Gravity waves in the mesopause region observed by meteor radar: 1. A simple measurement technique

A simple new technique for measuring gravity-wave activity using meteor radars is described. The technique uses the variance of horizontal wind velocities measured by individual meteors as a proxy for the activity of the gravity-wave field. It is sensitive to gravity waves with horizontal wavelengths of up to about 400 km and periods up to about 3 h. The technique can be used to investigate the vertical structure of the gravity-wave field at heights between approximately 80 and 100 km and with a time resolution of approximately 6 h. The technique is demonstrated using data from an all-sky meteor radar based at Rothera, Antarctica (68°S, 68°W). Observations made over Rothera for 2006 and 2007 reveal a seasonal behaviour with a semi-annual cycle in wave activity. Wave activity maximises in summer and winter and minimises at the equinoxes. Monthly mean gravity-wave activity increases with height in all seasons except in summer when gravity-wave variances show little or no increase with height below 90 km. Comparisons between the gravity-wave activity determined by this meteor-variance technique and other measurements at similar latitudes in the Antarctic reveal generally good agreement.     

中层大气重力波的全球分布特征

从2002年1月到2009年12月的SABER温度剖面数据提取了可以反映重力波活动的垂直尺度2～10 km的中尺度温度扰动,分析了全球中层大气重力波的分布.重力波扰动在夏季和冬季明显强于春季和秋季,而冬季与夏季相比,在70 km以下的高度夏季弱于冬季,在70 km以上夏季比冬季要强.从全球重力波分布来看,较大值分布在冬...     

北极地区低平流层惯性重力波的观测研究

南极地区重力波活动有大量报道,相对而言,北极地区重力波的研究还很少.本文利用极区Ny-Alesund站点（78.9°N,11.9°E）无线电探空仪从2012年4月1日到2017年3月31日共5年的观测数据,统计分析了北极地区低平流层惯性重力波的特征.观测显示,月平均纬向风在20 km以下盛行东向风,再随着高度增加,逐渐呈现出半年振荡现象.对流层顶高度在5~13 km范围内变化,其月平均高度显示出年循环,最高出现在夏季,约为10 km,最低出现在冬季,约为8.5 km.对流层和低平流层月平均温度都显示出明显的年周期变化,这与中低纬度观测结果有所不同.结合Lomb-Scargle谱分析和矢端曲线方法,估算了准单色惯性重力波参数.个例研究表明,低平流层惯性重力波呈现出远离源区的自由传播性质.统计结果显示,惯性重力波的水平和垂直波长分别集中在50~450 km和1~4 km范围内,本征频率集中在1~2.5倍惯性频率间,这些值都比中低纬度观测值稍小.垂直方向本征相速度主要集中在-0.3~0 m·s^-1,而纬向和经向本征相速度集中在-40~40 m·s^-1之间.在5年的观测中,大约91.5%的惯性重力波向上传播.在冬季和早春,由于极地平流层极涡活动,激发出向下传播的惯性重力波,因此,向下传播的比例上升到相应月份的20%左右.由于低层大气盛行的东向风的滤波效应,低平流层大部分惯性重力波向西传播.波能量呈现出明显的年周期变化,最大值在冬季、最小值在夏季,与北半球中低纬度观测结果一致,表明北半球重力波活动普遍冬季强、夏季弱.     

Gravity wave observations in the summertime polar mesosphere from the Cloud Imaging and Particle Size (CIPS) experiment on the AIM spacecraft

We present the first results of gravity wave signatures on polar mesospheric clouds (PMCs) during the summer of 2007, in the northern hemisphere polar region. The Cloud Imaging and Particle Size (CIPS) experiment has one of the three instruments on board the NASA Aeronomy of Ice in the Mesosphere (AIM) spacecraft, which was launched into a sun-synchronous orbit on April 25, 2007. CIPS is a four-camera, wide-field (120°×80°) imager designed to measure PMC morphology and particle properties. One of the objectives of AIM is to investigate gravity wave effects on PMC formation and evolution. CIPS images show distinct wave patterns and structures in PMCs that are similar to ground-based photographs of noctilucent clouds (NLCs). The observed horizontal wavelengths of the waves were found to vary between 15 and 320 km, with smaller-wavelength structures of less than 50 km being the most common. In this paper we present examples of individual quasi-monochromatic wave events observed by CIPS and statistics on the wave patterns observed in the northern hemisphere during the summer months of 2007, together with a map showing the geographic locations of gravity wave events observed from CIPS.     

First results of warm mesospheric temperature over Gadanki (13.5°N, 79.2°E) during the sudden stratospheric warming of 2009

Rayleigh lidar observations at Gadanki (13.5°N, 79.2°E) show an enhancement of the nightly mean temperature by 10–15 K at altitudes 70–80 km and of gravity wave potential energy at 60–70 km during the 2009 major stratospheric warming event. An enhanced quasi-16-day wave activity is observed at 50–70 km in the wavelet spectrum of TIMED–SABER temperatures, possibly due to the absence of a critical level in the low-latitude stratosphere because of less westward winds caused by this warming event. The observed low-latitude mesospheric warming could be due to wave breaking, as waves are damped at 80 km.     

Planetary waves observed by TIMED/SABER in coupling the stratosphere–mesosphere–lower thermosphere during the winter of 2003/2004: Part 2—Altitude and latitude planetary wave structure

Part 2 of the present paper is focused on the planetary wave coupling from the stratosphere to the lower thermosphere (30–120 km) during the Arctic winter of 2003/2004. The planetary waves seen in the TIMED/SABER temperature data in the latitudinal range 50°N–50°S are studied in detail. The altitude and latitude structures of the planetary wave (stationary and travelling) clearly indicate that the stratosphere and mesosphere (30–90 km) are coupled by direct vertical propagation of the planetary waves, while the lower thermosphere (above 90–95 km altitude) is only partly connected with the lower levels probably indirectly through in-situ generation of disturbances by the dissipation and breaking of gravity waves filtered by lower atmospheric planetary waves. A peculiar feature of the thermal regime in the lower thermosphere is that it is dominated by zonally symmetric planetary waves.     

Mesospheric bore formation from large-scale gravity wave perturbations observed by collocated all-sky OH imager and sodium lidar

On 9 October 2007, long-horizontal-wavelength gravity waves were observed for the first time to steepen and form mesospheric bores at the altitude of ～87 km, by an all-sky OH imager located at Fort Collins (41°N, 105°W), Colorado. The collocated sodium lidar simultaneously observed the presence of a temperature inversion layer as the ducting region. One mesospheric bore uniquely later evolved into a large-amplitude soliton-like perturbation. When the gravity wave and the associated soliton-like perturbation passed through the lidar beams, the lidar detected strong vertical disturbance at 90 km, indicating convective instability. A large cold front system recorded several hours before in the troposphere was aligned to phase fronts of these large gravity waves. For all of the 7 mesospheric bores observed over a 5 year period, we found a similar alignment with a cold front 1000–1500 km away as the likely source of these large-scale gravity waves.     

Gravity wave reflection: Case study based on rocket data

Since gravity waves significantly influence the atmosphere by transporting energy and momentum, it is important to study their wave spectrum and their energy dissipation rates. Besides that, knowledge about gravity wave sources and the propagation of the generated waves is essential. Originating in the lower atmosphere, gravity waves can move upwards; when the background wind field is equal to their phase speed a so-called critical layer is reached. Their breakdown and deposition of energy and momentum is possible. Another mechanism which can take place at critical layers is gravity wave reflection.In this paper, gravity waves which were observed by foil chaff measurements during the DYANA (DYnamics Adapted Network for the Atmosphere) campaign in 1990 in Biscarrosse (44°N, 1°W)—as reported by Wüst and Bittner [2006. Non-linear wave–wave interaction: case studies based on rocket data and first application to satellite data. Journal of Atmospheric and Solar-Terrestrial Physics 68, 959–976]—are investigated to look for gravity wave reflection processes. Following nonlinear theory, energy dissipation rates according to Weinstock [1980. Energy dissipation rates of turbulence in the stable free atmosphere. Journal of the Atmospheric Sciences 38, 880–883] are calculated from foil chaff cloud and falling sphere data and compared with the critical layer heights. Enhanced energy dissipation rates are found at those altitudes where the waves’ phase speed matches the zonal background wind speeds. Indication of gravity wave trapping is found between two altitudes of around 95 and 86 km.     

Global distribution and climatological features of the 5–6-day planetary waves seen in the SABER/TIMED temperatures (2002–2007)

The paper is focused on the global spatial structure, seasonal and interannual variability of the ～5-day Rossby (W1) and ～6-day Kelvin (E1) waves derived from the SABER/TIMED temperature measurements for 6 full years (January 2002–December 2007). The latitude structure of the ～5-day W1 wave is related to the gravest symmetric wave number 1 Rossby wave. The vertical structure of the ～5-day Rossby wave amplitude consists of double-peaked maxima centred at ～80–90 km and ～105–110 km. This wave has a vertically propagating phase structure from the stratosphere up to 120 km altitude with a mean vertical wavelength of ～50–60 km. The ～6-day E1 wave is an equatorially trapped wave symmetric about the equator and located between 20°N and 20°S. Its seasonal behaviour indicates some equinoctial and June solstice amplifications, while the vertical phase structure indicates that this is a vertically propagating wave between 20–100 km altitudes with a mean vertical wavelength of ～25 km.     

VHF radar observed characteristics of convectively generated gravity waves during wet and dry spells of Indian summer monsoon

A powerful VHF radar observed characteristics of Convectively generated Gravity Waves (CGW) excited during the wet and dry spells of Indian summer monsoon over a tropical station Gadanki (13.5°N, 79.2°E) are discussed. The characteristics of gravity waves in the lower stratosphere during these two spells are discussed in terms of their wavelet spectra along with height–time sections of vertical velocity. A total of 31 events are analyzed and in more than 50% of the events, the lower stratospheric gravity wave amplitudes were found to be relatively large in dry spell compared to that in the wet spell. The wavelet analysis of lower stratospheric vertical velocities showed a dominant periodicity of about ～20–40 min in wet spell and ～10–20 min in dry spell. The analysis also indicates that wet spell is found to be more conducive for the generation of gravity waves. However, the propagation of these waves into the stratosphere is found to be more efficient in dry spell of monsoon. The strengthening/weakening of the tropical easterly jet during wet/dry spell of monsoon is found to be the main reason for the inhibited/enhanced wave activity in the lower stratosphere during wet/dry spell. The present analysis also suggests that the static stability of the mid- and upper-troposphere during these two spells have implications in the observed frequency of the CGW. Thus, the present analyses brought out for the first time the features of CGW during two distinctive regimes of convective systems and emphasized the importance of prevailing background conditions in exciting/filtering them.     

Fast meridional transport in the lower thermosphere by planetary-scale waves

Observations showed that the main engine water exhaust plumes from space shuttles released at ～110 km altitude from Florida could be transported over thousands of kilometers northward or southward, reaching the Arctic after a day or so, and in one case Antarctica after three days (Stevens et al., 2003, Stevens et al., 2005). In this work, we study the meridional transport associated with the quasi-two-day wave (QTDW) and migrating tides. Diagnostic calculations are performed to trace the particle trajectories using winds from the Thermosphere–Ionosphere–Mesosphere-Electrodynamics General Circulation Model (TIME-GCM) simulations for January, when the amplitude of the QTDW usually peaks. The calculations demonstrate that the mean meridional circulation, a QTDW or a migrating tide cannot individually sustain planetary-scale meridional transport for one to three days, but the combined effects of a QTDW and a tide can. In particular, when the QTDW and the tides are scaled according to the observed amplitudes, particles released at ～110 km and appropriate longitudes/local times can undergo transport fast enough to reach Antarctica within three days as observed. The magnitude and direction of the transport depend on the amplitudes and phases of the tides and the QTDW. These simulations thus suggest that the observed rapid planetary-scale meridional transport of the shuttle main engine plume can be driven by planetary waves and tides.     

Equatorial and low latitude spread-F irregularity characteristics over the Indian region and their prediction possibilities

To study the occurrence characteristics of equatorial spread-F irregularities and their latitudinal extent, simultaneous digital ionosonde data (January–December 2001) from Trivandrum (8.2°N), Waltair (17.7°N) and Delhi (28.6°N) and 4 GHz scintillation data from Sikandarabad (26.8°N) and Chenglepet (10.4°N), and 250 MHz scintillation data from Bhopal (23.2°N) for equinoxes period are analysed. It is noted that except summer months, occurrence of spread F is always maximum at Trivandrum, minimum at Delhi and moderate at Waltair. During equinoxes and winter months. Their occurrences at higher latitude station are always conditional to their prior occurrences at lower latitudes indicating their association with the generation of equatorial plasma bubble and associated irregularities. Scintillation occurrences also follow the similar pattern. During the summer months, the spread-F occurrences are highest at equatorial location Trivandrum, moderate at Delhi and minimum at Waltair and seem to be caused by irregularities generated locally especially over Delhi.To gain forecasting capability, night-to-night occurrences of spread-F/scintillation at these locations are examined in relation to post sunset rise of h’F and upward ExB drift velocity over the magnetic equator using Trivandrum ionosonde data. It is noted that except the summer months, the spread-F at Trivandrum, Waltair and Delhi are observed only when equatorial ExB (h’F) is more than about 15 m/s (325 km), 20 m/s (350 km) and 25 m/s (375 km), respectively. With these threshold values their corresponding success rate of predictions are more than 90%, 50% and 15% at the respective locations. Whereas in the case of GHz scintillations near equator are observed only when ExB (h’F) is more than 15 m/s (325 km), whereas for low latitude, the same should be 30 m/s (400 km) and their success rate of prediction is about 90% and 30%, respectively. The intensity of 4 GHz scintillation at low latitude is also found to be positively correlated with equatorial upward ExB drift velocity values, whereas correlation is poor with that of equatorial scintillations. In conclusions, near magnetic equator threshold values of ExB or h’F can be successfully used for the night-to-night prediction of spread-F/scintillations occurrences, whereas these are necessary but not sufficient for their prediction at higher latitudes. For that some other controlling parameters like background electron density, neutral winds, gravity waves, etc. should also be examined.     

Observations of gravity waves in the upper and lower stratosphere by lidar and ozonesondes

Lidar observations of Rayleigh backscatter and temperature profiles measured by ozonesondes have been used to investigate gravity waves in the upper and lower stratosphere, respectively, over Aberystwyth (52.4°N, 4.1°W). Both data sets have been used to investigate the vertical wavenumber spectrum of the wave field at high wavenumbers. Similar analytic techniques applied to each data set enable direct comparison of the spectra. The possibility of laminar structures generated by differential advection contaminating gravity-wave fields deduced from temperature and/or density measurement is discussed and the behaviour of the wave-field mean potential energy reveals a seasonal cycle throughout the stratosphere with a late winter maximum and a summer minimum.     

Evidence of mesospheric bore formation from a breaking gravity wave event: simultaneous imaging and lidar measurements

A large wave event was observed in the three upper-mesospheric (80–105 km) airglow emissions of O(¹S), Na and OH by the Boston University all-sky imager, at the Arecibo Observatory, during the night of 3 May 2003. The airglow structures appeared to be due to a large upward propagating internal gravity wave, which subsequently became unstable near the 95 km height level and produced large-scale vertical motions and mixing. Simultaneous density and temperature lidar measurements indicated the presence of a large temperature inversion of 80 K valley-to-peak between 88 and 96 km during the time of the event. Near-simultaneous temperature profiles, made by the TIMED SABER instrument, provided evidence that the horizontal extent of the inversion was localized to within 500 km of Arecibo during the wave event. As the gravity wave dissipated, an internal bore was generated, apparently due to the deposition of momentum and energy into the region by the original wave. Although mesospheric gravity wave breaking has been reported previously (Swenson and Mende, 21(1994); Hecht et al., 102(1997); Yamada et al., 28(2001), for example), this was the first time that the phenomenon has been associated with the generation of an internal mesospheric bore. The event suggested that the breaking of a large mesospheric gravity wave can lead to the generation of an internal bore, as suggested by Dewan and Picard 106(2001). Such behavior is of particular interest since little is known of their origins.     

Acoustic energy transfer to the upper atmosphere from sinusoidal sources and a role of nonlinear processes

In the framework of solving the problem of acoustic energy transfer from near-surface sources through the upper atmosphere, the propagation of sinusoidal signals of different origin is studied. All calculations are made by means of a model that takes into account the inhomogeneity of the atmosphere, nonlinear effects, absorption, divergence of wave front due to long-range acoustic wave propagation, etc., but does not include the effect of gravity. Infrasonic waves of various periods and their absorption at various heights of the atmosphere are investigated. The calculations show that a sinusoidal signal is destroyed by nonlinear processes during its upward propagation; it transforms into two, initial and final, impulses. The location of the “transformation zone” depends on frequency; its height increases with decreasing frequency. The acoustic waves can heat the upper atmosphere, for example, waves with a period of 3 min generated by thunderstorms can heat the atmosphere by up to ΔT_a=13.08 K/day in the region of 323–431 km. The efficiency of a point artificial emitter is too weak to heat the atmosphere significantly.     

Statistics of gravity waves in the lower stratosphere over Beijing based on high vertical resolution radiosonde

Atmospheric gravity waves, with small to medium scales, prevail in the atmosphere and have global ef- fects. Many researches show that gravity waves are the main source that causes the variation of wind and temperature field in the stratosphere, and that the break-up of upward propagating gravity waves is the dominant sources of small scale turbulent and mixing processes in the middle atmosphere. Theories and ob- servations indicate that the redistribution of momen- tum, caused by the generati…     

Variability of equatorial ionospheric electron density at fixed heights below the F2 peak

A study on variability of the equatorial ionosphere was carried out at fixed heights below the F2 peak for two different levels of solar activity. The study covered height range of 100 km up to the peak of F2 layer using a real height step increase of 10 km. The variability index used is the percentage ratio of standard deviation over the average value for the month. Daytime minimum variability of between 3% and 10% was observed at height range of about 150–210 km during low solar activity and between 2% and 7% at height range of 160–220 km during high solar activity. The nighttime maximum of between 70% and 187% was observed at height range of about 210–250 km during low solar activity and between 42% and 127% at height range of 210–250 km during high solar activity. The height range at which daytime minimum was observed falls within the F1 height of the ionosphere. The result obtained is consistent with previous works carried out in the low latitude locations for American sector.     

The seismically active Andean and Central American margins: Can satellite gravity map lithospheric structures?

The spatial resolution and quality of geopotential models (EGM2008, EIGEN-5C, ITG-GRACE03s, and GOCO-01s) have been assessed as applied to lithospheric structure of the Andean and Central American subduction zones. For the validation, we compared the geopotential models with existing terrestrial gravity data and density models as constrained by seismic and geological data. The quality and resolution of the downward continued geopotential models in the Andes and Central America decrease with increasing topography and depend on the availability of terrestrial gravity data. High resolution of downward continued gravity data has been obtained over the Southern Andes where elevations are lower than 3000 m and sufficient terrestrial gravity data are available. The resolution decreases with an increase in elevation over the north Chilean Andes and Central America. The low resolution in Central America is mainly attributed to limited surface gravity data coverage of the region.To determine the minimum spatial dimension of a causative body that could be resolved using gravity gradient data, a synthetic gravity gradient response of a spherical anomalous mass has been computed at GOCE orbit height (254.9 km). It is shown that the minimum diameter of such a structure with density contrast of 240 kg m⁻³ should be at least ∼45 km to generate signal detectable at orbit height. The batholithic structure in Northern Chile, which is assumed to be associated with plate coupling and asperity generation, is about 60–120 km wide and could be traceable in GOCE data. Short wavelength anomalous structures are more pronounced in the components of the gravity gradient tensor and invariants than in the gravity field.As the ultimate objective of this study is to understand the state of stress along plate interface, the geometry of the density model, as constrained by combined gravity models and seismic data, has been used to develop dynamic model of the Andean margin. The results show that the stress regime in the fore-arc (high and low) tends to follow the trend of the earthquake distributions.     

基于OH全天空气辉成像仪观测的中国低纬地区的重力波传播统计特征

利用位于海南富克(19.5°N,109.1°E)和广西桂平(23.4°N,110.1°E)两个台站两年多的OH全天空气辉成像仪观测数据,对中国低纬地区的重力波传播统计特征进行了研究.从富克和桂平的气辉成像观测中, 分别提取了65和86个重力波事件.研究结果表明,观测水平波长,观测周期和水平相速度分别集中分布在10~35 km, 4~14 min和20~90 m·s^-1范围.重力波传播方向,在夏季表现出很强的东北方向传播.然而,在冬季主要沿东南和西南方向传播. 同时,结合流星雷达风场观测和TIMED/SABER卫星的温度数据,也发现在中层-低热层中传播的大多数重力波表现为耗散传播.且低层-中层大气中背景风场的滤波作用和多普勒频移可能对纬向方向传播的重力波产生的各向异性起到重要的调制作用.然而,经向方向传播的重力波产生的各向异性可能同时被低层大气中波源的非均匀分布以及潮汐变化所影响.     

Large internal waves in Massachusetts Bay transport sediments offshore

A field experiment was carried out in Massachusetts Bay in August 1998 to assess the role of large-amplitude internal waves (LIWs) in resuspending bottom sediments. The field experiment consisted of a four-element moored array extending from just west of Stellwagen Bank (90-m water depth) across Stellwagen Basin (85- and 50-m water depth) to the coast (24-m water depth). The LIWs were observed in packets of 5–10 waves, had periods of 5–10 min and wavelengths of 200–400 m, and caused downward excursions of the thermocline of as much as 30 m. At the 85-m site, the current measured 1 m above bottom (mab) typically increased from near 0 to 0.2 m/s offshore in a few minutes upon arrival of the LIWs. At the 50-m site, the near-bottom offshore flow measured 6 mab increased from about 0.1 to 0.4–0.6 m/s upon arrival of the LIWs and remained offshore in the bottom layer for 1–2 h. The near-bottom currents associated with the LIWs, in concert with the tidal currents, were directed offshore and sufficient to resuspend the bottom sediments at both the 50- and 85-m sites. When LIWs are present, they may resuspend sediments for as long as 5 hours each tidal cycle as they travel westward across Stellwagen Basin. At 85-m water depth, resuspension associated with LIWs is estimated to occur for about 0.4 days each summer, about the same amount of time as caused by surface waves.