Multi-Scale Kelvin-Helmholtz Instability Dynamics Observed by PMC Turbo on 12 July 2018: 2. DNS Modeling of KHI Dynamics and PMC Responses

Kjellstrand et al. (2021) describes the evolution and dynamics of a strong, large-scale Kelvin-Helmholtz instability (KHI) event observed in polar mesospheric clouds (PMCs) on 12 July 2018 by high-resolution imagers aboard the PMC Turbulence (PMC Turbo) stratospheric long-duration balloon experiment. The imaging provides evidence of KH billow interactions and instabilities that are strongly influenced by gravity waves at larger scales. Specific features include initially separated regions of KHI, secondary convective and KH instabilities of individual billows, and “tubes” and “knots” that arise where billow cores are mis-aligned or discontinuous along their axes. This study describes a direct numerical simulation of KH billow interactions in a periodic domain seeded with random initial noise that enables excitation of multiple KH billows exhibiting variable phase structures that capture multiple features of the observed KHI dynamics. Variable KH billow phases along their axes yield initial vortex tubes having diagonal alignments that link adjacent, but mis-aligned, billow cores. Weak initial vortex tubes and billow cores having nearly orthogonal alignments amplify, interact strongly, and drive intense vortex knots at these sites. These vortex tube & knot (T&K) dynamics excite “twist waves” that unravel the initial vortex tubes, and drive increasingly strong vortex interactions and a cascade of energy and enstrophy to successively smaller scales in the turbulence inertial range. The implications of T&K dynamics are much more rapid and intense breakdown and decay of the KH billows, and significantly enhanced energy dissipation rates, where these interactions occur

Mesospheric Bore Evolution and Instability Dynamics Observed in PMC Turbo Imaging and Rayleigh Lidar Profiling over Northeastern Canada on 13 July 2018

Two successive mesospheric bores were observed over northeastern Canada on 13 July 2018 in high-resolution imaging and Rayleigh lidar profiling of polar mesospheric clouds (PMCs) performed aboard the PMC Turbo long-duration balloon experiment. Four wide field-of-view cameras spanning an area of ~75x150 km at PMC altitudes captured the two evolutions occurring over ~2 hr and resolved bore and associated instability features as small as ~100 m. The Rayleigh lidar provided PMC backscatter profiling that revealed vertical displacements, evolving brightness distributions, evidence of instability character and depths, and insights into bore formation, ducting, and dissipation. Both bores exhibited variable structure along their phases, suggesting variable gravity wave (GW) source and bore propagation conditions. Both bores also exhibited small-scale instability dynamics at their leading and trailing edges. Those at the leading edges comprised apparent Kelvin-Helmholtz instabilities that were advected downward and rearward beneath the bore descending phases extending into an apparently intensified shear layer. Instabilities at the trailing edges exhibited alignments approximately orthogonal to the bore phases that resembled those seen to accompany GW breaking or intrusions arising in high-resolution modeling of GW instability dynamics. Collectively, PMC Turbo bore imaging and lidar profiling enabled enhanced definition of bore dynamics relative to what has been possible by previous ground-based observations, and a potential to guide new, three-dimensional modeling of bore dynamics. The observed bore evolutions suggest potentially important roles for bores in the deposition of energy and momentum transported into the mesosphere and to higher altitudes by high-frequency GWs achieving large amplitudes

PMC Turbo : Studying Gravity Wave and Instability Dynamics in the Summer Mesosphere Using Polar Mesospheric Cloud Imaging and Profiling From a Stratospheric Balloon

The Polar Mesospheric Cloud Turbulence (PMC Turbo) experiment was designed to observe and quantify the dynamics of small-scale gravity waves (GWs) and instabilities leading to turbulence in the upper mesosphere during polar summer using instruments aboard a stratospheric balloon. The PMC Turbo scientific payload comprised seven high-resolution cameras and a Rayleigh lidar. Overlapping wide and narrow camera field of views from the balloon altitude of ~38 km enabled resolution of features extending from ~20 m to ~100 km at the PMC layer altitude of ~82 km. The Rayleigh lidar provided profiles of temperature below the PMC altitudes and of the PMCs throughout the flight. PMCs were imaged during an ~5.9-day flight from Esrange, Sweden, to Northern Canada in July 2018. These data reveal sensitivity of the PMCs and the dynamics driving their structure and variability to tropospheric weather and larger-scale GWs and tides at the PMC altitudes. Initial results reveal strong modulation of PMC presence and brightness by larger-scale waves, significant variability in the occurrence of GWs and instability dynamics on time scales of hours, and a diversity of small-scale dynamics leading to instabilities and turbulence at smaller scales. At multiple times, the overall field of view was dominated by extensive and nearly continuous GWs and instabilities at horizontal scales from ~2 to 100 km, suggesting sustained turbulence generation and persistence. At other times, GWs were less pronounced and instabilities were localized and/or weaker, but not absent. An overview of the PMC Turbo experiment motivations, scientific goals, and initial results is presented here. © 2019. The Authors

Gravity Wave Breaking and Vortex Ring Formation Observed by PMC Turbo

Polar mesospheric cloud (PMC) imaging and lidar profiling performed aboard the 5.9 day PMC Turbo balloon flight from Sweden to northern Canada in July 2018 revealed a wide variety of gravity wave (GW) and instability events occurring nearly continuously at approximately 82 km. We describe one event exhibiting GW breaking and associated vortex rings driven by apparent convective instability. Using PMC Turbo imaging with spatial and temporal resolution of 20 m and 2 s, respectively, we quantify the GW horizontal wavelength, propagation direction, and apparent phase speed. We identify vortex rings with diameters of 2‐5 km and horizontal spacing comparable to their size. Lidar data show GW vertical displacements of ±0.3 km. From the data, we find a GW intrinsic frequency and vertical wavelength of 0.009 ± 0.003 rad s‐1 and 9 ± 4 km, respectively. We show that these values are consistent with the predictions of numerical simulations of idealized GW breaking. We estimate the momentum deposition rate per unit mass during this event to be 0.04 ± 0.02 m s‐2 and show that this value is consistent with the observed GW. Comparison to simulation gives a mean energy dissipation rate for this event of 0.05‐0.4 W kg‐1, which is consistent with other reported in‐situ measurements at the Arctic summer mesopause

PMC-Turbo: a balloon-borne Mission to image gravity waves and turbulence in polar mesospheric clouds

PMC-Turbo is a balloon-borne experiment that will fly at an altitude between 35 and 40 km. It is designed to record gravity wave events in polar mesospheric clouds with high spatial and temporal resolution as they unfold across a large field of the sky. The project is motivated by the serendipitous observation of PMCs during the balloon flight of EBEX, an observational cosmology experiment which flew in 2013 at an altitude of about 35 km. EBEX included two star cameras, each of which had a field of view of 4 by 3 degrees, a resolution of 2.5 m at 80 km altitude, and an image cadence of 30 seconds. Even though EBEX was not designed to observe PMCs, instability and turbulent structures were visible with features at scales down to 20 m in the star camera images. However, it is difficult to put the images in context due to the inconsistent pointing, slow image cadence, and the narrow field of view. PMC-Turbo was designed leverage the strengths of the EBEX star cameras to observe gravity wave events at various length scales. This requires capturing a wide view while remaining sensitive to small features, as well as recording images at a high cadence. It carries seven cameras, four of which are wide field cameras that together cover a field of view of about 150 by 40 degrees with an 8 m per pixel resolution. Cameras with narrow field lenses provide smaller fields of view of 10 by 15 degrees with about 3 m per pixel resolution and are situated within in the larger field of view. The cameras can sustain 3.5 frames per second and can capture bursts of images up to 8 frames per second. The payload also carries BOLIDE, a Rayleigh lidar from the DLR Institute of Atmospheric Physics and an airglow camera from Utah State University. These instruments will provide additional context to observed events in the form of thermal profiles and infrared mapping. The Balloon Lidar Experiment BOLIDE is a miniaturized Rayleigh backscatter lidar developed for PMC-Turbo that will provide observations of PMC with unprecedented resolution and signal to noise ratio. PMC-Turbo is scheduled to fly next year from either Sweden or Antarctica. We anticipate a fourteen day flight over Antarctica, and we expect to capture about 14 million images. An arctic flight would last around 5 days, but we anticipate several gravity wave events during this time. In addition to lab testing of our equipment, we have had opportunities to collect data with the PMC-Turbo instruments in the field. This December we will fly one camera as a piggyback on the Super Tiger payload from Antarctica. In July, we used several cameras on the ground to capture PMC images in High Level, Alberta. We hope to resolve tomography from the images captured during that campaign. If we fly from Sweden, we plan to coordinate ground-based tomographic imaging with the balloon flight

PMC Turbo: Studying Gravity Wave and Instability Dynamics in the Summer Mesosphere Using Polar Mesospheric Cloud Imaging and Profiling From a Stratospheric Balloon

The Polar Mesospheric Cloud Turbulence (PMC Turbo) experiment was designed to observe and quantify the dynamics of small‐scale gravity waves (GWs) and instabilities leading to turbulence in the upper mesosphere during polar summer using instruments aboard a stratospheric balloon. The PMC Turbo scientific payload comprised seven high‐resolution cameras and a Rayleigh lidar. Overlapping wide and narrow camera field of views from the balloon altitude of ~38 km enabled resolution of features extending from ~20 m to ~100 km at the PMC layer altitude of ~82 km. The Rayleigh lidar provided profiles of temperature below the PMC altitudes and of the PMCs throughout the flight. PMCs were imaged during an ~5.9‐day flight from Esrange, Sweden, to Northern Canada in July 2018. These data reveal sensitivity of the PMCs and the dynamics driving their structure and variability to tropospheric weather and larger‐scale GWs and tides at the PMC altitudes. Initial results reveal strong modulation of PMC presence and brightness by larger‐scale waves, significant variability in the occurrence of GWs and instability dynamics on time scales of hours, and a diversity of small‐scale dynamics leading to instabilities and turbulence at smaller scales. At multiple times, the overall field of view was dominated by extensive and nearly continuous GWs and instabilities at horizontal scales from ~2 to 100 km, suggesting sustained turbulence generation and persistence. At other times, GWs were less pronounced and instabilities were localized and/or weaker, but not absent. An overview of the PMC Turbo experiment motivations, scientific goals, and initial results is presented here

Kelvin Helmholtz Instability “Tube” & “Knot” Dynamics, Part I: Expanding Observational Evidence of Occurrence and Environmental Influences

Multiple recent observations in the mesosphere have revealed large-scale Kelvin-Helmholtz instabilities (KHI) exhibiting diverse spatial features and temporal evolutions. The first event reported by Hecht et al. (2021) exhibited multiple features resembling those seen to arise in early laboratory shear-flow studies described as “Tube” and “Knot” (T&K) dynamics by Thorpe (1985, 1987). The potential importance of T&K dynamics in the atmosphere, and in the oceans and other stratified and sheared fluids, is due to their accelerated turbulence transitions and elevated energy dissipation rates relative to KHI turbulence transitions occurring in their absence. Motivated by these studies, we survey recent observational evidence of multi-scale Kelvin-Helmholtz instabilities throughout the atmosphere, many features of which closely resemble T&K dynamics observed in the laboratory and idealized initial modeling. These efforts will guide further modeling assessing the potential importance of these T&K dynamics in turbulence generation, energy dissipation, and mixing throughout the atmosphere and other fluids. We expect these dynamics to have implications for parameterizing mixing and transport in stratified shear flows in the atmosphere and oceans that have not been considered to date. Companion papers describe results of a multi-scale gravity wave direct numerical simulation (DNS) that serendipitously exhibits a number of KHI T&K events and an idealized multi-scale DNS of KHI T&K dynamics without gravity wave influences

Lidar Soundings of Noctilucent Clouds during the PMC Turbo Balloon Mission

Noctilucent clouds are optically thin layers of ice particles occuring around 83 km altitude during polar summer. Their intriguing fine-scale structure provides a means to study atmospheric waves and instabilities in the sensitive mesopause region of our atmosphere. For the first time, noctilucent clouds were observed using a backscatter lidar and multiple cameras from a balloon platform. The NASA long-duration balloon PMC-Turbo was launched in July 2018 from Kiruna, Sweden, and floated at 40 km altitude during six days to northern Canada. During the mission, a large dataset with unprecedented high-resolution soundings of noctilucent clouds down to scales of few meters were obtained. The combination of near-vertical lidar soundings with horizontal structures visible in narrow- and wide-field of view cameras allows to fully characterize the morphological structures of noctilucent clouds that are modulated by gravity waves, and reveal dynamic processes such as the breaking of these waves, the generation of various types of instabilities and transitions to turbulence

Mesospheric Bore Evolution and Instability Dynamics Observed in PMC Turbo Imaging and Rayleigh Lidar Profiling Over Northeastern Canada on 13 July 2018

Two successive mesospheric bores were observed over northeastern Canada on 13 July 2018 in high-resolution imaging and Rayleigh lidar profiling of polar mesospheric clouds (PMCs) performed aboard the PMC Turbo long-duration balloon experiment. Four wide field-of-view cameras spanning an area of ~75x150 km at PMC altitudes captured the two evolutions occurring over ~2 hr and resolved bore and associated instability features as small as ~100 m. The Rayleigh lidar provided PMC backscatter profiling that revealed vertical displacements, evolving brightness distributions, evidence of instability character and depths, and insights into bore formation, ducting, and dissipation. Both bores exhibited variable structure along their phases, suggesting variable gravity wave (GW) source and bore propagation conditions. Both bores also exhibited small-scale instability dynamics at their leading and trailing edges. Those at the leading edges comprised apparent Kelvin-Helmholtz instabilities that were advected downward and rearward beneath the bore descending phases extending into an apparently intensified shear layer. Instabilities at the trailing edges exhibited alignments approximately orthogonal to the bore phases that resembled those seen to accompany GW breaking or intrusions arising in high-resolution modeling of GW instability dynamics. Collectively, PMC Turbo bore imaging and lidar profiling enabled enhanced definition of bore dynamics relative to what has been possible by previous ground-based observations, and a potential to guide new, three-dimensional modeling of bore dynamics. The observed bore evolutions suggest potentially important roles for bores in the deposition of energy and momentum transported into the mesosphere and to higher altitudes by high-frequency GWs achieving large amplitudes