Observations performed with a Rayleigh lidar and an Advanced Mesosphere Temperature Mapper aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V research ...aircraft on 13 July 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) measurement program revealed a large‐amplitude, multiscale gravity wave (GW) environment extending from ~20 to 90 km on flight tracks over Mount Cook, New Zealand. Data from four successive flight tracks are employed here to assess the characteristics and variability of the larger‐ and smaller‐scale GWs, including their spatial scales, amplitudes, phase speeds, and momentum fluxes. On each flight, a large‐scale mountain wave (MW) having a horizontal wavelength ~200–300 km was observed. Smaller‐scale GWs over the island appeared to correlate within the warmer phase of this large‐scale MW. This analysis reveals that momentum fluxes accompanying small‐scale MWs and propagating GWs significantly exceed those of the large‐scale MW and the mean values typical for these altitudes, with maxima for the various small‐scale events in the range ~20–105 m2 s−2.
Key Points
Mountain waves penetrate the mesosphere under suitable propagation conditions
Small‐scale gravity waves can attain very large momentum fluxes
Occurrence of peak momentum fluxes is often dictated by multiscale environments
A companion paper describes high‐resolution, ground‐based imaging of apparent Kelvin‐Helmholtz instabilities (KHI) observed in noctilucent clouds (NLCs) near the polar summer mesopause. Here we ...employ direct numerical simulations of KHI at Richardson numbers from Ri = 0.05 to 0.20 and relatively high Reynolds numbers to illustrate the dependence of KHI and secondary instabilities on these quantities and interpret and quantify the KHI events described by Baumgarten and Fritts (2014). We conclude that one event triggered by small‐scale gravity waves provides clear evidence of strong KHI initiated at Ri ~0.05–0.10. Events arising in a more uniform shear environment exhibit KHI and small‐scale dynamics that compare reasonably well with modeled KHI initiated at Ri ~0.20. Our application of numerical modeling in quantifying KHI dynamics observed in NLCs suggests that characteristics of KHI, and perhaps other small‐scale dynamics, that are defined well in NLC displays can be used to quantify the dynamics and spatial scales of such events with high confidence. Specifically, our comparisons of KHI observations and modeling appear to indicate a “turbulent” viscosity ~5–40 times the true kinematic viscosity at the NLC altitude. This offers an alternative, or an augmentation, to more traditional radar, lidar, and/or airglow measurements employed for such studies of small‐scale dynamics at coarser spatial scales during polar summer.
Key Points
NLC observations are a valuable tracer of small‐scale dynamics in the MLTModeling can greatly aid the interpretation of KHI observed in NLCObservations and modeling suggest a turbulent viscosity that may be large
We address the sources and dynamics of vorticity and helicity and their relations in transitions to turbulence arising due to Kelvin–Helmholtz instability (KHI) “Tube” and “Knot” (T&K) events. Such ...events are common in the atmosphere and oceans, and initial numerical simulations reveal that T&K dynamics significantly accelerate turbulence transitions and enhance KHI peak and mean energy dissipation rates. KHI T&K events arise where emerging KH billows exhibit varying wavelengths, phases, amplitudes, and/or discontinuities along their axes. As the KH billows intensify, these regions evolve roughly orthogonal billow cores and induced vortex tubes in close proximity. Their mutual advection as they intensify induces large-amplitude Kelvin vortex waves, or “twist waves”, that arise where locally uniform vortices are distorted by axial or radial advection. The twist waves propagate along, and fragment, the vortex tubes and billow cores, thus accounting for the emergence of helicity and the down-scale energy, enstrophy, and helicity fluxes within the turbulence inertial range. We describe the results of four direct numerical simulations (DNS) addressing KHI T&K dynamics in large and idealized small domains. The large-domain vorticity fields reveal the character and diversity of KHI T&K dynamics, the emergence of twist waves at larger and smaller scales, and their driving of turbulence transitions. Two small-domain DNS exhibit idealized KHI T&K events arising from KH billows that are mis-aligned and that exhibit phase variability along their axes. A third examines the interactions of two vortex tubes in close proximity. These reveal that twist waves drive the character and evolutions of the vorticity and helicity fields.
Abstract
Thorpe analysis has been used to study turbulence in the atmosphere and ocean. It is clear that Thorpe analysis applied to individual soundings cannot be expected to give quantitatively ...reliable measurements of turbulence parameters because of the instantaneous nature of the measurement. A critical aspect of this analysis is the assumption of the linear relationship
C
=
L
O
/
L
T
between the Thorpe scale
L
T
, derived from the sounding measurements, and the Ozmidov scale
L
O
. It is the determination of
L
O
that enables determination of the dissipation rate of turbulence kinetic energy
ε
. Single atmospheric and oceanic soundings cannot indicate either the source of turbulence or the stage of its evolution; different values of
C
are expected for different turbulence sources and stages of the turbulence evolution and thus cannot be expected to yield quantitatively reliable turbulence parameters from individual profiles. The variation of
C
with the stage of turbulence evolution is illustrated for direct numerical simulation (DNS) results for gravity wave breaking. Results from a DNS model of multiscale initiation and evolution of turbulence with a Reynolds number Re (which is defined using the vertical wavelength of the primary gravity wave and background buoyancy period as length and time scales, respectively) of 100 000 are sampled as in sounding of the atmosphere and ocean, and various averaging of the sounding results indicates a convergence to a well-defined value of
C
, indicating that applying Thorpe analysis to atmospheric or oceanic soundings and averaging over a number of profiles gives more reliable turbulence determinations. The same averaging study is also carried out when the DNS-modeled turbulence is dominated by turbulence growing from the initial instabilities, when the turbulence is fully developed, when the modeled turbulence is decaying, and when the turbulence is in a still-later decaying stage. These individual cases converge to well defined values of
C
, but these values of
C
show a large variation resulting from the different stages of turbulence evolution. This study gives guidance as to the accuracy of Thorpe analysis of turbulence as a function of the number of profiles being averaged. It also suggests that the values of
C
in different environments likely depend on the dominant turbulence initiation mechanisms and on the Reynolds number of the environment.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Numerical simulations are used to study gravity wave (GW) propagation, instability, and breaking in the lower thermosphere. Compressible effects are accounted for via an anelastic formulation of the ...equations of motion and we employ a realistic description of the background thermodynamic state. An initially low‐amplitude, monochromatic GW with horizontal wavelength 60 km and intrinsic frequencyN/3.7 is introduced at the lower boundary and allowed to propagate to higher altitudes. The GW steepens as it propagates upward and displays instability and breaking over the altitude range ∼120–160 km. The effects of momentum deposition due to viscous attenuation and wave breaking are studied by comparing simulations which either include or exclude induced changes to the mean wind. These two cases also bound the range of expected behavior for horizontally localized GW packets. When induced changes to the mean wind are excluded, instability and turbulence occur over a broad altitude range spanning several vertical wavelengths. In contrast, the region of instability and turbulence is confined to a much more limited altitude range when induced mean wind effects are included. Wave breaking and turbulence in this case are largely confined within a shear layer formed by GW momentum transport. In time, the shear layer evolves into a critical level which consumes nearly all of the incident GW energy.
Key Points
Numerical simulation of gravity wave breaking in compressible atmosphere
Wave breaking is predicted to exist above 150 km
Wave momentum deposition has a profound effect on the mean wind
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.
Results of two‐dimensional and narrow three‐dimensional (2‐D and 2.5‐D) simulations of a gravity wave (GW) packet localized in altitude and along its propagation direction employing a new, versatile ...compressible model are described. The simulations explore self‐acceleration and instability dynamics in an idealized atmosphere at rest under mean solar conditions in a domain extending to an altitude of 260 km and 1,800 km horizontally without artificial dissipation. High resolution in the central 2.5‐D domain enables the description of 3‐D instability dynamics accounting for breaking, dissipation, and momentum deposition within the GW packet. 2‐D results describe responses to localized self‐acceleration effects, including generation of secondary GWs (SGWs) at larger scales able to propagate to much higher altitudes. 2.5‐D results exhibit instability forms consistent with previous 3‐D simulations of instability dynamics and cause SGW generation and propagation at smaller spatial scales to weaken significantly compared to the 2‐D results. SGW responses at larger scales are driven primarily by GW/mean flow interactions arising at early stages of the self‐acceleration dynamics prior to strong GW instabilities and dissipation. As a result, they exhibit similar responses in both the 2‐D and 2.5‐D simulations and readily propagate to high altitudes at large distances from the initial GW packet. A companion paper examines these dynamics for an initial GW packet localized in three dimensions and evolving in a representative 3‐D tidal wind field.
Key Points
Localized GW packets exhibit strong self‐acceleration dynamics as their amplitudes increase
Self‐acceleration causes cessation of vertical propagation, local 3‐D instabilities and dissipation, and SGW and AW generation
SGW has spatial scales determined largely by the packet scales and thus have larger scales and phase speeds that easily reach higher altitudes
ABSTRACT We present an initial survey in the southern sky of the sporadic meteoroid orbital environment obtained with the Southern Argentina Agile MEteor Radar (SAAMER) Orbital System (OS), in which ...over three-quarters of a million orbits of dust particles were determined from 2012 January through 2015 April. SAAMER-OS is located at the southernmost tip of Argentina and is currently the only operational radar with orbit determination capability providing continuous observations of the southern hemisphere. Distributions of the observed meteoroid speed, radiant, and heliocentric orbital parameters are presented, as well as those corrected by the observational biases associated with the SAAMER-OS operating parameters. The results are compared with those reported by three previous surveys performed with the Harvard Radio Meteor Project, the Advanced Meteor Orbit Radar, and the Canadian Meteor Orbit Radar, and they are in agreement with these previous studies. Weighted distributions for meteoroids above the thresholds for meteor trail electron line density, meteoroid mass, and meteoroid kinetic energy are also considered. Finally, the minimum line density and kinetic energy weighting factors are found to be very suitable for meteroid applications. The outcomes of this work show that, given SAAMER's location, the system is ideal for providing crucial data to continuously study the South Toroidal and South Apex sporadic meteoroid apparent sources.
Abstract
A new platform for high-resolution in situ measurements in the lower troposphere is described and its capabilities are demonstrated. The platform is the small GPS-controlled DataHawk ...unmanned aerial system (UAS), and measurements were performed under stratified atmospheric conditions at Dugway Proving Ground, Utah, on 11 October 2012. The measurements included spiraling vertical profiles of temperature and horizontal wind vectors, from which the potential temperature
θ
, mechanical energy dissipation rate
ε
, Brunt–Väsälä frequency
N
, temperature structure parameter
C
T
2
, Thorpe and Ozmidov scales
L
T
and
L
O
, and Richardson number Ri were inferred. Profiles of these quantities from ~50 to 400 m reveal apparent gravity wave modulation at larger scales, persistent sheet-and-layer structures at scales of ~30–100 m, and several layers exhibiting significant correlations of large
ε
,
C
T
2
,
L
T
, and small Ri. Smaller-scale flow features suggest local gravity waves and Kelvin–Helmholtz instabilities exhibiting strong correlations, yielding significant vertical displacements and inducing turbulence and mixing at smaller scales. Comparisons of these results with a direct numerical simulation (DNS) of similar multiscale dynamics indicate close agreement between measured and modeled layer character and evolution, small-scale dynamics, and turbulence intensities. In particular, a detailed examination of the potential biases in inferred quantities and/or misinterpretation of the underlying dynamics as a result of the specific DataHawk sampling trajectory is carried out using virtual sampling paths through the DNS and comparing these with the DataHawk measurements.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Dong et al. (2020, https://doi.org/10.1029/2019JD030691) employed a new compressible model to examine gravity wave (GW) self‐acceleration dynamics, instabilities, secondary gravity wave (SGW) ...generation, and mean forcing for GW packets localized in two dimensions (2D). This paper extends the exploration of self‐acceleration dynamics to a GW packet localized in three dimensions (3D) propagating into tidal winds in the mesosphere and thermosphere. As in the 2D packet responses, 3D GW self‐acceleration dynamics are found to be significant and include 3D GW phase distortions, stalled GW vertical propagation, local instabilities, and SGW and acoustic wave generation. Additional 3D responses described here include refraction by tidal winds, localized 3D instabilities, asymmetric SGW propagation, reduced SGW and acoustic wave responses at higher altitudes relative to 2D responses, and forcing of transient, large‐scale, 3D mean responses that may have implications for chemical and microphysical processes operating on longer time scales.
Key Points
3D gravity wave packets exhibit strong self‐acceleration, mean‐flow interactions, and instability dynamics
3D gravity wave packets yield strong local mean‐flow forcing and secondary gravity wave and acoustic wave generation
Secondary gravity waves are modulated by tidal winds and have large scales and large influences extending into the thermosphere
PDF
