Thermal Duct Research Articles

A Statistical Study of Polar Mesospheric Cloud Fronts in the Northern Hemisphere

AbstractComplex spatial structures in polar mesospheric cloud (PMC) images provide visual clues to the dynamics that occur in the summer mesosphere. In this study, we document one such structure, a PMC front, by analyzing PMC images in the northern hemisphere from the Cloud Imaging and Particle Size (CIPS) instrument onboard the aeronomy of ice in the mesosphere (AIM) satellite. A PMC front is defined as a sharp boundary that separates cloudy and mostly clear regions, and where the clouds at the front boundary are brighter than the clouds in the cloudy region. We explore the environment that supports the formation of PMC fronts using near‐coincident temperature and water vapor observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instrument. A comparison of PMC front locations to near‐coincident temperature profiles reveals the presence of inversion layers at PMC altitudes. The adiabatic and superadiabatic topside lapse rates of these temperature inversions indicate that some of the identified inversion layers may have been formed by gravity wave (GW) dissipation. The structure of the squared buoyancy frequency profiles indicates a stable layer or thermal duct that can be associated with large‐amplitude mesospheric inversion layers (MILs) that extend large distances. These inversion layers may be conducive to horizontal wave propagation. We hypothesize that ducted GWs may be a formation mechanism of PMC fronts.

Computation of non-Newtonian quadratic convection in electro-magneto-hydrodynamic (EMHD) duct flow with temperature-dependent viscosity

Motivated by novel developments in smart non-Newtonian thermal duct systems, a theoretical study has been presented in this article for electro-magneto-hydrodynamic (EMHD) buoyancy-driven flow of a fourth-grade viscoelastic fluid in a vertical duct with quadratic convection. The viscosity of the fourth-grade fluid model is assumed to be temperature-dependent, and the Reynolds exponential model is deployed. Viscous heating and Joule dissipation effects are included. The duct comprises a pair of parallel electrically insulated vertical flat plates located a finite distance apart. Via suitable scaling transformations, a nonlinear boundary value problem is derived for the momentum and heat transport. A homotopy perturbation method (HPM) solution is obtained coded in Mathematica symbolic software. There is a considerable enhancement in wall skin friction with an increment in fourth-grade fluid parameter, Brinkman number, electrical field parameter, thermal buoyancy parameter, and quadratic thermal convection parameter. However, skin friction is strongly reduced with a rise in variable viscosity parameter, Hartmann (magnetic) number, and electromagnetic heat generation to conduction ratio. Nusselt number magnitudes are elevated with increase in variable viscosity parameter, thermal buoyancy parameter, and quadratic thermal convection parameter, whereas they are significantly suppressed with increment in fourth-grade fluid parameter, Brinkman number, and Hartmann magnetic number.

A case study of mesospheric frontal interaction and associated processes over the western Himalaya

An interesting event of interaction between a mesospheric bore and a front is observed in O(1S) 557.7 nm airglow images over the western Himalayan region on the night of 25 April 2022. The event is unique as it is the first report of a mesospheric bore interacting with a typical mesospheric front. The vertical profiles of temperature and Brunt Vaisala frequency indicate presence of a strong mesospheric inversion layer (MIL) which acts as a stable thermal duct for propagation of the mesospheric bore. Chemical heating is believed to be a causative mechanism for sustaining such strong MIL/thermal duct, although the presence of Doppler duct could not be verified due to a lack of wind measurement. The bore front shows an anti-clockwise rotation with time which is attributed to the differential phase speed between the extreme parts of the bore due to variation of duct depth. The bore propagation above is believed to push the underlying OH layer downward, resulting in a maximum horizontal slope of the peak height of the OH volume emission rate (VER) on the event day. The results highlight the characteristics of bore-front interaction, bore propagation supported by the thermal duct, and its effect on the vertical displacement of the airglow emission layers.

Investigation of a Dissipating Mesospheric Bore Using Airglow Imager and Direct Numerical Simulation

AbstractAtmospheric gravity waves play an important role in driving the dynamics of the Mesosphere and Lower Thermosphere and the basic structure of this region is determined by momentum deposition of these waves. Mesospheric bores are a type of non‐linear response that cause the amplification of gravity wave, due to trapping, that is characterized by a propagating step‐like jump followed by undulating waves. They require a stable layer or duct to travel horizontally with little attenuation thereby capable of transporting wave energy and momentum over larger distances. We present a prominent bright undular bore event observed in the mesospheric O(1S), O2, and OH emission layers on 16 March 2021 over Germany. A striking feature of this observation is the capture of bore's rapid dissipation around the center of the imager's field of view. The vertical temperature profile obtained from the satellite data indicates the presence of temperature inversion layer which acted as a thermal duct for the bore propagation. In addition, we have performed idealized two dimensional direct numerical simulations (DNS) of Navier‐Stokes equations under Boussinesq approximation. The DNS results reproduce many important characteristics of the observed airglow event like the nonlinear wave‐steepening, number of trailing waves, and its dissipation by implementing a thermal duct and a wave‐like perturbation. Furthermore, the DNS results also indicate that the duct width and amplitude of the initial perturbation have a considerable effect on the bore morphology.

Electro-magnetohydrodynamics hybrid nanofluid flow with gold and magnesium oxide nanoparticles through vertical parallel plates

The hybrid nanofluid flow under suspension of Gold and Magnesium oxide nanoparticles (Au/MgO-NPs) propagating between vertical parallel plates is investigated. Sodium alginate third-grade non-Newtonian fluid is used as the base fluid. The effect of electro-magnetohydrodynamics is also taken into account. The energy equation also includes the effect of Joule heating and viscous dissipation. Due to the nonlinear nature of the formulated differential equations, perturbation strategy is utilized to acquire the analytical solutions. Discussion and plotting are presented with respect to most significant parameters. It is analyzed that the rate of heat transfer is dramatically increased, and this is owing to an increase in the thermal conductivity of the fluid due to hybrid nanofluid. With increment in buoyancy convection parameter and electric field parameter, the flow is accelerated. It is also noted that the temperature is boosted with increasing nanoparticle volume fraction of both magnesium oxide and gold nanoparticles. A comparison with previously studied results is also included. The applications of the work include novel thermal duct processing technologies in biomedical, nuclear and process engineering.

A case study of a ducted gravity wave event over northern Germany using simultaneous airglow imaging and wind-field observations

Abstract. An intriguing and rare gravity wave event was recorded on the night of 25 April 2017 using a multiwavelength all-sky airglow imager over northern Germany. The airglow imaging observations at multiple altitudes in the mesosphere and lower thermosphere region reveal that a prominent upward-propagating wave structure appeared in O(1S) and O2 airglow images. However, the same wave structure was observed to be very faint in OH airglow images, despite OH being usually one of the brightest airglow emissions. In order to investigate this rare phenomenon, the altitude profile of the vertical wavenumber was derived based on colocated meteor radar wind-field and SABER temperature profiles close to the event location. The results indicate the presence of a thermal duct layer in the altitude range of 85–91 km in the southwest region of Kühlungsborn, Germany. Utilizing these instrumental data sets, we present evidence to show how a leaky duct layer partially inhibited the wave progression in the OH airglow emission layer. The coincidental appearance of this duct layer is responsible for the observed faint wave front in the OH airglow images compared O(1S) and O2 airglow images during the course of the night over northern Germany.

Signature of a mesospheric bore in 557.7 nm airglow emission using all-sky imager at Hanle (32.7oN, 78.9oE)

A prominent signature of dark bore front in the mesospheric O( 1 S) airglow emission is observed on a night in the late winter at Hanle (32.7 o N, 78.9 o E) located in the western Himalaya. The leading front was followed by a series of evident trailing waves and the event lasts for more than two hours. The characteristic features of the bore indicate it to be linear undular type. Instantaneous temperature profile shows presence of stable region through formation of thermal duct in the upper mesosphere possibly supported by chemistry and/or dynamics. With time, the bore fronts became faint in presence of ripples as they reached other side of imager field of view (FOV). During the initial period around 3–4 waves h −1 appear to enter the imager FOV. The observed average phase speed, period and horizontal wavelength are found to be 40 m/s, 12 min and 29 km, respectively. The bore fronts exhibit a clockwise rotation at a rate of around 5° h −1 . The front edge perpendicular to the direction of propagation shows small-amplitude undulation indicating nonuniform duct structure. The tropospheric meteorological conditions may indicate plausible contribution from jet stream, weather front (linked with the Himalayan orography), and nonmigrating tides to excite and sustain the mesospheric bore event although further investigations in this direction are being sought to understand the actual underlying physical processes.

Augmentation of heat transfer via nanofluids in duct flows using Fourier-type conditions: Theoretical and numerical study

Motivated by developments in thermal duct processing, an investigation is presented to study the behavior of viscous nanoparticle suspensions flowing in a vertical duct subject to Fourier-type conditions. The left wall temperature is kept lower than that of the right wall. Brownian motion and thermophoresis which are invoked via the presence of nanoparticles are incorporated in the study. Numerical solutions with an efficient Runge–Kutta shooting method are also presented at all values of the control parameters. The impact of thermal Grashof number [Formula: see text], Eckert number [Formula: see text], thermophoresis [Formula: see text], and Brownian motion parameters [Formula: see text] on the velocity, temperature, and nanoparticle concentration distributions for identical [Formula: see text] and differing Biot numbers [Formula: see text] (at the duct walls) are computed and visualized graphically. With vanishing thermophoresis and Brownian motion parameters, the solutions match exactly with the earlier Newtonian viscous flow computations. Symmetric and asymmetric wall heat conditions are also acknowledged. Intensifying the thermal Grashof number, Eckert number, thermophoresis parameter, and Brownian parameter serve to amplify magnitudes of the velocity and temperature, whereas the nanoparticle concentration field is suppressed. The skin friction and Sherwood number are also computed with various combinations of the flow control parameters. Nusselt number values at the hot duct wall are enhanced with an increase in thermal buoyancy parameter, Eckert number, Brownian motion parameter, and thermophoresis parameter for equal Biot numbers. The opposite trend is computed for different Biot numbers. For any given values of Biot numbers, the mean velocity and bulk temperature are boosted with increase in thermal buoyancy parameter, Eckert number, Brownian motion parameter, and thermophoresis parameter. Hence, it may be inferred that the transport characteristics computed using Fourier-type boundary conditions are substantially different from those based on isothermal boundary conditions in nanofluid duct flows.

A Numerical Study of Gravity Waves Propagation Characteristics in the Mesospheric Doppler Duct

AbstractDong et al. (2018, https://doi.org/10.1029/2018JD029190, hereafter D18) employed a compressible model to examine the long‐distance horizontal propagation of gravity waves (GWs) in a stratospheric thermal duct in a two‐dimensional domain. Here, we extend this previous work to explore the ducting of GWs in the mesospheric Doppler duct. While most of the characteristics of the thermal duct described in D18 are found to be operative in the Doppler duct, there are some important differences. The propagation velocity of the fundamental (0th) wave mode is always largest in the Doppler duct and smallest in the thermal duct. We find that regardless of the type of duct (Doppler or thermal duct) the energy dissipation rates of wave modes in the duct region are inversely proportional to their horizontal group velocities. Our results suggest that observations of ducted GWs, particularly those made from multiple observation sites, should reveal characteristics that are germane to the particular duct type. For example, we find that the frequency of the dominant wave mode in the Doppler duct increases with increasing distance from the wave source. This behavior is exactly the opposite of what was obtained for the thermal duct by D18.

A case study of a thermally ducted undular mesospheric bore accompanied by ripples over the western Himalayan region

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.

Characteristics of a mesospheric front observed in Polar Mesospheric Cloud fields

We report on a Polar Mesospheric Cloud (PMC) front structure observed on July 2, 2007 over Greenland. This structure appears to be localized solitary wave, with a sharp boundary that separates a cloud and cloud-free region. Near-coincident temperature measurements indicate a 20 K temperature difference between these two regions that likely contributed to the sharp PMC front boundary. Gravity wave (GW) temperature amplitude and buoyancy frequency show that large amplitude GWs and the formation of a stable atmospheric layer between two unstable regions supported the formation of a pronounced mesospheric temperature inversion that destroyed PMCs. Given the absence of an inversion layer close to the location of PMC front, it is not clear if a similar thermal duct but with colder temperatures supported the formation of a single wave resulting in the formation of the observed PMC front. The buoyancy frequency structure with stable and unstable regions also indicates mesospheric wave propagation, and is present in both the cloud and cloud-free regions. We identify a tropospheric low-pressure area and a frontal system as potential sources of these mesospheric GWs. Ray-tracing simulations indicate that GWs from these sources propagated to the mesosphere and may have contributed to the observed PMC variability.

Formation of an additional density peak in the bottom side of the sodium layer associated with the passage of multiple mesospheric frontal systems

Abstract. We present a detailed investigation of the formation of an additional sodium density peak at altitudes of 79–85 km below the main peak of the sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromsø, Norway, on the night of 19 December 2014. The airglow imager observations of OH emissions revealed four passing frontal systems that resembled mesospheric bores, which typically occur in ducting regions of the upper mesosphere. For about 1.5 h, the lower-altitude sodium peak had densities similar to that of the main peak of the layer around 90 km. The lower-altitude sodium peak weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column-integrated sodium densities increased gradually during the formation of the lower-altitude sodium peak. Temperatures measured with the lidar indicate that there was a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in the OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km, wherein the temperature was also higher. Both factors would have liberated sodium atoms from the reservoir species and suppressed the reconversion of atomic sodium into reservoir species so that the lower-altitude sodium peak could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals.

Mathematical Modelling of Triple Diffusion in Natural Convection Flow in a Vertical Duct with Robin Boundary Conditions, Viscous Heating, and Chemical Reaction Effects

The triple-diffusive convective flow (thermal diffusion and dual species diffusion) in a viscous fluid flowing within a vertical duct is investigated subject to Robin boundary conditions at the duct walls. Viscous heating and homogenous chemical reaction effects are included. The mass transfer (solutal) buoyancy effects due to concentration gradients of the dispersed components are taken into account using the Boussinesq approximation. Symmetric and asymmetric wall conditions for the temperature are taken into account. The conservation equations are rendered into dimensionless form via suitable transformations and the emerging ordinary differential equations feature a number of dimensionless parameters including thermal Grashof number, two solutal Grashof numbers (one for each of the diffusing components i.e. species 1 and species 2), left and right duct wall thermal Biot numbers, species 1 and species 2 chemical reaction parameters, Brinkman number and temperature difference ratio.These coupled and nonlinear dimensionless conservation equations are solved numerically using theRunge-Kutta shooting method. The solutions obtained numerically are validated with approximate analytical solutions obtained via a regular perturbation method which are valid for small values of Brinkman number.The impact of selected parameters on velocity, temperature and dual species concentration distributions is visualized graphically. Furthermore, the variation of skin friction and Nusselt number with these parameters is also tabulated. The solutions obtained numerically and analytically are found to be equal in the absence of viscous dissipation. However, the deviation is magnified with large values of Brinkman number. In the absence of chemical reaction, the results concur with the earlier computations of Zanchini (1998). Increasing second species solutal Grashof number is observed to decelerate the flow in the left duct half space, to accelerate the flow in the right duct half space and consistently reduce temperatures across the entire duct width. With increasing species 1 chemical reaction parameter the concentration magnitudes are elevated in the left duct half space whereas they are depressed in the right duct half space. A similar response is computed for the influence of species 2 reaction parameter on the concentration profile. Temperatures are strongly enhanced across the duct width with increasing Brinkman number and are symmetric in nature about the channel centerline for the symmetric Biot number case (equal thermal Biot numbers at the left and right walls). These profiles are morphed for the asymmetric Biot number case (equal thermal Biot numbers at the left and right walls). Temperatures descend from the left wall to the right wall, although they are still enhanced with increasing Brinkman number. The simulations are relevant to geochemical transport phenomena, industrial materials processing and thermal duct design.

A Case Study of the Stratospheric and Mesospheric Concentric Gravity Waves Excited by Thunderstorm in Northern China

In this paper, the complete process in which a concentric gravity wave (CGW), excited by a tropospheric thunderstorm, propagated into the stratosphere and mesosphere in Northern China is investigated. A strong thunderstorm developed in the middle of the Inner Mongolia autonomous region on the night of 10th August 2013. The stratospheric temperature perturbation, caused by the CGW, was observed by the Atmospheric Infrared Sounder (AIRS) at 02:11 LT 11th August 2013. An all-sky OH imager at the Shuozhou station (39.8° N, 112.1° E), supported by the Meridian Space Weather Monitoring Project, measured the mesospheric CGW between 22:00 LT to 23:00 LT on the night. It was certified that both the stratospheric and mesospheric CGWs were triggered by the aforementioned thunderstorm, and the excitation source was calculated to be located at (40.59° N, 108.67° E) by employing the dispersion relation. The CGWs were excited in the initial stage of the thunderstorm. The temperature and wind field data obtained by SABER and meteoric radar, respectively, were used to evaluate the background properties of the respective propagation regions. The result shows that an obvious thermal duct structure, with a positive squared vertical wavenumber (m2) existed around the OH layer.

A Numerical Study of Gravity Wave Propagation Characteristics in the Stratospheric Thermal Duct

AbstractWe developed a fully nonlinear, two‐dimensional, numerical model to simulate the long horizontal distance propagations of atmospheric gravity waves in a stratospheric thermal duct. The numerical results show that after the wind disturbance excited by the initial wave forcing enters the duct completely, symmetric and antisymmetric modes can be identified in the horizontal direction. The frequency‐wave number spectrum indicates that various modes with different spatial structures can exist simultaneously in the duct. The primary parameters (horizontal wavelength, vertical wavelength and wave frequency) of the ducted wave packets in a given thermal duct are mainly determined by the horizontal wavelength of the initial wave forcing. The mean ratio of the upward propagating energy to the downward propagating energy in the thermal duct was regarded as an approximation of the power standing wave ratio (PSWR). The time variation of PSWR indicates that the standing wave was formed with the propagation of the ducted waves. During the gravity waves ducted propagation, most wave energy is restricted to the ducting region and dissipates slowly with time. The higher the wave frequency (or the shorter the horizontal wavelength) of the initial wave forcing, the slower the wave energy dissipates.

Low‐latitude gravity wave variances in the mesosphere and lower thermosphere derived from SABER temperature observation and compared with model simulation of waves generated by deep tropical convection

AbstractA portion of waves generated by deep convection have scales and amplitudes large enough to be detected by spaceborne instruments. We have analyzed temperature data from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite for subtidal‐scale fluctuations. Filtering was applied both vertically and horizontally to extract wave variances. We have analyzed the altitude region between 70 and 130 km and focus on the variances at equatorial latitudes for the altitude region between 70 and 120 km as a function of season, local time intervals, geographical location, and altitude. We find significant variances where convection is particularly prolific (Intertropical Convergence Zone) and at altitudes where wave trapping is known to be favored (e.g., the lower thermospheric duct). The locations of significant variances persist from year to year. Standard deviations of a few tens of kelvins are found. We have also performed simulations of the response to deep tropical convection with a time‐dependent, high‐resolution fully compressible dynamical model. Our simulations give wave amplitudes that agree reasonably well with the observed amplitudes and show layering that is consistent with the observations. Our main finding is that significant variations seen in TIMED/SABER temperature data have a convective wave source and are concentrated in layers where thermal ducts occur.

Coordinated airglow observations between IMAP/VISI and a ground‐based all‐sky imager on concentric gravity wave in the mesopause

AbstractWe present a study of concentric gravity waves (CGWs) event from the coordinated observation between Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping (IMAP)/Visible and near‐Infrared Spectral Imager (VISI), all‐sky camera at Rikubetsu, Multi‐functional Transport Satellite (MTSAT), Tropical Rainfall Measuring Mission, and MF radar at Wakkanai combined with Modern‐Era Retrospective Analysis for Research and Application data. IMAP/VISI is the first space‐based imager that capable of imaging the airglow in the mesosphere and lower thermosphere region in the nadir‐looking direction. Therefore, it has a unique ability to observe a great extend of CGWs propagation. Arc‐like shaped, part of CGWs pattern was observed around the mesopause (~95 km) in the O2 762 nm airglow emission obtained by IMAP/VISI at 1204 UT on 18 October 2012. Similar patterns were also observed by the all‐sky imager at Rikubetsu (43.5°N, 143.8°E) in OI 557.7 nm and OH band airglow emissions from ~1100 to 1200 UT. Horizontal wavelengths of the observed small‐scale gravity waves are ~50 km (OH band and OI 557.7 nm) and ~67 km (O2 762 nm). The source is suggested to be a deep convective activity over Honshu Island which likely was an enhanced convective activity related to a typhoon in the south of Japan. The data showed that the CGWs could propagate up to ~1400–1500 km horizontally from the source to the mesopause but not farther away. Using atmospheric temperature profiles obtained by Thermospheric Ionosphere Mesosphere Energetics Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry, we conclude that this long‐distance propagation of the waves could be caused by thermal duct in the middle atmosphere. The arc‐like shaped instead of full circle pattern points out that the wind filtering effect is significant for the particular direction of wave propagation.

Numerical and statistical evidence for long‐range ducted gravity wave propagation over Halley, Antarctica

Abundant short‐period, small‐scale gravity waves have been identified in the mesosphere and lower thermosphere over Halley, Antarctica, via ground‐based airglow image data. Although many are observed as freely propagating at the heights of the airglow layers, new results under modeled conditions reveal that a significant fraction of these waves may be subject to reflections at altitudes above and below. The waves may at times be trapped within broad thermal ducts, spanning from the tropopause or stratopause to the base of the thermosphere (∼140 km), which may facilitate long‐range propagation (∼1000s of km) under favorable wind conditions.

Generalized Models for Laminar Developing Flows in Heat Sinks and Heat Exchangers

Recent models for laminar friction and heat transfer in non-circular ducts and channels are reviewed. Models for both hydrodynamically and thermally developing flows are presented. These models are based on the superposition of asymptotic characteristics for short and long ducts. The non-dimensional mean wall shear stress (friction factor-Reynolds number product) and non-dimensional heat transfer coefficient (Nusselt number) are shown to be only functions of the dimensionless hydrodynamic or thermal duct length, respectively, and the duct aspect ratio. This is achieved by means of using a new transversal length scale, the square root of cross-sectional area, rather than the hydraulic diameter. Additional definitions more appropriate to single fluid devices such as heat sinks are also discussed.

Horizontal extension of acoustic resonance between the ground and the lower thermosphere

A time-dependent, two-dimensional, nonlinear, non-hydrostatic, compressible, numerical model is used to investigate the horizontal extension of acoustic waves trapped in a thermal duct between the lower thermosphere and the ground or the stratosphere, and evanescent and gravity waves around them. The simulation shows that an impulsive point source on the ground excites acoustic waves ducted in the mesosphere. The ducted acoustic waves have horizontal wavelengths longer than 130km and frequencies of 3.7–3.8mHz. This ducted acoustic waves are identified as acoustic resonance modes expected in previous studies. They extend to 300km distance from the source after 2h. In the case of the source with a finite width, the acoustic resonance would extend to 300km out of the edge of the source. The temporal variation of two resonance modes is also declared. In the vicinity of the source, two acoustic resonance modes are formed which have frequencies of 3.7 and 4.4mHz. The mode of 4.4mHz is dominant above the mesopause for about 50min after the impulsive perturbation of the source since the mode is leaky. On the other hand, the mode of 3.7mHz is well trapped in the thermal duct and persist over 2h. In addition to the acoustic waves, evanescent gravity waves with horizontal wavelengths shorter than 100km and with frequencies between 2.4 and 3.6mHz are excited in the stratosphere. Gravity waves freely propagating are also excited with frequencies lower than 2mHz and with horizontal wavelengths shorter than 130km.