Gravity Wave Breaking Research Articles

Tropical Aviation Turbulence Induced by the Interaction Between a Jet Stream and Deep Convection

AbstractOn 18 December 2022, Hawaiian Airlines flight HA35 encountered severe turbulence without warning in a cloud‐free height. We reproduced this incident using the Weather Research and Forecasting Model (WRF) at a convection‐permitting resolution. We found that this case of tropical upper‐level turbulence occurred primarily due to the fast‐growing convective tower in the unstable layer created by gravity wave breaking. At lower altitudes, a mesoscale convective system (MCS) caused a decrease in wind speed in both upstream and downstream regions. At upper levels, a large‐scale jet descended and accelerated after flowing over the top of the MCS, which acted like a barrier and produced a situation similar to a downslope windstorm due to mountain terrain. Upper‐level turbulence is 2–3 km higher than the top of the MCS. The critical level above the jet and the locally self‐induced critical level created the locally enhanced descending jet stream, which destabilized the flow through Kelvin–Helmholtz instabilities. The convective tower existed near the flight route and played an important role in triggering turbulence in the unstable environment through its convective gravity waves.

On the Gravity Wave‐Seeded Ionospheric Irregularities in the Martian Ionosphere

AbstractFor the past few decades, it has been demonstrated that gravity waves (GWs) and neutral winds can drive ionospheric irregularities on Earth. Still, as far as we know, the formation of ionospheric irregularities on Mars due to GWs has not been well studied. In this study, we use data from the NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission to show evidence of an irregularity event in the Martian ionosphere, potentially seeded by the GWs break (GWB). Statistical findings indicate that the observed ratio of GWB‐related irregularity events varies from ∼0.25 to ∼0.47 each year, and the average ratio in 2015–2020 is ∼0.37. We perform a numerical simulation to provide further insight into the processes behind irregularity formation, which employs neutral wind shear as a source of perturbation in the context of the GWB. The simulations yield results fundamentally aligned with the observed characteristics of ionospheric irregularities in the 2018 event by considering the wind shear as the disturbance source. This study provides supplementary insights into the perturbation sources involved in shaping irregularities within the Martian ionosphere and presents valuable information about the coupling between the Martian ionosphere and the lower atmosphere.

An Efficient Tidal Dissipation Mechanism via Stellar Magnetic Fields

Recent work suggests that inwardly propagating internal gravity waves (IGWs) within a star can be fully converted to outward magnetic waves if they encounter a sufficiently strong magnetic field. The resulting magnetic waves dissipate as they propagate outward to regions with lower Alfvén velocity. While tidal forcing is known to excite IGWs, this conversion and subsequent damping of magnetic waves have not been explored as a tidal dissipation mechanism. In particular, stars with sufficiently strong magnetic fields could fully dissipate tidally excited waves, yielding the same tidal evolution as the previously studied “traveling wave regime.” Here, we evaluate the viability of this mechanism using stellar models of stars with convective cores (F-type stars in the mass range of 1.2–1.6 M ⊙), which were previously thought to be weakly tidally dissipative (due to the absence of nonlinear gravity-wave breaking). The criterion for wave conversion to operate is evaluated for each stellar mass using the properties of each star’s interior along with estimates of the magnetic field produced by a convective core dynamo under the assumption of equipartition between kinetic (convective) and magnetic energies. Our main result is that this previously unexplored source of efficient tidal dissipation can operate in stars within this mass range for significant fractions of their lifetimes. This tidal dissipation mechanism appears to be consistent with the observed inspiral of WASP-12b and more generally could play an important role in the orbital evolution of hot Jupiters—and to lower-mass ultra-short-period planets—orbiting F-type stars.

Measurements of nearshore ocean-surface kinematics through coherent arrays of free-drifting buoys

Abstract. Surface gravity wave breaking occurs along coastlines in complex spatial and temporal patterns that significantly impact erosion, scalar transport, and flooding. Numerical models are used to predict wave breaking and associated processes but many lack sufficient evaluation with observations. To fill the need for more nearshore wave measurements, we deployed coherent arrays of small-scale, free-drifting buoys named microSWIFTs. The microSWIFT is a small buoy equipped with a GPS module to measure the buoy's position, horizontal velocities, and an inertial measurement unit (IMU) to directly measure the buoy's rotation rates, accelerations, and heading. Measurements were collected over a 27 d field experiment in October 2021 at the US Army Corps of Engineers Field Research Facility in Duck, NC. The microSWIFTs were deployed as a series of coherent arrays, meaning they all sampled simultaneously with a common time reference, leading to a rich spatial and temporal dataset during each deployment. Measurements spanned offshore significant wave heights ranging from 0.5 to 3 m and peak wave periods ranging from 5 to 15 s over the entire experiment. The completed dataset consists of 67 deployment files that contain 971 drift tracks that contain all associated data. We use an attitude and heading reference system (AHRS) 9-degrees-of-freedom Kalman filter to rotate the measured accelerations from the reference frame of the buoy to the Earth reference frame. We then use the corrected accelerations to compute the vertical velocity and sea-surface elevation. We give example evaluations of wave spectral energy density estimates from individual microSWIFTs compared with a nearby acoustic wave and current (AWAC) sensor. A zero-crossing algorithm is applied to each buoy time series of sea-surface elevation to extract realizations of measured surface gravity waves, yielding 116 307 wave realizations throughout the experiment. We also compute significant wave height estimates from the aggregate wave realizations and compare these estimates with the nearby AWAC estimates. An example of spatial variability in cross-shore velocity and vertical acceleration is explored. Wave-breaking events, detected by high-intensity vertical acceleration peaks, are explored, and the cross-shore distribution of all breaking events detected in the experiment is shown. A total of 3419 wave-breaking events were detected across the entire experiment. These data are available at https://doi.org/10.5061/dryad.hx3ffbgk0 (Rainville et al., 2023) and will be used to investigate nearshore wave kinematics, transport of buoyant particles, and wave-breaking processes.

Kelvin–Helmholtz Instability “Tube” and “Knot” Dynamics. Part II: KHI T&K Dynamics in a Multiscale Gravity Wave Direct Numerical Simulation

Abstract A companion paper by Fritts et al. reviews evidence for Kelvin–Helmholtz instability (KHI) “tube” and “knot” (T&K) dynamics that appear to be widespread throughout the atmosphere. Here we describe the results of an idealized direct numerical simulation of multiscale gravity wave dynamics that reveals multiple larger- and smaller-scale KHI T&K events. The results enable assessments of the environments in which these dynamics arise and their competition with concurrent gravity wave breaking in driving turbulence and energy dissipation. A larger-scale event is diagnosed in detail and reveals diverse and intense T&K dynamics driving more intense turbulence than occurs due to gravity wave breaking in the same environment. Smaller-scale events reveal that KHI T&K dynamics readily extend to weaker, smaller-scale, and increasingly viscous shear flows. Our results suggest that KHI T&K dynamics should be widespread, perhaps ubiquitous, wherever superposed gravity waves induce intensifying shear layers, because such layers are virtually always present. A second companion paper demonstrates that KHI T&K dynamics exhibit elevated turbulence generation and energy dissipation rates extending to smaller Reynolds numbers for relevant KHI scales wherever they arise. These dynamics are suggested to be significant sources of turbulence and mixing throughout the atmosphere that are currently ignored or underrepresented in turbulence parameterizations in regional and global models. Significance Statement Atmospheric observations reveal that Kelvin–Helmholtz instabilities (KHI) often exhibit complex interactions described as “tube” and “knot” (T&K) dynamics in the presence of larger-scale gravity waves (GWs). These dynamics may prove to make significant contributions to energy dissipation and mixing that are not presently accounted for in large-scale modeling and weather prediction. We explore here the occurrence of KHI T&K dynamics in an idealized model that describes their behavior and character arising at larger and smaller scales due to superposed, large-amplitude GWs. The results reveal that KHI T&K dynamics arise at larger and smaller scales, and that their turbulence intensities can be comparable to those of the GWs.

Physics-informed learning for modeling infrasound propagation in extreme weather conditions

Extreme events in fluid flows are characterized by the coexistence of complex nonlinear dynamics, high intrinsic dimensionality and intermittency, which often results in spatially localized disturbances (turbulence spots, gravity wave breaking). Although many studies have shown that atmospheric ducting of infrasound is sensitive to these disturbances, yet the link between their statistical properties and that of the infrasound wavefield remains an open question, mainly because very little data are available for extreme events. The present work focuses on catastrophic events in climate systems where the amount of data available (typically a few decades) is not sufficient to extrapolate the PDFs. This class of problem involves geophysical fluid flows over climate scales where reanalysis data are a reliable source of information. In contrast to methods that rely on standard models to compute the PDFs from available data, the focus here is on data-driven methods that encode some information about the wave dynamics. The idea behind this approach is to combine two sources of information (reanalysis data and wave theory) using physics-informed neural networks to extrapolate the PDFs. The performance of this approach is illustrated around two types of events that affect infrasound propagation: sudden stratospheric warmings and mountain-induced extreme weathers.

Probing Gravity Waves in the Middle Atmosphere Using Infrasound From Explosions

AbstractThis study uses low‐frequency, inaudible acoustic waves (infrasound) to probe wind and temperature fluctuations associated with breaking gravity waves (GWs) in the middle atmosphere. Building on an approach introduced by Chunchuzov et al., infrasound recordings are used to retrieve effective sound‐speed fluctuations in an inhomogeneous atmospheric layer that causes infrasound backscattering. The infrasound was generated by controlled blasts at Hukkakero, Finland, and recorded at the IS37 infrasound station, Norway in the late summers 2014–2017. Our findings indicate that the analyzed infrasound scattering occurs at mesospheric altitudes of 50–75 km, a region where GWs interact under non‐linearity, forming thin layers of strong wind shear. The retrieved fluctuations were analyzed in terms of vertical wave number spectra, resulting in an approximate power law that corresponds to the “universal” saturated spectrum of atmospheric GWs. The power law wavenumber range corresponds to vertical atmospheric scales of 33–625 m. The fluctuation spectra were compared to theoretical GW saturation theories as well as to independent wind measurements by the Saura medium‐frequency radar near Andøya Space Center around 100 km west of IS37, yielding a good agreement in terms of vertical wavenumber spectrum amplitudes and slopes. This suggests that the radar and infrasound‐based effective sound‐speed profiles represent low‐ and high‐wavenumber regimes of the same “universal” GW spectrum. The results illustrate that infrasound allows for probing fine‐scale dynamics not well captured by other techniques, suggesting that infrasound can provide a complementary technique to probe atmospheric GWs.

Potential Vorticity Generation in Breaking Gravity Waves

Potential vorticity (PV) is an important quantity in stratified flows because it is conserved following the flow in the absence of forcing and viscous and diffusive effects. However, as shown by previous work for unstratified turbulence, viscosity and diffusion, when present, are not purely dissipative and can create potential vorticity even when none is present initially. In this work, we use direct numerical simulations to investigate the viscous and diffusive generation of potential vorticity and potential enstrophy (integrated square PV) in stratified turbulence. Simulations are initialized with a two-dimensional standing internal gravity wave, which has no potential vorticity apart from some low-level random noise; as a result, all potential vorticity and enstrophy comes from viscous and diffusive effects. Significant potential enstrophy is found when the standing wave breaks, and the maximum potential enstrophy increases with increasing Reynolds number. The mechanism for the initial PV generation is spanwise diffusion of buoyancy perturbations, which grow as the standing wave three-dimensionalizes, into the direction of spanwise vorticity. The viscous and diffusive terms responsible are small-scale and are sensitive to under-resolution, so high resolution is required to obtain robust results.

Deep Kernel Dictionary Learning for detection of wave breaking features in Atmospheric Gravity Waves

Atmospheric Gravity Waves play a significant role in the Middle Atmosphere Dynamics and breaking of Gravity Waves leads to turbulence. To confirm the breaking of Atmospheric Gravity Waves, we require Wind Velocity profiles with high accuracy and the problem in the existing scenario is the Wind velocity measuring instruments are not accurate at the altitude of interest and are very sparse. We came up with a solution by taking advantage of Dictionary Learning and Deep Learning approach for the detection of the Wave Breaking events from atmospheric temperature perturbations instead of looking for wind velocity profiles.In the present work, we discuss incorporating Kernels into Deep Dictionary Learning. The Deep Dictionary Learning algorithm, introduced recently, shows an improvement in feature detection in comparison to k-means Singular Value Decomposition (KSVD) Dictionary Learning, Convolution Neural Network, and LSTM Auto-Encoders. Incorporating Kernels into Deep Dictionary Learning increases the effectiveness of the detection of the Wave Breaking events. The potential of the proposed method is demonstrated with a case study on the detection of atmospheric Gravity Wave Breaking event leading to turbulence in the atmosphere using satellite data (Aura-Microwave Limb Sounder) and is validated using data available from the ground-based instruments.

Aircraft observations of gravity wave activity and turbulence in the tropical tropopause layer: prevalence, influence on cirrus clouds, and comparison with global storm-resolving models

Abstract. The tropical tropopause layer (TTL) is a sea of vertical motions. Convectively generated gravity waves create vertical winds on scales of a few to thousands of kilometers as they propagate in a stable atmosphere. Turbulence from gravity wave breaking, radiatively driven convection, and Kelvin–Helmholtz instabilities stirs up the TTL on the kilometer scale. TTL cirrus clouds, which moderate the water vapor concentration in the TTL and stratosphere, form in the cold phases of large-scale (> 100 km) wave activity. It has been proposed in several modeling studies that small-scale (< 100 km) vertical motions control the ice crystal number concentration and the dehydration efficiency of TTL cirrus clouds. Here, we present the first observational evidence for this. High-rate vertical winds measured by aircraft are a valuable and underutilized tool for constraining small-scale TTL vertical wind variability, examining its impacts on TTL cirrus clouds, and evaluating atmospheric models. We use 20 Hz data from five National Aeronautics and Space Administration (NASA) campaigns to quantify small-scale vertical wind variability in the TTL and to see how it varies with ice water content, distance from deep convective cores, and height in the TTL. We find that 1 Hz vertical winds are well represented by a normal distribution, with a standard deviation of 0.2–0.4 m s−1. Consistent with a previous observational study that analyzed two out of the five aircraft campaigns that we analyze here, we find that turbulence is enhanced over the tropical west Pacific and within 100 km of convection and is most common in the lower TTL (14–15.5 km), closer to deep convection, and in the upper TTL (15.5–17 km), further from deep convection. An algorithm to classify turbulence and long-wavelength (5 km < λ < 100 km) and short-wavelength (λ < 5 km) gravity wave activity during level flight legs is applied to data from the Airborne Tropical TRopopause EXperiment (ATTREX). The most commonly sampled conditions are (1) a quiescent atmosphere with negligible small-scale vertical wind variability, (2) long-wavelength gravity wave activity (LW GWA), and (3) LW GWA with turbulence. Turbulence rarely occurs in the absence of gravity wave activity. Cirrus clouds with ice crystal number concentrations exceeding 20 L−1 and ice water content exceeding 1 mg m−3 are rare in a quiescent atmosphere but about 20 times more likely when there is gravity wave activity and 50 times more likely when there is also turbulence, confirming the results of the aforementioned modeling studies. Our observational analysis shows that small-scale gravity waves strongly influence the ice crystal number concentration and ice water content within TTL cirrus clouds. Global storm-resolving models have recently been run with horizontal grid spacing between 1 and 10 km, which is sufficient to resolve some small-scale gravity wave activity. We evaluate simulated vertical wind spectra (10–100 km) from four global storm-resolving simulations that have horizontal grid spacing of 3–5 km with aircraft observations from ATTREX. We find that all four models have too little resolved vertical wind at horizontal wavelengths between 10 and 100 km and thus too little small-scale gravity wave activity, although the bias is much less pronounced in global SAM than in the other models. We expect that deficient small-scale gravity wave activity significantly limits the realism of simulated ice microphysics in these models and that improved representation requires moving to finer horizontal and vertical grid spacing.

An energetic signature for breaking inception in surface gravity waves

A dynamical understanding of the physical process of surface gravity wave breaking remains an unresolved problem in fluid dynamics. Conceptually, breaking can be described by inception and onset, where breaking inception is the initiation of unknown irreversible processes within a wave crest that precede the visible manifestation of breaking onset. In the search for an energetic indicator of breaking inception, we use an ensemble of non-breaking and breaking crests evolving within unsteady wave packets simulated in a numerical wave tank to investigate the evolution of each term in the kinetic energy balance equation. We observe that breaking onset is preceded by around one quarter of a wave period by a rapid increase in the rate of convergence of kinetic energy that triggers an irreversible acceleration of the kinetic energy growth rate. This energetic signature, which is present only for crests that subsequently break, arises when the kinetic energy growth rate exceeds a critical threshold. At this point the additional kinetic energy convergence cannot be offset by converting excess kinetic energy to potential energy or by dissipation through friction. Our results suggest that the ratio of the leading terms of the kinetic energy balance equation at the time of this energetic signature is proportional to the strength of the breaking crest. Hence this energetic inception point both predicts the occurrence of breaking onset and indicates the strength of the breaking event.

Increased vertical resolution in the stratosphere reveals role of gravity waves after sudden stratospheric warmings

Abstract. Sudden stratospheric warmings (SSWs) have a long-lasting effect within the stratosphere as well as impacts on the underlying troposphere. However, sub-seasonal forecasts of the winter polar stratosphere fail to use their full potential for predictability as they tend to underestimate the magnitude and persistence of these events already within the stratosphere. The origin of this underestimation is unknown. Here, we demonstrate that the associated polar stratospheric cold bias following SSW events in sub-seasonal hindcasts can be halved by increasing vertical model resolution, suggesting a potential sensitivity to gravity wave forcing. While the predictability of the planetary Rossby wave flux into the stratosphere at lead times longer than a week is limited, the existence of a critical layer for gravity waves with a low zonal phase speed caused by the disturbed polar vortex provides predictability to the upper stratosphere. Gravity wave breaking near that critical layer can, therefore, decelerate the zonal flow consistently with anomalous subsidence over the polar cap leading to warmer temperatures in the middle polar stratosphere. Since the spectrum of gravity waves involves vertical wavelengths of less than 4000 m, as estimated by wavelet analysis, a high vertical model resolution is needed to resolve the positive feedback between gravity wave forcing and the state of the polar vortex. Specifically, we find that at a spectral resolution of TCo639 (approximate horizontal grid spacing of 18 km) at least 198 levels are needed to correctly resolve the spectrum of gravity waves in the ECMWF Integrated Forecasting System. Increasing vertical resolution in operational forecasts will help to mitigate stratospheric temperature biases and improve sub-seasonal predictions of the stratospheric polar vortex.

Analysis of the Bidirectional Characteristic of Radiation of Flat and Rough Water–Air Interfaces Based on the Theory of Radiative Transfer

The Lambertian property of objects is one of the basic hypotheses in remote sensing research. However, the spectral radiance of natural objects is always anisotropic. On the sea surface, a large amount of sea foam is generated at the water–air interface, induced by wind speed and breaking gravity waves. Additionally, the scattering characteristic at the water–air interface significantly influences the accuracy of ocean color remote sensing and its output. The bidirectionality of the water light field is one of the sources of errors in ocean color inversion. Therefore, the knowledge of the bidirectional reflectance distribution of water surfaces is of great significance in quantitative remote sensing or for the evaluation of measurement errors in surface optical parameters. To clarify the bidirectional reflectance distribution, we used the coupled ocean–atmosphere radiative transfer (COART) model to simulate the bidirectional radiance of water bodies and explored the anisotropy of radiance at the water–air interface. The results indicate that the downward and upward irradiance just below the water surface and the water-leaving radiance changed with the sun-viewing geometry. The downward and upward radiance just below the water surface decreased as the zenith angle of the incident light increased. This effect can be mitigated using a function of the viewing angle. Additionally, the viewing azimuth angle and rough sea surface had no significant effect on the downward and upward radiance. The water-leaving radiance had an obvious bidirectional reflectance characteristic. Additionally, a backward hotspot was found in the simulated results. Then, the transmission coefficient was calculated, and the bidirectional distribution characteristic was found for flat and rough sea surfaces. This study can be used as a reference to correct bidirectional errors and to guide the spectral measurements of water and its error control for rough sea surfaces.

Three-Dimensional Static Instability of Gravity Waves and a Possible Parameterization of the Associated Wave Breaking

Abstract Parameterizations of subgrid-scale gravity waves (GWs) in atmospheric models commonly involve the description of the dissipation of GWs. Where they dissipate, GWs have an increased effect on the large-scale flow. Instabilities that trigger wave breaking are an important starting point for the route to dissipation. Possible destabilizing mechanisms are numerous, but the classical vertical static instability is still regarded as a key indicator for the disposition to wave breaking. In this work, we investigate how the horizontal variations associated with a GW could alter the criterion for static instability. To this end, we use an extension of the common parcel displacement method. This three-dimensional static stability analysis predicts a significantly larger range of instability than does the vertical static stability analysis. In this case, the Lindzen-type saturation adjustment to a state of marginal stability is perhaps a less suitable ansatz for the parameterization of the GW breaking. To develop a possible ansatz for the GW dissipation due to three-dimensional instability, we apply the methods of irreversible thermodynamics, which are embedded in the Gibbs formalism of dynamics. In this way, the parameterization does not only satisfy the second law of thermodynamics, but it can also be made consistent with the conservation of energy and further (non-)conservation principles. We develop the parameterization for a discrete spectrum of GW packets. Offline computations of GW drag and dissipative heating rates are performed for two vertical profiles of zonal wind and temperature for summer and winter conditions from CIRA data. The results are compared to benchmarks from the literature.

Signature of gravity wave propagations from the troposphere to ionosphere

Abstract. We observed a gravity wave (GW) signature in the OH emission layer in the upper mesosphere, and 4 h later, a medium-scale travelling ionospheric disturbance (MSTID) in the OI 630 nm emission layer. Spectral analysis of the two waves showed that both have almost the same wave characteristics: wavelength, period, phase speed and propagation direction, respectively, 200 km, 60 min, 50 m s−1, toward the southeast. From the gravity wave ray-tracing simulation for the mesospheric gravity wave, we found that the wave came from a tropospheric deep convection spot and propagated up to the 140 km altitude. Regarding the same wave characteristics between mesospheric GW and ionospheric MSTID, the two possible cases are investigated: a direct influence of the GW oscillation in the OI 630 nm emission height and the generation of a secondary wave during the GW breaking process. This is the first time to report an observational event of gravity wave propagation from the troposphere, mesosphere to thermosphere–ionosphere in the South American region.

Stratospheric water vapour and ozone response to the quasi-biennial oscillation disruptions in 2016 and 2020

Abstract. The quasi-biennial oscillation (QBO) is a major mode of climate variability in the tropical stratosphere with quasi-periodically descending westerly and easterly winds, modulating transport and distributions of key greenhouse gases such as water vapour and ozone. In 2016 and 2020, anomalous QBO easterlies disrupted the QBO's mean period of about 28 months previously observed. Here, we quantify the impact of these two QBO disruption events on the Brewer–Dobson circulation and respective distributions of water vapour and ozone using the ERA5 reanalysis and Microwave Limb Sounder (MLS) satellite observations, respectively. In 2016, both water vapour and ozone in the lower stratosphere decreased globally during the QBO disruption event by up to about 20 %. In 2020, the lower-stratospheric ozone only weakly decreased during the QBO disruption event, by up to about 10 %, while the lower-stratospheric water vapour increased by up to about 15 %. These dissimilarities in the anomalous circulation and the related ozone response between the year 2016 and the year 2020 result from differences in the tropical upwelling and in the secondary circulation of the QBO caused by differences in anomalous planetary and gravity wave breaking in the lower stratosphere near the equatorward upper flanks of the subtropical jet. The anomalous planetary and gravity wave breaking was stronger in the lower stratosphere between the tropopause and the altitude of about 23 km during the QBO disruption events in 2016 than in 2020. However, the differences in the response of lower-stratospheric water vapour to the QBO disruption events between the year 2016 and the year 2020 are mainly due to the differences in cold-point temperatures induced by Australian wildfire, which moistened the lower stratosphere, thereby obscuring the impact of the QBO disruption event in 2020 on water vapour in the lower stratosphere. Our results highlight the need for a better understanding of the causes of the QBO disruption, their interplay with other modes of climate variability in the Indo-Pacific region, including the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), and their impacts on water vapour and ozone in the upper troposphere/lower stratosphere in the face of a changing climate.

The polar mesospheric cloud dataset of the Balloon Lidar Experiment (BOLIDE)

Abstract. The Balloon Lidar Experiment (BOLIDE) observed polar mesospheric clouds (PMCs) along the Arctic circle between Sweden and Canada during the balloon flight of PMC Turbo in July 2018. The purpose of the mission was to study small-scale dynamical processes induced by the breaking of atmospheric gravity waves by high-resolution imaging and profiling of the PMC layer. The primary parameter of the lidar soundings is the time- and range-resolved volume backscatter coefficient β. These data are available at high resolutions of 20 m and 10 s (Kaifler, 2021, https://doi.org/10.5281/zenodo.5722385). This document describes how we calculate β from the BOLIDE photon count data and balloon floating altitude. We compile information relevant for the scientific exploration of this dataset, including statistics, mean values, and temporal evolution of parameters like PMC brightness, altitude, and occurrence rate. Special emphasis is given to the stability of the gondola pointing and the effect of resolution on the signal-to-noise ratio and thus the detection threshold of PMC. PMC layers were detected during 49.7 h in total, accounting for 36.8 % of the 5.7 d flight duration and a total of 178 924 PMC profiles at 10 s resolution. Up to the present, published results from subsets of this dataset include the evolution of small-scale vortex rings, distinct Kelvin–Helmholtz instabilities, and mesospheric bores. The lidar soundings reveal a wide range of responses of the PMC layer to larger-scale gravity waves and breaking gravity waves, including the accompanying instabilities, that await scientific analysis.

The 2017 Split wildfire in Croatia: evolution and the role of meteorological conditions

Abstract. The Split wildfire in July 2017, which was one of the most severe wildfires in the history of this Croatian World Heritage Site, is the focus in this study. The Split fire is a good example of a wildfire–urban interface, with unexpected fire behavior including rapid downslope spread to the coastal populated area. This study clarifies the meteorological conditions behind the fire event, those that have limited the effectiveness of firefighting operations, and the rapid escalation and expansion of the fire zones within 30 h. The Split fire propagation was first reconstructed using radio logs, interviews with firefighters and pilots involved in the intervention, eyewitness statements, digital photographs from fire detection cameras, media, and the monthly firefighting journal. Four phases of fire development have been identified. Then, weather observations and numerical simulations using an enhanced-resolution operational model are utilized to analyze the dynamics in each phase of the fire runs. The synoptic background of the event includes large surface pressure gradient between the Azores anticyclone accompanied by a cold front and a cyclone over the southeastern Balkan Peninsula. At the upper level, there was a deep shortwave trough extending from the Baltic Sea to the Adriatic Sea, which developed into a cut-off low. Such synoptic conditions have resulted in the maximum fire weather index in 2017. Combined with topography, they also locally provoke the formation of the strong northeasterly bura wind along the Adriatic coast, which has been accompanied by a low-level jet (LLJ). The bura (downslope wind), with mid- to low-level gravity-wave breaking and turbulence mixing (as in the hydraulic jump theory), also facilitated the subsidence of dry air from the upper troposphere and rapid drying at the surface. This study demonstrates that numerical guidance that indicates the spatial and temporal occurrence of a LLJ is highly capable of explaining the Split fire evolution from the ignition potential to its extinguishment stage. Thus, in addition to the conventional fire weather indices, such products are able to improve fire weather behavior forecasting and in general more effective decision-making in fire management.

Effects of Distant Organized Convection on Forecasts of Widespread Clear-Air Turbulence

Abstract Two cases of observed widespread moderate-or-greater (MOG) clear-air turbulence (CAT) in different synoptic patterns are investigated using a nested high-resolution NWP model. Both of these cases occurred in confluent entrance regions of upper-tropospheric–lower-stratospheric (UTLS) jet streaks, where large-scale anticyclonic outflow from distant organized moist convection strengthened the UTLS jet. Both the strength and vertical sharpness of the resulting jet influence the altitudes of MOG turbulence and the details of simulated turbulence onset mechanisms. In a strong and narrow UTLS jet downstream of a weak synoptic ridge, MOG turbulence arises from Kelvin–Helmholtz (KH) waves that overturn in opposite directions on the vertical flanks of the jet. In broader UTLS jets, MOG turbulence arising from KH waves was favored in strong vertical shear layers beneath the wind maximum, but was inhibited above it due to static stability increases near the tropopause. However, vertically propagating internal gravity waves initiated above KH wave breaking beneath the UTLS jet amplify within the lower stratosphere above the jet, constituting another possible source of turbulence. Turbulence onset mechanisms were often apparent in simulations with minimum horizontal grid spacings of Δx = 1 km. However, amplitudes of the associated grid-resolved vertical motions were unreliable when compared with simulations having minimum horizontal grid spacings of Δx = 1/3 km. In spite of this, turbulence forecasting systems driven by input from coarser-resolution operational NWP models are demonstrated to provide good diagnoses of this type of convectively influenced CAT when the NWP model accurately forecasts upstream convection. Significance Statement In this study we document the role of distant convection on observed widespread strong clear-air turbulence near upper-level jet streams in different weather patterns. Results from nested NWP model simulations, together with similar examples from previous case studies, suggest a strong association of widespread turbulence with jet stream enhancements related to outflow from upstream convection. We also demonstrate how model horizontal grid spacings of <1 km are required to adequately resolve common turbulence onset mechanisms (e.g., Kelvin–Helmholtz instability, gravity wave breaking). Nevertheless, examples are provided showing that turbulence forecasting systems driven by lower-resolution operational NWP models can provide potentially valuable guidance on these widespread turbulence events when those models accurately forecast the timing and locations of upstream convection.

Modeling Studies of Gravity Wave Dynamics in Highly Structured Environments: Reflection, Trapping, Instability, Momentum Transport, Secondary Gravity Waves, and Induced Flow Responses.

A compressible numerical model is applied for three‐dimensional (3‐D) gravity wave (GW) packets undergoing momentum deposition, self‐acceleration (SA), breaking, and secondary GW (SGW) generation in the presence of highly‐structured environments enabling thermal and/or Doppler ducts, such as a mesospheric inversion layer (MIL), tidal wind (TW), or combination of MIL and TW. Simulations reveal that ducts can strongly modulate GW dynamics. Responses modeled here include reflection, trapping, suppressed transmission, strong local instabilities, reduced SGW generations, higher altitude SGW responses, and induced large‐scale flows. Instabilities that arise in ducts experience strong dissipation after they emerge, while trapped smaller‐amplitude and smaller‐scale GWs can survive in ducts to much later times. Additionally, GW breaking and its associated dynamics enhance the local wind along the GW propagation direction in the ducts, and yield layering in the wind field. However, these dynamics do not yield significant heat transport in the ducts. The failure of GW breaking to induce stratified layers in the temperature field suggests that such heat transport might not be as strong as previously assumed or inferred from observations and theoretical assessments. The present numerical simulations confirm previous finding that MIL generation may not be caused by the breaking of a transient high‐frequency GW packet alone.