany was investigated. The results of former gravity wave investigations were used for the Hamburg area. For other parts use was made of published data. A detailed analysis is made with the "Hamburg Morning Wave" of 7 May 1986. In a period of 7 years 6 clear events passed over Hamburg. Most of them happened in spring, and together with a nocturnal temperature inversion in the lower atmosphere. Coherent small amplitude gravity wave activity occurred in about 10 % of the time. Examples of recordings at an 250 m instrumented (wind, temperature) antenna mast up to 250 m are presented. Evident are the often strong changes of the wind direction during these disturbances. The horizontal wind has been decomposed into a mean back-ground wind, an additional mean wind (adm) during the passage of the group and a quasi sinusoidal periodic part. The "adm" points into the direction of the wave propagation, indicating a mean mass flux together with the waves.

The evolution of a gust front to bore to solitary wave transition, and comprehensive information on the evolving nocturnal boundary layer (NBL) associated with this change, are described with analysis of radar and profiler measurements. The observations were obtained on 21 June 2002 in the Oklahoma panhandle during the International H2O Project. The evolution of this system, from a strong bore (initiated by a vigorous gust front) to a solitary wave, was observed over a 4-h period with Doppler radar and surface measurements. Detailed information on the mature bore structure was obtained by a cluster of profiling instruments including two boundary layer wind profilers, a lidar ceilometer, and a microwave profiling radiometer. A strong bore was initiated by an extensive gust front that perturbed an incipient NBL whose development (prior to sunset) was enhanced by shading from the parent mesoscale convective system. At the time of bore formation, the NBL was about 300 m deep and exhibited a surface temperature about 4 K less than the afternoon maximum. Initially, the bore assumed kinematic properties similar to those of a gust front. As the NBL stabilized, the bore matured and exhibited undular formations over 30–60-km segments along the bore axis. A 30-km-wide cloud field accompanied the mature bore system within three hours of its formation. System-relative airflow within the cloud field was front-to-rear and exhibited a primary hydraulic jump updraft (4–5 m s−1 magnitude) within the bore core. The bore core exhibited a low, smooth cloud base, a cloud depth of 2.5 km, nearly adiabatic liquid water content, and pronounced turbulence. The maximum parcel displacements within the bore were about 2 km (sufficient for marginal convective initiation), and the net parcel displacement from before to after bore passage was 0.6–0.9 km.

Detailed observations of the interactions of a cold front and a dryline over the central United States that led to dramatic undulations in the boundary layer, including an undular bore, are investigated using high-resolution water vapor mixing ratio profiles measured by Raman lidars. The lidar-derived water vapor mixing ratio profiles revealed the complex interaction between a dryline and a cold-frontal system. An elevated, well-mixed, and deep midtropospheric layer, as well as a sharp transition (between 5- and 6-km altitude) to a drier region aloft, was observed. The moisture oscillations due to the undular bore and the mixing of the prefrontal air mass with the cold air at the frontal surface are all well depicted. The enhanced precipitable water vapor and roll clouds, the undulations associated with the bore, the strong vertical circulation and mixing that led to the increase in the depth of the low-level moist layer, and the subsequent lifting of this moist layer by the cold-frontal surface, as well as the feeder flow behind the cold front, are clearly indicated. A synthesis of the Raman lidar–measured water vapor mixing ratio profiles, satellite, radiometer, tower, and Oklahoma Mesonet data indicated that the undular bore was triggered by the approaching cold front and propagated south-southeastward. The observed and calculated bore speeds were in reasonable agreement. Wave-ducting analysis showed that favorable wave-trapping mechanisms existed; a low-level stable layer capped by an inversion, a well-mixed midtropospheric layer, and wind curvature from a low-level jet were found.

The outbreak of tornadoes from the Mississippi River to just east of the Appalachian Mountains on 2–5 April 1974 is analyzed using conventional techniques and the Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5). The MM5 was run for 48 h using the NCEP–NCAR reanalysis dataset for initial conditions. It is suggested that the first damaging squall line within the storm of 2–5 April 1974 (herein referred to as the Super Outbreak storm) was initiated by updrafts associated with an undular bore. The bore resulted from the forward advance of a Pacific cold front into a stable air mass. The second major squall line within the Super Outbreak storm, which produced the strongest and most numerous tornadoes, was directly connected with the lifting associated with a cold front aloft. This second squall line was located along the farthest forward protrusion of a Pacific cold front as it occluded with a lee trough/dryline. An important factor in the ...

Observations are presented of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study conducted between July 26 and August 5, 1996. Current and temperature measurements were made with an upward looking acoustic Doppler current profiler (ADCP) located on the 147 m isobath near the shelfbreak and three vertical thermistor moorings located upshelf. Data from the ADCP and two nearby thermistor chains show energetic internal tides propagating at roughly 0.9 m s 21 to the north-northwest, nearly perpendicular to the local topography with 10 -15 cm s 21 horizontal currents and 15-30 m vertical displacements. These waves evolve rapidly within a 5.8 km range into an undular internal tidal bore. Cross-isobath barotropic tidal currents, responsible for generating the internal tides are in the 5-12 cm s 21 range. The bore formation is highly variable. There is evidence of a correlation between internal tide steepening and a shelfbreak front jet orientation that is oppositely directed to the internal tide propagation. There is no correlation between steepening and the jet's vertical shear. Statistics of the undular bores show rms travel time fluctuations from 0.8 to 1.7 hours and average tidal bore durations from 12 to 9 hours. The average undular bore speed is 0.9 m s 21 , with an rms fluctuation of 0.4 m s 21 . The number of high-frequency waves in the bore varies from 0 to 8 near the shelfbreak and increases to 30 waves 26.7 km upshelf. The observed distribution function of temporal spacing between high-frequency internal waves is spread between 4 and 20 min.

A dramatic front of airglow radiance and wave structure reported by Taylor et al. 1995 was attributed, in a previous paper of ours Dewan and Picard, 1998, to a manifestation of an internal undular bore in the mesosphere. In the current paper we address the question of what physical process could be responsible for generating such a bore at that altitude. While it is relatively easy to find sources of internal tropospheric bores, including the dramatic "morning glory," the same cannot be said for mesospheric bores. It will be argued here that a likely candidate for the generator of such bores is the interaction of gravity waves with the mean flow at a critical layer. This interaction could take place within an already existing inversion layer, the latter playing the role of the "channel" in which the bore propagates. As Huang et al 1998 have shown, a similar wave/critical-level interaction may be responsible for the inversion layer in question. Hence we are proposing that the physical process producing the channel is the same as the one responsible for subsequently generating the bore.

The Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) is used in conjunction with satellite observations and data from the Coastal Waves 1996 experiment to investigate the dynamics of unusual wave clouds that occur upwind and offshore of orographic features along the California coast. Results indicate that supercritical flow within the marine boundary layer, interacting with blocking coastal orography, is forced to decelerate and an atmospheric bow shock forms. The location and orientation of the COAMPS forecast shock matches well with the leading edge of the wave clouds in satellite imagery, and the modeled jump in boundary layer depth across the shock is in good agreement with the aircraft observations. In the parameter space of Froude number and jump strength that develops within the flow (observed and modeled), the shock manifests itself as an undular bore. On the innermost grid (Δx = ⅓ km), long, lineal variations in the wind, temperature, and moisture...

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 cl...

A midlevel, coastally trapped atmospheric event occurred along the California coast 10–11 June 1994. This feature reversed the surface wind field along the coast in a northerly phase progression. Along the central California coast, the winds at the coastal stations reverse before the corresponding coastal buoy offshore, then followed hours later by passage of the leading edge of an overcast stratus cloud. The sea surface temperature was much colder in the narrow strip along the coast. The cloud characteristics may be accounted for by a sea surface mixed layer (SSML) model beginning with the wind reversal and growing with the square root of time. Heat is lost from the SSML to the sea surface. A cloud forms when the air temperature at the top of the SSML is equal to the dewpoint. It is suggested that a bore develops on the top of the SSML, increasing the thickness of the SSML and the progression speed of the cloud to 8 m s−1. There is evidence that an undular bore with a leading cloud develops in t...

A nonlinear wave disturbance in the marine atmosphere associated with an atmospheric cold front has been identified on a spaceborne radar image of the North Sea by its imprint on the sea surface. This nonlinear wave disturbance is either a solitary wave disturbance or an undular bore propagating in the lower marine atmosphere. The radar image showing sea surface manifestations of this atmospheric wave disturbance was acquired over the North Sea by the synthetic aperture radar (SAR) aboard the first European Remote Sensing Satellite ERS 1 on March 8, 1992. Atmospheric waves become visible on radar images of the sea surface because they are associated with a varying wind field at the sea surface, which modulates the sea surface roughness, and, thus, the backscattered radar power. The identification of the wave pattern visible on the ERS 1 SAR image as a sea surface manifestation of an atmospheric nonlinear wave disturbance has been possible because this disturbance was also captured by meteorological sensors mounted on an 80-m high meteorological mast located on the island of Heligoland. Geophysical parameters characterizing the atmospheric wave disturbance are derived from the ERS 1 SAR image and from the in situ meteorological data. The wave period in the reference frame moving with the average wind is 9.5 min, the average wavelength is 2300 m, and the propagation velocity is 4.5–4.9 m s−1. Estimates of the amplitude of the wind speed fluctuations in the wave propagation direction derived from the meteorological measurements at Heligoland and from the gray level variations in the SAR image yield values between 1.3 and 3.0 m s−1. The experimental data are interpreted in terms of theories of atmospheric nonlinear gravity waves.

A case study of a sea-breeze front originating from the east coast of Florida colliding with a gust front moving toward the southeast is presented. Single- and multi-Doppler radar analyses combined with serial balloon ascents suggest that the denser sea-breeze flow undercut the cold pool behind the gust front and may have generated a westward-propagating undular bore. In addition, another undular bore was generated by the collision and propagated eastward through the CaPE network. The latter bore propagated upstream, against the southeasterly flow behind the sea-breeze front and was apparent as a narrow band, 5–6 km wide, of near-zero radial velocities embedded within an overall flow that was positive as seen by the CP-4 Doppler radar. As the eastern bore propagated through the dual-Doppler lobe defined by the NCAR CP-3 and CP-4 radars, a detailed and unique wind synthesis revealed its three-dimensional kinematic structure. Significant along-bore variability was shown in the convergence and verti...

[1] The Aerospace Corporation's Nightglow Imager observed a large step function change in airglow in the form of a traveling front in the OH Meinel (OHM) and O2atmospheric (O2A) airglow emissions over Alice Springs, Australia, on 2 February 2003. The front exhibited nearly a factor of 2 stepwise increase in the OHM brightness and a stepwise decrease in the O2A brightness. There was significant (∼25 K) cooling behind the airglow fronts. The OHM airglow brightness behind the front was among the brightest for Alice Springs that we have measured in 7 years of observations. The event was associated with a strong phase-locked 2 day wave (PL/TDW). We have analyzed the wave trapping conditions for the upper mesosphere and lower thermosphere using a combination of data and empirical models and found that the airglow layers were located in a region of ducting. The PL/TDW-disturbed wind profile was effective in supporting a high degree of ducting, whereas without the PL/TDW the ducting was minimal or nonexistent. The change in brightness in each layer was associated with a strong leading disturbance followed by a train of weak barely visible waves. In OHM the leading disturbance was an isolated disturbance resembling a solitary wave. The characteristics of the wave train suggest an undular bore with some turbulent dissipation at the leading edge.

All-sky CCD observations of mesospheric gravity waves have been made from Halley Station Antarctica (75.5°S, 26.7°W) as part of a collaborative research program between British Antarctic Survey, U.K. and Utah State University, USA. A mesospheric bore event was observed in the nightglow emissions over a period of several hours on the 27th of May, 2001. Two dimensional S-Transform (ST) analysis is applied to the airglow images of this bore event. This local spectral technique allows one to calculate the wave parameters as a function of time and space. It is observed that the horizontal phase speed and wavelength decrease over time as the amplitude attenuates. Simultaneously with this wave event the background wind experiences a large acceleration in the direction of the wave propagation. Mesospheric bore theory calculations are used to estimate the bore duct depth and it is shown that as the wave packet evolves, the bore duct collapses (decreasing in its vertical extent). As the bore duct shrinks, the wave's group velocity decelerates, the amplitude attenuates, and the wave dissipates.

A detailed and unique multisensor observation of an undular bore is presented. The data include those from rawinsonde, satellite, two Doppler radars, and a tall instrumented tower. Noteworthy are Doppler radar images that resolve the wave's characteristics and capture a good part of its spatial extent. The basic parameters of the wave train are established from the observations.

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 o...

[1] Deformation of the Indian Ocean tsunami moving into the shallow Strait of Malacca and formation of undular bores and solitary waves in the strait are simulated in a model study using the fully nonlinear dispersive method (FNDM) and the Korteweg-deVries (KdV) equation. Two different versions of the incoming wave are studied where the waveshape is the same but the amplitude is varied: full amplitude and half amplitude. While moving across three shallow bottom ridges, the back face of the leading depression wave steepens until the wave slope reaches a level of 0.0036–0.0038, when short waves form, resembling an undular bore for both full and half amplitude. The group of short waves has very small amplitude in the beginning, behaving like a linear dispersive wave train, the front moving with the shallow water speed and the tail moving with the linear group velocity. Energy transfer from long to short modes is similar for the two input waves, indicating the fundamental role of the bottom topography to the formation of short waves. The dominant period becomes about 20 s in both cases. The train of short waves, emerging earlier for the larger input wave than for the smaller one, eventually develops into a sequence of rank-ordered solitary waves moving faster than the leading depression wave and resembles a fission of the mother wave. The KdV equation has limited capacity in resolving dispersion compared to FNDM.

[1] The shoaling of the nonlinear internal tide in Massachusetts Bay is studied with a fully nonlinear and nonhydrostatic model. The results are compared with current and temperature observations obtained during the August 1998 Massachusetts Bay Internal Wave Experiment and observations from a shorter experiment which took place in September 2001. The model shows how the approaching nonlinear undular bore interacts strongly with a shoaling bottom, offshore of where KdV theory predicts polarity switching should occur. It is shown that the shoaling process is dominated by nonlinearity, and the model results are interpreted with the aid of a two-layer nonlinear but hydrostatic model. After interacting with the shoaling bottom, the undular bore emerges on the shallow shelf inshore of the 30-m isobath as a nonlinear internal tide with a range of possible shapes, all of which are found in the available observational record.

During the summer, nonlinear internal waves (NLIWs) are commonly observed propagating in Massachusetts Bay. The topography of the area is unique in the sense that the generation area (over Stellwagen Bank) is only 25 km away from the shoaling area, and thus it represents an excellent natural laboratory to study the life cycle of NLIWs. To assist in the interpretation of the data collected during the 1998 Massachusetts Bay Internal Wave Experiment (MBIWE98), a fully nonlinear and nonhydrostatic model covering the generation/shoaling region was developed, to investigate the response of the system to the range of background and driving conditions observed. Simplified models were also used to elucidate the role of nonlinearity and dispersion in shaping the NLIW field. This paper concentrates on the generation process and the subsequent evolution in the basin. The model was found to reproduce well the range of propagation characteristics observed (arrival time, propagation speed, amplitude), and provided a coherent framework to interpret the observations. Comparison with a fully nonlinear hydrostatic model shows that during the generation and initial evolution of the waves as they move away from Stellwagen Bank, dispersive effects play a negligible role. Thus the problem can be well understood considering the geometry of the characteristics along which the Riemann invariants of the hydrostatic problem propagate. Dispersion plays a role only during the evolution of the undular bore in the middle of Stellwagen Basin. The consequences for modeling NLIWs within hydrostatic models are briefly discussed at the end.

An unusual cloud feature was noted over the south Texas coast on the morning of 14 March 1997. This feature shares some characteristics with the “morning glory” bores that are seen along the coast of northern Australia.

[1] We reported the observation of a mesospheric front with the properties of an undular bore by an OH imager, over the Fort Collins/Platteville area on 6/7 October 2002. Unlike the earlier bore observations, a Na lidar capable of measuring mesopause region temperature, zonal, and meridional winds was in operation concurrently. The lidar data confirm, for the first time, the existence of a collocated temperature inversion layer to serve as the ducting region for bore propagation, as required by the simple theory proposed by Dewan and Picard 6 years ago. In addition, the lidar data in principle provide sufficient information for the determination of all parameters of the bore as suggested by the simple theory. The parameters so determined are compared to two bores previously studied. Like the earlier cases, the horizontal wavelength estimated from the theory is in good agreement with the observation. The lifetime of this undular bore, ∼120 min, was considerably shorter than the other two. Continued lidar observation after the bore event reveals that the ducting region may be controlled by a long-period wave, most likely related to a semidiurnal tide, and that atmospheric dynamic instability occurs simultaneously with the destruction of the wave train associated with the bore. It is possible that this constitutes, for the first time, the observation of the transition from an undular to a turbulent, or foaming, internal bore predicted by the theory.

Results of some idealized numerical experiments of strong tidal flow of a stratified fluid across a finite amplitude bank edge are presented. These experiments were motivated by a need to develop an understanding of some of the complex internal wave phenomena observed on Georges Bank (Loder et al., 1992) and at other locations where tidal forcing is strong. The numerical model solves the fully nonlinear, nonhydrostatic Boussinesq equations on an ƒ plane. The model is two-dimensional, with spatial variation in the vertical and cross-bank directions only. Model forcings are based on the Georges Bank observations. A horizontally uniform stratification is used. The model successfully reproduces some observed features including the formation of a large depression and a hydraulic jump over the bank edge during off-bank flow and two on-bank propagating depressions every tidal period. An undular bore propagating away from the bank is in agreement with other observations (La Violette et al., 1990). Rotational effects are shown to be responsible for the formation of the second of the on-bank propagating depressions. Sensitivity of the results to the topographic slope, tidal current strength, stratification, and model initialization is explored.

Observational data are presented on southerly nocturnal wind surges, which sometimes give rise to spectacular “morning glory”-type roll clouds in the southern part of the Gulf of Carpentaria region of northern Australia. Like their more frequent northeasterly counterparts in the region, southerly surges are shown to have the character of an undular bore propagating on the nocturnal low-level stable layer. Synoptic mean sea level pressure patterns conducive to the formation of southerly surges are identified and possible mechanisms for the genesis of surges are discussed. The possibility that some southerly surges are generated by the movement of a front across central Australia, as suggested by Smith et al., does not appear to be the most usual generation mechanism. In fact, an analysis of time-height cross sections of velocity components derived from six hourly rawinsoundings from Mount Isa point to a strong association between the occurrence of southerly surges and the formation of the nocturna...

This paper presents the results of a field expedition mounted in late September/early October 1979 to investigate the structure and origin of the “morning glory” of the Gulf of Carpentaria in northern Australia. The morning glory is a line wind squall, accompanied by a pressure jump, and often by a long roll-cloud or series of such clouds. It frequently occurs in the early morning, especially in October, in the Gulf area. A light aircraft, fitted with a temperature and humidity probe, was flown in two glories to determine their thermodynamic structure, and wind fields wore obtained principally by tracking pilot balloons using the double theodolite method. Data also were obtained from a network of surface stations, recording wind velocity and pressure, installed at locations across Cape York Peninsula, which is believed to be the area of genesis. The morning glory is identified as an internal undular bore propagating on the nocturnal and/or maritime inversion. Its origin appears to lie frequently ...

Early in the morning on 9 June 1982 a system of traveling wave cloud lines passed over 0klahoma, and in particular, over the relatively dense mesonetwork of surface stations, including the instrumented 444 m KTVY television tower, operated by the U.S. National Severe Storms Laboratory. An analysis of the network and other data presented herein shows that, in structure, the associated disturbance was an internal undular bore propagating on a low-level stable layer, similar to “morning glory”-type disturbances, which are common at certain times of the year in parts of northern Australia. Moreover, the speed of propagation of the component bore-waves is in broad agreement with theoretical calculations. There is evidence that the disturbance emanated late the previous evening from an area of rapidly growing intense thunderstorms located more than 400 km north-northwest of the surface mesonetwork. Possible mechanisms for generation are discussed, but the data pertaining to genesis are insufficient to ...

SUMMARY In this paper the formation and structure of an atmospheric undular bore is examined with the aid of a numerical model and theoretical analysis. The numerical model is first used to simulate a density current propagating into an unstratified environment. A detailed comparison of the model results with existing theory shows that for a given pressure head the current can travel at a greater speed than that predicted by irrotational theory. When the density current is allowed to propagate into a low-level stable layer it is found that for certain parameter regimes an undular bore forms ahead of the current. The results for two different types of stable layers are examined. For a layer with a step profile of potential temperature, comparison is made with both classical bore theory and the theory of density currents in two-layer fluids and encouraging agreement is found. When a linear profile is specified in the stable layer it is found that energy-conserving solutions can be obtained. The numerical fields are then compared with measurements of the morning glory, an undular bore observed in north-eastern Australia, and again agreement is good. Finally, preliminary results of model simulations with stratification above the stable layer are discussed.

A numerical model is used to examine the effects of ambient stratification on the behavior of an atmospheric undular bore. It is shown that stratification reduces the amplitude of the disturbance at low levels by allowing energy to propagate upward. This reduction of amplitude can be inhibited by specifying winds opposing the wave motion in the middle and upper troposphere. Mean observations in the region where the Morning Glory is prevalent support this result. The effects of moisture are also examined. If condensation does not occur, moisture increases the disturbance amplitude by reducing, through virtual temperature effects, the stability of the atmosphere. However, if condensation occurs, the wave amplitude is decreased compared with a dry atmosphere with the same effective stability. Finally, it is shown that cloud formation increases the wavelength of the disturbance.

SUMMARY Observations made during an expedition in October 1980 to study ‘morning glory’ wind squalfs in the Gulf of Carpentaria region of northern Australia are described. During a ten day period in Burketown, north Queensland, six squalls were documented on consecutive mornings (8-1 3 October), including two moving from the south, an apparently unusual direction. The unique data gathered from.the latter confirm that these have the same character as their north-easterly counterparts; that is, they are undular bores. The origin of southerly glories remains uncertain, but, at least, in some cases, appears to be. linked with the northeast advance of a frontal trough across central Australia. While the data are equivocal, it seems possible that the motion of the front into the developing nqcturnal inversion could generate waves, or hydraulic-type disturbances, which run along the inversion layer, ahead of the front, as proposed by Tepper (1950). The efficacy of such a mechanism is demonstrated by a simple laboratory experiment. Evidence is presented which suggests that morning glory type events occur elsewhere in the world, sometimes as precursors to cold fronts. Observations in the Gulf of Carpentaria region show that they are effective triggers of deep convection when they advance into air with a sufficient degree of conditional instability.

Using the laser-induced fluorescent method, the transition process from bore to runup mode, i.e., “bore collapse,” is investigated experimentally. The observed process appears to be different from both previous analytical and numerical predictions. The results indicate that momentum exchange takes place between the incident bore and the quiescent water body along the shoreline. Turbulence generated in a bore nearshore is highly three-dimensional and sporadic. Very close to the shore, turbulence is advected with the bore front, and consequently, the bore collapse process involves strong turbulent action onto the dry beach bed.