Propagation Of Gravity Waves Research Articles

A new method for detecting solar atmospheric gravity waves.

Internal gravity waves have been observed in the Earth's atmosphere and oceans, on Mars and Jupiter, and in the Sun's atmosphere. Despite ample evidence for the existence of propagating gravity waves in the Sun's atmosphere, we still do not have a full understanding of their characteristics and overall role for the dynamics and energetics of the solar atmosphere. Here, we present a new approach to study the propagation of gravity waves in the solar atmosphere. It is based on calculating the three-dimensional cross-correlation function between the vertical velocities measured at different heights. We apply this new method to a time series of co-spatial and co-temporal Doppler images obtained by SOHO/MDI and Hinode/SOT as well as to simulations of upward propagating gravity wave-packets. We show some preliminary results and outline future developments. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

Gravity Wave Breaking and Vortex Ring Formation Observed by PMC Turbo

AbstractPolar 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.

Generation of Internal Gravity Waves Far from Moving Non-Local Source

We consider analytical solutions describing the generation of internal gravity waves far from a non-local source of disturbances. We suppose that the source moves on the surface of stratified medium of a finite depth. A model distribution of the non-local source shape with radial symmetry is used. This approximation correctly describes (qualitatively) the main spatiotemporal characteristics of natural sources of generation of internal gravity waves in the ocean. The resulting solution is the sum of wave modes. The solution is presented as a series of eigenfunctions of the spectral problem of internal gravity waves. The results of numerical calculations of internal gravity waves components at different depths are presented and discussed.

Detection of stratospheric gravity waves induced by the total solar eclipse of July 2, 2019

Atmospheric gravity waves generated by an eclipse were first proposed in 1970. Despite numerous efforts since, there has been no definitive evidence for eclipse generated gravity waves in the lower to middle atmosphere. Measuring wave characteristics produced by a definite forcing event such as an eclipse provides crucial knowledge for developing more accurate physical descriptions of gravity waves. These waves are fundamental to the transport of energy and momentum throughout the atmosphere and their parameterization or simulation in numerical models provides increased accuracy to forecasts. Here, we present the findings from a radiosonde field campaign carried out during the total solar eclipse of July 2, 2019 aimed at detecting eclipse-driven gravity waves in the stratosphere. This eclipse was the source of three stratospheric gravity waves. The first wave (eclipse wave #1) was detected 156 min after totality and the other two waves were detected 53 and 62 min after totality (eclipse waves #2 and #3 respectively) using balloon-borne radiosondes. Our results demonstrate both the importance of field campaign design and the limitations of currently accepted balloon-borne analysis techniques for the detection of stratospheric gravity waves.

Influence of gravity wave temperature anomalies and their vertical gradients on cirrus clouds in the tropical tropopause layer – a satellite-based view

Abstract. Negative temperature perturbations (T′) from gravity waves are known to be favorable to tropical tropopause layer (TTL) clouds, and recent studies have further suggested a possible role of dT′/dz in facilitating TTL cloud formation and maintenance. With a focus on exploring the influence of dT′/dz on TTL clouds, this study utilizes radio occultation temperature retrievals and cloud layers from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) to understand how gravity wave perturbations modulate cloud occurrence in the tropics. Cloud populations were evaluated in four phases corresponding to positive or negative T′ and dT′/dz. We find that 55 % of TTL clouds are found where T′ and dT′/dz are both negative. Regions of frequent convection are associated with higher cloud populations in the warm phase T′>0. We show that the partitioning of cloud population among wave phases exhibits dependence on background relative humidity. In the phase where T′ and dT′/dz are both negative, the mean cloud effective radius is the smallest of all four phases, but the differences are small. It is shown that the strongest mean negative T′ anomaly is centered on the cloud top, resulting in positive dT′/dz above the cloud top and negative dT′/dz below. This negative T′ anomaly propagates downward with time, characteristic of upward propagating gravity waves. Negative (positive) T′ anomalies are associated with increased (decreased) probability of being occupied by clouds. The magnitude of T′ correlates with the increase or decrease in cloud occurrence, giving evidence that the wave amplitude influences the probability of cloud occurrence. While the decrease in cloud occurrence in the warm phase is centered on the altitude of T′ maxima, we show that the increase in cloud occurrence around T′ minima occurs below the minima in height, indicating that cloud formation or maintenance is facilitated mainly inside negative dT′/dz. Together with existing studies, our results suggest that the cold phase of gravity waves is favorable to TTL clouds mainly through the region where dT′/dz is negative.

Quasi-Equilibrium and Weak Temperature Gradient Balances in an Equatorial Beta-Plane Model

AbstractConvective quasi-equilibrium (QE) and weak temperature gradient (WTG) balances are frequently employed to study the tropical atmosphere. This study uses linearized equatorial beta-plane solutions to examine the relevant regimes for these balances. Wave solutions are characterized by moisture–temperature ratio (q–T ratio) and dominant thermodynamic balances. An empirically constrained precipitation closure assigns different sensitivities of convection to temperature (εt) and moisture (εq). Longwave equatorial Kelvin and Rossby waves tend toward the QE balance with q–T ratios of εt:εq that can be ~1–3. Departures from strict QE, essential to both precipitation and wave dynamics, grow with wavenumber. The propagating QE modes have reduced phase speeds because of the effective gross moist stability meff, with a further reduction when εt > 0. Moisture modes obeying the WTG balance and with large q–T ratios (>10) emerge in the shortwave regime; these modes exist with both Kelvin and Rossby wave meridional structures. In the υ = 0 case, long propagating gravity waves are absent and only emerge beyond a cutoff wavenumber. Two bifurcations in the wave solutions are identified and used to locate the spatial scales for QE–WTG transition and gravity wave emergence. These scales are governed by the competition between the convection and gravity wave adjustment times and are modulated by meff. Near-zero values of meff shift the QE–WTG transition wavenumber toward zero. Continuous transitions replace the bifurcations when meff < 0 or moisture advection/WISHE mechanisms are included, but the wavenumber-dependent QE and WTG balances remain qualitatively unaltered. Rapidly decaying convective/gravity wave modes adjust to the slowly evolving QE/WTG state in the longwave/shortwave regimes, respectively.

Generation of Internal Gravity Waves in the Thermosphere during Operation of the SURA Facility under Parametric Resonance Conditions

The problem of excitation of internal gravity waves (IGWs) in the upper atmosphere by an external source of a limited duration of operation is investigated. An isothermal atmosphere was chosen as the propagation environment of IGWs in the presence of a uniform wind that changes over time according to the harmonic law. For the vertical component of the displacement of an environment, the Mathieu equation with zero initial conditions was solved with the right part simulating the effect of a powerful heating facility on the ionosphere. In the case of a small amplitude of the variable component of the wind, the time dependence of the vertical displacement under parametric resonance conditions using the perturbation method is obtained. The obtained dependence of the solution of the differential equation on the parameters allows us to perform a numerical analysis of the problem in the case of variable wind of arbitrary amplitude. For practical estimations of the obtained values, data on the operating modes of the SURA heating facility (56.15° N, 46.11° E) with periodic (15–30 min) switching on during of 2–3 h for ionosphere impact were used.

Observations of Stratospheric Gravity Waves Over Europe on 12 January 2016: The Role of the Polar Night Jet

AbstractObservations during 12 January 2016 revealed a series of events of significant gravity wave (GW) activity over Europe. Analysis of derived temperatures from the Atmospheric InfraRed Sounder (AIRS) provides insight into the sources of these GWs, and include a new observation of stratosphere polar night jet (PNJ) generated GWs. Mountain waves were present during this time as well over the French Alps and the Carpathian Mountains and had maximum temperature perturbations, T′, as large as 27 K over the French Alps. Further investigation of the mountain waves that demonstrated their presence in the stratosphere was determined not only by stratospheric conditions but also by strong winds in the troposphere and at the surface. GWs generated in the stratosphere by the PNJ had maximum T′ of 7 K. These observations demonstrate multiple sources of GWs during a dynamically active period and implicate the role of the PNJ in both the vertical propagation of GWs generated in the troposphere and the generation of GWs from the PNJ itself.

Radiosonde Observations of a Wintertime Meridional Convergence of Gravity Waves Around 60°S in the Lower Stratosphere

AbstractSatellite observations show that there is a wintertime hotspot of gravity wave activity, located mainly over the ocean, around 60°S in the stratosphere. However, the sources of the gravity waves making up this hotspot are varied and complex and remain unclear. Here we use radiosonde observations from 11 Antarctic stations and selected small islands close to 60°S to examine the horizontal directional pseudo‐momentum flux and energy density distributions of upward propagating gravity waves in the lower stratosphere. This paper shows, for the first time, that short vertical wavelength gravity waves in the lower stratosphere clearly propagate meridionally toward 60°S during the winter months. This result supports previous studies that show that this belt of gravity wave activity over the ocean is contributed to by wave sources outside 60°S.

Evolution of errors in the global multiresolution model for prediction across scales: Shallow water (MPAS–SW)

AbstractThe global Model for Prediction Across Scales (MPAS) with shallow‐water (SW) dynamics is taken as the forecast model to characterize the errors under a variable‐resolution (VR) mesh. An idealized experiment featuring gravity and Rossby waves triggered by orography is conducted with two meshes consisting of the same number of grid cells, which directly indicates the computational cost. One mesh is of 120‐km uniform resolution (UR), the other has 53–210 km VR. Both simulations are compared with the solutions from a 60‐km uniform high‐resolution (HR) mesh serving as the reference. The differences with respect to the HR results for both UR and VR experiments are manifested as rapidly propagating gravity waves circling the Earth about every two days. These signals are regarded as errors due to insufficient resolution. Over most of the Earth, the resolution of the VR mesh is coarser than that of the UR mesh. The magnitudes of the errors in the VR experiment are found to grow larger than those in the UR case shortly after the simulation starts. The sensitivities to the errors in the eight‐day forecast calculated with the MPAS–SW adjoint model show similar propagation patterns, following a nonlinear state trajectory. The sensitivities under VR suggest that little contribution to the errors throughout the simulation process is made within the finely resolved areas. In the initial conditions under VR, the error signals come primarily from the coarse‐resolution regions immediately outside the areas with enhanced resolution. This finding implies that, in simulations under VR, error signals generated in the coarsely resolved regions can be propagated into the finely resolved areas when conveyed by wave types allowed in the model, that is, gravity waves in the case of this study, the rate of which depends on the fluid mean height.

Thermospheric gravity wave characteristics in the daytime over low-latitudes during geomagnetic quiet and disturbed conditions

The importance of gravity waves (GWs) in the redistribution of energy and momentum in the Earth's atmosphere is well known. It is extremely important to understand the nature of variation in the GW characteristics to address issues of coupling in the Earth's atmosphere. We have used digisonde measurements to investigate the variation in thermospheric wave dynamics in the daytime over low-latitudes (Ahmedabad, India) during geomagnetic quiet and disturbed times. Vertical phase speeds and wavelengths of GWs are calculated by monitoring the phase offsets in the digisonde derived height variations of constant electron densities throughout the day. During geomagnetic quiet times, the magnitudes of vertical propagation speeds of GWs and their vertical wavelengths show seasonal variation and vary in the range of 10–70 ms−1 and 70–520 km, respectively. The distributions of vertical propagation parameters of GWs for geomagnetic quiet days show two peaks annually and are anti-correlated with the thermospheric horizontal winds. As these winds over a given location flow in different directions in different seasons, this information can be used to infer the season dependent preferential direction of propagation of GWs. Changes in the vertical propagation characteristics of GWs during geomagnetic disturbed days as compared to their average quiet time values show good semblance with the integrated values of the AE index. A linear relation has been obtained between the changes in vertical phase speeds of GWs and the integrated values of AE index during geomagnetic disturbed days. This linear relation, in combination with empirical formulation arrived at for averaged values of vertical propagation speeds during geomagnetic quiet times, yields comprehensive information on propagation characteristics of GWs as a function of day of the year that includes geomagnetic disturbance conditions as well. Such systematic information on the GW characteristics in the daytime thermosphere and their quantification during geomagnetic quiet times and their changes due to enhancement in energy input at high-latitudes (as characterized by the AE index) also provides a strong and direct information on high-to low-latitude coupling.

Observational evidence of equatorial ionospheric response to severe cyclonic storms ‘AILA’ and ‘WARD’ observed over the North Indian Ocean

This work addresses the vertical atmosphere –ionosphere coupling through two cyclonic events AILA (23–26 May 2009) and ‘WARD’ (10–16 December 2009) observed over the North Indian Ocean. Combined ground Digisonde Tirunelveli (8.7oN, 77.8oE) observation, ECMWF reanalysis, and NOAA Outgoing Longwave Radiation data used to reveal the connection between atmosphere –ionosphere system. At the time of tropical cyclone, we observed a decrease in surface pressure and an increase of sustained oceanic winds. Thereby increases the tropical convection observed from the lowered OLR (Outgoing Longwave Radiation). The existence of Gravity wave oscillations of period ~4 h at the stratosphere and ionosphere regions indicate the propagation of low-frequency GWs (Gravity Waves) from the lower atmosphere. Weak eastward stratospheric winds observed at the time of onset of the cyclonic storm in both AILA and WARD events. The Ionospheric parameter foF2 and h’F2 show strong modulation at the time of cyclones AILA and WARD.

Superposition of gravity waves with different propagation characteristics observed by airborne and space-borne infrared sounders

Abstract. Many gravity wave analyses, based on either observations or model simulations, assume the presence of only a single dominant wave. This paper shows that there are much more complex cases with gravity waves from multiple sources crossing each others' paths. A complex gravity wave structure consisting of a superposition of multiple wave packets was observed above southern Scandinavia on 28 January 2016 with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA). The tomographic measurement capability of GLORIA enabled a detailed 3-D reconstruction of the gravity wave field and the identification of multiple wave packets with different horizontal and vertical scales. The larger-scale gravity waves with horizontal wavelengths of around 400 km could be characterised using a 3-D wave-decomposition method. The smaller-scale wave components with horizontal wavelengths below 200 km were discussed by visual inspection. For the larger-scale gravity wave components, a combination of gravity-wave ray-tracing calculations and ERA5 reanalysis fields identified orography as well as a jet-exit region and a low-pressure system as possible sources. All gravity waves are found to propagate upward into the middle stratosphere, but only the orographic waves stay directly above their source. The comparison with ERA5 also shows that ray tracing provides reasonable results even for such complex cases with multiple overlapping wave packets. Despite their coarser vertical resolution compared to GLORIA measurements, co-located AIRS measurements in the middle stratosphere are in good agreement with the ray tracing and ERA5 results, proving once more the validity of simple ray-tracing models. Thus, this paper demonstrates that the high-resolution GLORIA observations in combination with simple ray-tracing calculations can provide an important source of information for enhancing our understanding of gravity wave propagation.

The time fractional D(m,n) system: invariant analysis, explicit solution, conservation laws and optical soliton

ABSTRACT In this article, a generalized Drinfeld–Sokolov system, also called system which describes the anomalous propagation of nonlinear surface gravity waves over horizontal seabed, is investigated. The Lie classical method is performed on the governing model. The obtained symmetries are used to reduce the model into the fractional ordinary differential equations and to derive the local conservation laws. To make the use of the reduced system, an explicit series solution is derived. Also, it is shown that the bright and singular solitons exist only for the time fractional equation by means of a transformation. The considered system does not give rise to the dark soliton.

Equatorial Plasma Bubble Occurrence Under Propagation of MSTID and MLT Gravity Waves

AbstractWe observed a signature of equatorial plasma bubbles (EPBs) followed by a medium‐scale traveling ionospheric disturbance (MSTID) crossing the solar terminator over the geomagnetic equator during the period of post‐sunset rise (PSSR) of the F layer. Simultaneous observations of the EPBs and the MSTIDs by ground‐based Global Navigation Satellite System receivers and OI 630‐nm imagers, ionospheric parameters by ionosondes, and mesosphere–lower thermosphere (MLT) gravity waves (MLT‐GWs) by OH airglow imagers have been carried out in the equatorial and low‐latitude region of Brazil. On 16–17 September 2015, MSTIDs with very fast horizontal phase speeds, propagating toward northeast (NE), were observed over the latitude region of 0–30°S in the afternoon to evening time zone. It was geomagnetically quiet time (Kp: 2–3). An EPB development was observed after one of the MSTIDs crossed the solar terminator at 22:00 UT. Large‐scale MLT‐GWs with slower phase speeds were also observed both at São João do Cariri (7.5°S, 35.0°W) propagating from southwest (SW) to NE and Cachoeira Paulista (22.7°S, 45.0°W) propagating northward. This is to report on what we consider as a signature of direct seeding of EPBs by an MSTID that was likely induced by a secondary GW from tropospheric deep cloud convections to the south. This study highlights a possible dynamical process from the troposphere to ionosphere via this vertical coupling process over the 2,500 km of horizontal distance.

Changes in the daytime thermospheric gravity wave propagation characteristics over low-latitudes in response to the variation in solar flux

Gravity waves play an important role in the propagation of energy and momentum from the lower atmospheric regions into the upper atmosphere. Depending on the varying dissipation conditions, these waves generated in the lower atmosphere can propagate higher into the atmosphere. As the neutrals and plasma share the same volume in the upper atmosphere, these waves affect both these densities as they propagate. In our analysis, using the information on the variations in phase offsets at different heights corresponding to isoelectron density contours at several defined transmission frequencies of digisonde, vertical phase propagation speeds, and vertical scale sizes of gravity waves in the daytime thermosphere have been estimated. Investigation of gravity waves by radio measurements, unlike the conventional optical techniques, which are limited to clear sky conditions alone, enable an unhindered study of gravity wave dynamics in all weather conditions. We have found a very good semblance between the variations in derived vertical phase speeds of gravity waves in the thermosphere and Solar F10.7cm flux. Further, gravity wave activity, i.e., the number of gravity waves present in the thermosphere in daytime increases with increasing Solar flux. We quantify these inter-relationships and present linear relations of gravity wave activity and their propagation speeds as a function of variation in Solar flux.

Realistic Simulation of Tropical Atmospheric Gravity Waves Using Radar-Observed Precipitation Rate and Echo Top Height.

Gravity waves (GWs) generated by tropical convection are important for the simulation of large‐scale atmospheric circulations, for example, the quasi‐biennial oscillation (QBO), and small‐scale phenomena like clear‐air turbulence. However, the simulation of these waves still poses a challenge due to the inaccurate representation of convection, and the high computational costs of global, cloud‐resolving models. Methods combining models with observations are needed to gain the necessary knowledge on GW generation, propagation, and dissipation so that we may encode this knowledge into fast parameterized physics for global weather and climate simulation or turbulence forecasting. We present a new method suitable for rapid simulation of realistic convective GWs. Here, we associate the profile of latent heating with two parameters: precipitation rate and cloud top height. Full‐physics cloud‐resolving WRF simulations are used to develop a lookup table for converting instantaneous radar precipitation rates and echo top measurements into a high‐resolution, time‐dependent latent heating field. The heating field from these simulations is then used to force an idealized dry version of the WRF model. We validate the method by comparing simulated precipitation rates and cloud tops with scanning radar observations and by comparing the GW field in the idealized simulations to satellite measurements. Our results suggest that including variable cloud top height in the derivation of the latent heating profiles leads to better representation of the GWs compared to using only the precipitation rate. The improvement is especially noticeable with respect to wave amplitudes. This improved representation also affects the forcing of GWs on large‐scale circulation.

Occurrence and Development of an Extreme Precipitation Event in the Ili Valley, Xinjiang, China and Analysis of Gravity Waves

We used observational data and the results from a high-resolution numerical simulation model to analyze the occurrence and development of an extreme precipitation event in the Ili Valley, Xinjiang, China on 26 June 2015. We analyzed the horizontal wavelength, period, speed, ducting, energy propagation and feedback mechanism of inertial gravity waves. A low-level convergence line was formed in the valley by the northerly and westerly winds as a result of Central Asian vortices and the trumpet-shaped topography of the Ili Valley. There was sufficient water vapor in the valley for the precipitation event to develop. A mesoscale vortex formed and developed on the low-level convergence line and the rainfall was distributed either near the convergence line or the mesoscale vortex. The low-level convergence line and the uplift caused by the terrain triggered convection, and then the convection triggered waves at lower levels. The combination of ascending motion induced by the lower level waves and the mesoscale vortex led to the development of convection, causing the precipitation to intensify. When the convection moved eastward to Gongliu County, it was coupled with the ascending phase of upper level waves, causing both the convection and precipitation to intensify again. We applied spectral analysis methods to verify that the waves were inertial gravity waves. The upper level inertial gravity waves propagated westward at a mean speed of −12 m s−1 with periods of 73–179 min and horizontal wavelengths of 50–55 km. The lower level inertial gravity waves propagated eastward at a mean speed of 8 m s−1 with periods of 73–200 min and a horizontal wavelength of 85 km. The more (less) favorable waveguide conditions determined whether the gravity waves persisted for a long (short) time and propagated for a longer (shorter) distance. Based on the mesoscale Eliassen–Palm flux theory, the wave energy of inertial gravity waves had an important effect on the maintenance and development of convection and precipitation by affecting wind strength and wind divergence. Feedback was mainly through the meridional and vertical transport of zonal momentum and the meridional transport of heat.

Propagation of gravity waves and its effects on pseudomomentum flux in a sudden stratospheric warming event

Abstract. Effects of realistic propagation of gravity waves (GWs) on distribution of GW pseudomomentum fluxes are explored using a global ray-tracing model for the 2009 sudden stratospheric warming (SSW) event. Four-dimensional (4D; x–z and t) and two-dimensional (2D; z and t) results are compared for various parameterized pseudomomentum fluxes. In ray-tracing equations, refraction due to horizontal wind shear and curvature effects are found important and comparable to one another in magnitude. In the 4D, westward pseudomomentum fluxes are enhanced in the upper troposphere and northern stratosphere due to refraction and curvature effects around fluctuating jet flows. In the northern polar upper mesosphere and lower thermosphere, eastward pseudomomentum fluxes are increased in the 4D. GWs are found to propagate more to the upper atmosphere in the 4D, since horizontal propagation and change in wave numbers due to refraction and curvature effects can make it more possible that GWs elude critical level filtering and saturation in the lower atmosphere. GW focusing effects occur around jet cores, and ray-tube effects appear where the polar stratospheric jets vary substantially in space and time. Enhancement of the structure of zonal wave number 2 in pseudomomentum fluxes in the middle stratosphere begins from the early stage of the SSW evolution. An increase in pseudomomentum fluxes in the upper atmosphere is present even after the onset in the 4D. Significantly enhanced pseudomomentum fluxes, when the polar vortex is disturbed, are related to GWs with small intrinsic group velocity (wave capture), and they would change nonlocally nearby large-scale vortex structures without substantially changing local mean flows.

Multilayer hazes over Saturn\u2019s hexagon from Cassini ISS limb images

In June 2015, Cassini high-resolution images of Saturn’s limb southwards of the planet’s hexagonal wave revealed a system of at least six stacked haze layers above the upper cloud deck. Here, we characterize those haze layers and discuss their nature. Vertical thickness of layers ranged from 7 to 18 km, and they extended in altitude ∼130 km, from pressure level 0.5 bar to 0.01 bar. Above them, a thin but extended aerosol layer reached altitude ∼340 km (0.4 mbar). Radiative transfer modeling of spectral reflectivity shows that haze properties are consistent with particles of diameter 0.07–1.4 μm and number density 100–500 cm−3. The nature of the hazes is compatible with their formation by condensation of hydrocarbon ices, including acetylene and benzene at higher altitudes. Their vertical distribution could be due to upward propagating gravity waves generated by dynamical forcing by the hexagon and its associated eastward jet.