Small-scale Gravity Waves Research Articles

Density Fluctuations in the Lower Thermosphere of Mars Retrieved From the ExoMars Trace Gas Orbiter (TGO) Aerobraking

The upper atmosphere of Mars is constantly perturbed by small-scale gravity waves propagating from below. As gravity waves strongly affect the large-scale dynamics and thermal state, constraining their statistical characteristics is of great importance for modeling the atmospheric circulation. We present a new data set of density perturbation amplitudes derived from accelerometer measurements during aerobraking of the European Space Agency’s Trace Gas Orbiter. The obtained data set presents features found by three previous orbiters: the lower thermosphere polar warming in the winter hemisphere, and the lack of links between gravity wave activity and topography. In addition, the orbits allowed for demonstrating a very weak diurnal variability in wave activity at high latitudes of the southern winter hemisphere for the first time. The estimated vertical damping rates of gravity waves agree well with theoretical predictions. No clear anticorrelation between perturbation amplitudes and the background temperature has been found. This indicates differences in dissipation mechanisms of gravity waves in the lower and upper thermosphere.

Seasonal and intra-diurnal variability of small-scale gravity waves in OH airglow at two Alpine stations

Abstract. Between December 2013 and August 2017 the instrument FAIM (Fast Airglow IMager) observed the OH airglow emission at two Alpine stations. A year of measurements was performed at Oberpfaffenhofen, Germany (48.09∘ N, 11.28∘ E) and 2 years at Sonnblick, Austria (47.05∘ N, 12.96∘ E). Both stations are part of the network for the detection of mesospheric change (NDMC). The temporal resolution is two frames per second and the field-of-view is 55 km × 60 km and 75 km × 90 km at the OH layer altitude of 87 km with a spatial resolution of 200 and 280 m per pixel, respectively. This resulted in two dense data sets allowing precise derivation of horizontal gravity wave parameters. The analysis is based on a two-dimensional fast Fourier transform with fully automatic peak extraction. By combining the information of consecutive images, time-dependent parameters such as the horizontal phase speed are extracted. The instrument is mainly sensitive to high-frequency small- and medium-scale gravity waves. A clear seasonal dependency concerning the meridional propagation direction is found for these waves in summer in the direction to the summer pole. The zonal direction of propagation is eastwards in summer and westwards in winter. Investigations of the data set revealed an intra-diurnal variability, which may be related to tides. The observed horizontal phase speed and the number of wave events per observation hour are higher in summer than in winter.

On the short-term variability of turbulence and temperature in the winter mesosphere

Abstract. Four mesosphere–lower thermosphere temperature and turbulence profiles were obtained in situ within ∼30 min and over an area of about 100 by 100 km during a sounding rocket experiment conducted on 26 January 2015 at Poker Flat Research Range in Alaska. In this paper we examine the spatial and temporal variability of mesospheric turbulence in relationship to the static stability of the background atmosphere. Using active payload attitude control, neutral density fluctuations, a tracer for turbulence, were observed with very little interference from the payload spin motion, and with high precision (<0.01 %) at sub-meter resolution. The large-scale vertical temperature structure was very consistent between the four soundings. The mesosphere was almost isothermal, which means more stratified, between 60 and 80 km, and again between 88 and 95 km. The stratified regions adjoined quasi-adiabatic regions assumed to be well mixed. Additional evidence of vertical transport and convective activity comes from sodium densities and trimethyl aluminum trail development, respectively, which were both observed simultaneously with the in situ measurements. We found considerable kilometer-scale temperature variability with amplitudes of 20 K in the stratified region below 80 km. Several thin turbulent layers were embedded in this region, differing in width and altitude for each profile. Energy dissipation rates varied between 0.1 and 10 mW kg−1, which is typical for the winter mesosphere. Very little turbulence was observed above 82 km, consistent with very weak small-scale gravity wave activity in the upper mesosphere during the launch night. On the other hand, above the cold and prominent mesopause at 102 km, large temperature excursions of +40 to +70 K were observed. Simultaneous wind measurements revealed extreme wind shears near 108 km, and combined with the observed temperature gradient, isolated regions of unstable Richardson numbers (0<Ri<0.25) were detected in the lower thermosphere. The experiment was launched into a bright auroral arc under moderately disturbed conditions (Kp∼5).

Seasonal characteristics of small- and medium-scale gravity waves in the mesosphere and lower thermosphere over the Brazilian equatorial region

Abstract. The present work reports seasonal characteristics of small- and medium-scale gravity waves in the mesosphere and lower thermosphere (MLT) region. All-sky images of the hydroxyl (NIR-OH) airglow emission layer over São João do Cariri (7.4∘ S, 36.5∘ W; hereafter Cariri) were obtained from September 2000 to December 2010, during a total of 1496 nights. For investigation of the characteristics of small-scale gravity waves (SSGWs) and medium-scale gravity waves (MSGWs), we employed the Fourier two-dimensional (2-D) spectrum and keogram fast Fourier transform (FFT) techniques, respectively. From the 11 years of data, we could observe 2343 SSGW and 537 MSGW events. The horizontal wavelengths of the SSGWs were concentrated between 10 and 35 km, while those of the MSGWs ranged from 50 to 200 km. The observed periods for SSGWs were concentrated around 5 to 20 min, whereas the MSGWs ranged from 20 to 60 min. The observed horizontal phase speeds of SSGWs were distributed around 10 to 60 m s−1, and the corresponding MSGWs were around 20 to 120 m s−1. In summer, autumn, and winter both SSGWs and MSGWs propagated preferentially northeastward and southeastward, while in spring the waves propagated in all directions. The critical level theory of atmospheric gravity waves (AGWs) was applied to study the effects of wind filtering on SSGW and MSGW propagation directions. The SSGWs were more susceptible to wind filtering effects than MSGWs. The average of daily mean outgoing longwave radiation (OLR) was also used to investigate the possible wave source region in the troposphere. The results showed that in summer and autumn, deep convective regions were the possible source mechanism of the AGWs. However, in spring and winter the deep convective regions did not play an important role in the waves observed at Cariri, because they were too far away from the observatory. Therefore, we concluded that the horizontal propagation directions of SSGWs and MSGWs show clear seasonal variations based on the influence of the wind filtering process and wave source location. Keywords. Atmospheric composition and structure (airglow and aurora) – electromagnetics (wave propagation) – history of geophysics (atmospheric sciences)

Observed Solar Cycle Variation of the Stratospheric QBO Generated in the Mesosphere and Amplified by Upward Propagating Waves

With an analysis of zonal wind observations over 40 years, Salby and Callaghan [1] showed that the Quasi-biennial Oscillation (QBO) at 20 km is modulated by 11-year solar cycle (SC) variations from about 12 to 20 m/s (Figure 2). The observations are reproduced qualitatively in a study with the 3D Numerical Spectral Model, which shows that the SC effect of the stratospheric QBO is produced by dynamical downward coupling originating in the mesosphere. In this modeling study, the SC period is taken to be 10 years, and a realistic heat source is applied varying exponentially with altitude: 0.2%, surface; 2%, 50 km; 20%, 100 km and above. The numerical results show that the variable solar radiation in the mesosphere around 65 km generates a hemispheric symmetric Equatorial Annual Oscillation (EAO), which is modulated by relatively large SC variations. Under the influence of wave mean flow interactions, the EAO propagates into the lower atmosphere and is the dynamical source or pacemaker for the large SC modulation of the QBO. The numerical results show that the upward propagating small-scale gravity waves from the troposphere amplify the SC modulations of the QBO and EAO in the stratosphere, part of the SC mechanism. The zonal winds of the equatorial QBO and EAO produce through the meridional circulation measurable SC variations in the temperature of the stratosphere and troposphere at high latitudes. Analysis of NCEP temperature and zonal wind data (1958 to 2006) provides observational evidence of the EAO with SC variations around 11 years.

Analysis of small-scale structures in lidar observations of noctilucent clouds using a pattern recognition method

Noctilucent clouds (NLC) have been observed with the ALOMAR Rayleigh/Mie/Raman lidar at 69° N using a temporal resolution of 30s since 2008. We present an approach to identify and analyze the localized small scale wave structures of the varying altitude of the NLC layers in the range of 5–30min that may be caused by gravity waves. Small scale gravity waves breaking in the mesopause region contribute notably to the momentum flux but are difficult to observe and to characterize. The approach is based on a template matching method using generalized structures to be identified in the NLC observations. The new method permits the identification of structures that are present in NLC only for a time too short to appear in a Fourier or wavelet spectrum. Without the need for a continuous time series the method can handle multiple NLC layers and data gaps. In the 2000h of NLC data from the years 2008–2015, we find almost 5000 single wave structures with a total length of 738h. The structures are found on average 400m below the NLC centroid altitude and a large number of the structures has a length at the lower limit of 5min. With the background wind from the meteor radar near ALOMAR a horizontal scale is estimated based on the length of the individual structures. The distribution of horizontal scales shows a peak of wave structures at 15–20km in accordance with the horizontal wavelengths found by ground-based camera observations of NLC.

Statistical Characteristics of Albedo Variation in Noctilucent Clouds Induced by Small-scale Gravity Waves

利用AIM卫星搭载的CIPS云成像探测器获得的云图数据，提取2008-2009年南北半球共6664个小尺度重力波（波长10～150km）个例，通过重力波区域与背景云层反照率变化值的对比分析，研究重力波引起云层反照率的变化特征.结果表明，重力波引起的反照率变化值以正值为主，最大平均值4.48&#215;10^-6sr^-1出现在南半球降交轨道.反照率变化值与IWC变化值正相关，相关系数均在0.85以上.重力波引起的反照率变化呈现出很强的纬度和时间依赖性，且几乎均为正值.反照率变化值在中期阶段（冬/夏至日之后的50天）的高纬地区（>70&#176;）更大，但在中期以外始末阶段的低纬地区（<70&#176;）逐渐变小，甚至开始出现负值.随着背景云层的增强，反照率平均值呈线性增大，小尺度重力波能够引起背景云层反照率约14.6%～28.8%的变化量.当重力波引起的反照率周期性变化的振幅逐渐增大时，反照率变化值也线性增大，变化率约为0.909%～1.194%.南半球的变化率整体比北半球稍小，这与背景大气条件的差异有关

High-resolution observations of small-scale gravity waves and turbulence features in the OH airglow layer

Abstract. A new version of the Fast Airglow Imager (FAIM) for the detection of atmospheric waves in the OH airglow layer has been set up at the German Remote Sensing Data Center (DFD) of the German Aerospace Center (DLR) at Oberpfaffenhofen (48.09° N, 11.28° E), Germany. The spatial resolution of the instrument is 17 m pixel−1 in zenith direction with a field of view (FOV) of 11.1 km × 9.0 km at the OH layer height of ca. 87 km. Since November 2015, the system has been in operation in two different setups (zenith angles 46 and 0°) with a temporal resolution of 2.5 to 2.8 s. In a first case study we present observations of two small wave-like features that might be attributed to gravity wave instabilities. In order to spectrally analyse harmonic structures even on small spatial scales down to 550 m horizontal wavelength, we made use of the maximum entropy method (MEM) since this method exhibits an excellent wavelength resolution. MEM further allows analysing relatively short data series, which considerably helps to reduce problems such as stationarity of the underlying data series from a statistical point of view. We present an observation of the subsequent decay of well-organized wave fronts into eddies, which we tentatively interpret in terms of an indication for the onset of turbulence. Another remarkable event which demonstrates the technical capabilities of the instrument was observed during the night of 4–5 April 2016. It reveals the disintegration of a rather homogenous brightness variation into several filaments moving in different directions and with different speeds. It resembles the formation of a vortex with a horizontal axis of rotation likely related to a vertical wind shear. This case shows a notable similarity to what is expected from theoretical modelling of Kelvin–Helmholtz instabilities (KHIs). The comparatively high spatial resolution of the presented new version of the FAIM provides new insights into the structure of atmospheric wave instability and turbulent processes. Infrared imaging of wave dynamics on the sub-kilometre scale in the airglow layer supports the findings of theoretical simulations and modellings.

On the Susceptibility of Cold Tropical Cirrus to Ice Nuclei Abundance

Abstract Numerical simulations of cirrus formation in the tropical tropopause layer (TTL) during boreal wintertime are used to evaluate the impact of heterogeneous ice nuclei (IN) abundance on cold cloud microphysical properties and occurrence frequencies. The cirrus model includes homogeneous and heterogeneous ice nucleation, deposition growth/sublimation, and sedimentation. Reanalysis temperature and wind fields with high-frequency waves superimposed are used to force the simulations. The model results are constrained by comparison with in situ and satellite observations of TTL cirrus and relative humidity. Temperature variability driven by high-frequency waves has a dominant influence on TTL cirrus microphysical properties and occurrence frequencies, and inclusion of these waves is required to produce agreement between the simulated and observed abundance of TTL cirrus. With homogeneous freezing only and small-scale gravity waves included in the temperature curtains, the model produces excessive ice concentrations compared with in situ observations. Inclusion of relatively numerous heterogeneous ice nuclei (NIN ≥ 100 L−1) in the simulations improves the agreement with observed ice concentrations. However, when IN contribute significantly to TTL cirrus ice nucleation, the occurrence frequency of large supersaturations with respect to ice is less than indicated by in situ measurements. The model results suggest that the sensitivity of TTL cirrus extinction and ice water content statistics to heterogeneous ice nuclei abundance is relatively weak. The simulated occurrence frequencies of TTL cirrus are quite insensitive to ice nuclei abundance, both in terms of cloud frequency height distribution and regional distribution throughout the tropics.

Coherent Structures in the Boundary and Cloud Layers: Role of Updrafts, Subsiding Shells, and Environmental Subsidence

Abstract Coherent structures, such as updrafts, downdrafts/shells, and environmental subsidence in the boundary and cloud layers of shallow convection, are investigated using a new classification method. Using large-eddy simulation data, the new method first filters out background turbulence and small-scale gravity waves from the coherent part of the flow, composed of turbulent coherent structures and large-scale transporting gravity waves. Then the algorithm divides this coherent flow into “updrafts,” “downdrafts/shells,” “subsidence,” “ascendance,” and four other flow structures using an octant analysis. The novel method can systematically track structures from the cloud-free boundary layer to the cloud layer, thus allowing systematic analysis of the fate of updrafts and downdrafts. The frequency and contribution of the coherent structures to the vertical mass flux and transport of heat and moisture can then be investigated for the first time. Updrafts, subsidence, and downdrafts/subsiding shells—to a lesser extent—are shown to be the most frequent and dominant contributors to the vertical transport of heat and moisture in the boundary layer. Contrary to previous perspective, environmental subsidence transport is shown to be weak in the cloud layer. Instead, downdrafts/shells are the main downward transport contributors, especially in the trade inversion layer. The newly developed method in this study can be used to better evaluate the entrainment and detrainment of individual—or an ensemble of—coherent structures from the unsaturated boundary layer to the cloud layer.

Mesospheric gravity wave characteristics and identification of their sources around spring equinox over Indian low latitudes

Abstract. We report OI557.7 nm night airglow observations with the help of a charged-couple device (CCD)-based all-sky camera from a low-latitude station, Gadanki (13.5° N; 79.2° E). Based on the data collected during March and April over 3 years, from 2012 to 2014 (except March 2013), we characterize the small-scale gravity wave properties. During this period, 50 gravity wave events were detected. The horizontal wavelengths of the gravity waves are found to ranging from 12 to 42 km with the phase velocity 20–90 m s−1. In most cases, these waves were propagating northward with only a few occurrences of southward propagation. In the present novel investigation from the Indian sector, each of the wave events was reverse-ray-traced to its source. The outgoing longwave radiation (OLR) suggested that tropospheric convection was a possible source for generation of the observed waves. It was found that approximately 66 % of the events were triggered directly by the convection.

The dynamics of the mesosphere and lower thermosphere: a brief review

The dynamics of the mesosphere-lower thermosphere (MLT) (60 to 110 km) is dominated by waves and their effects. The basic structure of the MLT is determined by momentum deposition by small-scale gravity waves, which drives a summer-to-winter pole circulation at the mesopause. Atmospheric tides are also an important component of the dynamics of the MLT. Observations from extended ground-based networks, satellites as well as numerical modelling show that non-migrating tidal modes in the MLT are more important than previously thought, with evidence for directly coupling into the thermosphere/ionosphere. Major disturbances lower in the atmosphere, such as wintertime sudden stratospheric warmings, temporarily disrupt the circulation pattern and thermal structure of the MLT. In the equatorial mesosphere, gravity wave driving leads to oscillations in the zonal wind on semiannual time scales, although variability on quasi-biennial time scales is also apparent. Planetary-scale waves such as the quasi-two-day wave temporarily dominate the dynamics of the summertime MLT, especially in the southern hemisphere. Impacts may include short-term changes to the thermal structure and physics of the high-latitude MLT. Here, we briefly review the dynamics of the MLT, with a particular emphasis on developments in the past decade.

The quasi-biennial oscillation in a warmer climate: sensitivity to different gravity wave parameterizations

In order to simulate the quasi-biennial oscillation (QBO) with a realistic period and amplitude, general circulation models commonly include parameterizations of small scale gravity waves (GW). In this work, we explore how different GW parameterization setups determine the response of QBO properties to a warmer climate. Atmosphere-only experiments in both present day and warmer climate serve as testbed to analyze the effect of four different GW parameterization setups, active in the tropics. Having tuned the GW parameterizations to produce a realistic QBO in present day climate, we analyze changes of QBO properties in the warmer climate. The QBO period decreases in two parameterization setups by ~30 %, while the QBO period remains unchanged in the remaining two parameterization setups. In all parameterization setups, the QBO amplitude in the warmer climate weakens below 10 hPa but shows different behaviour above 10 hPa. We show that changes in QBO amplitude and changes in QBO period are inconsistent among experiments. In the chosen experimental design, the inconsistent future change in QBO properties among the suite of experiments depends solely on the choice of the GW parameterization setup.

Simulation of non-hydrostatic gravity wave propagation in the upper atmosphere

Abstract. The high-frequency and small horizontal scale gravity waves may be reflected and ducted in non-hydrostatic simulations, but usually propagate vertically in hydrostatic models. To examine gravity wave propagation, a preliminary study has been conducted with a global ionosphere–thermosphere model (GITM), which is a non-hydrostatic general circulation model for the upper atmosphere. GITM has been run regionally with a horizontal resolution of 0.2° long × 0.2° lat to resolve the gravity wave with wavelength of 250 km. A cosine wave oscillation with amplitude of 30 m s−1 has been applied to the zonal wind at the low boundary, and both high-frequency and low-frequency waves have been tested. In the high-frequency case, the gravity wave stays below 200 km, which indicates that the wave is reflected or ducted in propagation. The results are consistent with the theoretical analysis from the dispersion relationship when the wavelength is larger than the cutoff wavelength for the non-hydrostatic situation. However, the low-frequency wave propagates to the high altitudes during the whole simulation period, and the amplitude increases with height. This study shows that the non-hydrostatic model successfully reproduces the high-frequency gravity wave dissipation.

On the simulation of the quasi-biennial oscillation in the Community Atmosphere Model, version 5

The quasi-biennial oscillation (QBO) of the tropical zonal mean wind is a prominent feature of the tropical stratosphere. The easterly and westerly wind regimes alternate with a period of about 28 months. The QBO is believed to be forced by a combination of equatorial waves, in particular, Kelvin and mixed Rossby-gravity waves, as well as smaller-scale gravity waves. Although the QBO is well observed and basic forcing mechanism well understood, it has been a challenge to simulate in General Circulation Models (GCMs). In this paper we examine the role of vertical resolution and gravity wave parameterization on the simulation of the QBO in the Community Atmosphere Model, version 5. We show that in this model vertical resolution of 500 m and adequate gravity wave drag are needed to obtain a realistic QBO. At 500 m vertical resolution, CAM5 generates significantly more mixed Rossby-gravity and Kelvin waves as compared to CAM5 with 700 m or 1200 m vertical resolution. These waves then contribute to the forcing of the easterly and westerly phases of the QBO, respectively. In this work, we also briefly explore the effects of horizontal resolution on the QBO and conclude that the QBO can be adequately represented with horizontal resolution of ∼200 km as long as vertical resolution of the model is fine enough.

Rapid‐run ionosonde observations of traveling ionospheric disturbances in the auroral ionosphere

AbstractSince 2007, the Sodankylä Geophysical Observatory routinely performs vertical ionosphere soundings once per minute, using a frequency‐modulated continuous‐wave chirp at the rate of 500 kHz/s from 500 kHz to 16 MHz. We used these data to study traveling ionospheric disturbances (TIDs) during 10–16 local time. The observations were made between April 2007 and June 2012, mostly during low solar activity. The TIDs were studied in five bands of periods corresponding to the following: infrasonic (acoustic) waves and the buoyancy cutoff (periods from 5 to 10 min); small‐scale gravity waves (GWs; 10–15 min); medium‐scale (MS; 15–30 min) GWs; medium‐large scale (MS‐LS; 30–60 min) GWs; and large‐scale (LS; 60–120 min) GWs. Relative contribution (with respect to LS TIDs) of the short‐period (5–15 min) and MS (15–30 min) TIDs shows minima in winter and maxima in summer. These annual variations anticorrelate with variations of true height, namely, the largest relative amplitudes occur in summer, when TIDs were observed at minimal heights. We suggest that the summer increase of shorter‐period TIDs is due to lowering reflection to the height where the Brunt‐Väisälä period is smaller and, hence, shorter‐period gravity waves exist. The summer maxima were most prominent during the 3 years of minimal solar activity (2008–2010). In 2011, when solar activity increased, the annual variation seems less prominent. Annual variations of the longer‐period (30–120 min) TIDs are essentially less significant. For all TIDs, no obvious dependences on the AE and Ap indices of magnetic activity were found.

A Parameterization for the Effects of Ozone on the Wave Driving Exerted by Equatorial Waves in the Stratosphere

An equatorial β-plane model of the tropical stratosphere is used to examine the effects of ozone on Kelvin, Rossby–gravity, equatorial Rossby, inertia–gravity, and smaller-scale gravity waves. The model is composed of coupled equations for wind, temperature, and ozone volume mixing ratio, which are linearized about a zonally averaged background state. Using the Wentzel–Kramers–Brillouin (WKB) formalism, equations are obtained for the vertical spatial scale, spatial damping rate, and amplitude of the waves. These equations yield an analytical expression for the ozone-modified wave driving of the zonal-mean circulation. The expression for the wave driving provides an efficient parameterization that can be implemented into models that are unable to spontaneously generate the ozone-modified, convectively coupled waves that drive the quasi-biennial and semiannual oscillations of the tropical stratosphere. The effects of ozone on the wave driving, which are strongly modulated by the Doppler-shifted frequency, are maximized in the upper stratosphere, where ozone photochemistry and vertical ozone advection combine to augment Newtonian cooling. The ozone causes a contraction in spatial scale and an increase in the spatial damping rate. In the midstratosphere to lower mesosphere, the ozone-induced increase in wave driving is about 10%–30% for all wave types, but it can be as large as about 80% over narrow altitude regions and for specific wave types. In the dynamically controlled lower stratosphere, vertical ozone advection dominates over meridional ozone advection and opposes Newtonian cooling, causing, on average, a 10%–15% reduction in the damping rate.

Small-scale temperature fluctuations seen by the VeRa Radio Science Experiment on Venus Express

The Venus Express Radio Science Experiment VeRa retrieves atmospheric profiles in the mesosphere and troposphere of Venus in the approximate altitude range of 40–90km. A data set of more than 500 profiles was retrieved between the orbit insertion of Venus Express in 2006 and the end of occultation season No. 11 in July 2011. The atmospheric profiles cover a wide range of latitudes and local times, enabling us to study the dependence of vertical small-scale temperature perturbations on local time and latitude.Temperature fluctuations with vertical wavelengths of 4km or less are extracted from the measured temperature profiles in order to study small-scale gravity waves. Significant wave amplitudes are found in the stable atmosphere above the tropopause at roughly 60km as compared with the only shallow temperature perturbations in the nearly adiabatic region of the adjacent middle cloud layer, below.Gravity wave activity shows a strong latitudinal dependence with the smallest wave amplitudes located in the low-latitude range, and an increase of wave activity with increasing latitude in both hemispheres; the greatest wave activity is found in the high-northern latitude range in the vicinity of Ishtar Terra, the highest topographical feature on Venus.We find evidence for a local time dependence of gravity wave activity in the low latitude range within ±30° of the equator. Gravity wave amplitudes are at their maximum beginning at noon and continuing into the early afternoon, indicating that convection in the lower atmosphere is a possible wave source.The comparison of the measured vertical wave structures with standard linear-wave theory allows us to derive rough estimates of the wave intrinsic frequency and horizontal wavelengths, assuming that the observed wave structures are the result of pure internal gravity waves. Horizontal wavelengths of the waves at 65km altitude are on the order of ≈300–450km with horizontal phase speeds of roughly 5–10m/s.

Three-dimensional properties of Andes mountain waves observed by satellite: A case study

The southern Andes region has been clearly identified in previous satellite and balloon observations and in global models as a hot spot of small-scale gravity wave activity, with monthly mean momentum fluxes exceeding 10 times background values in fall, winter, and spring seasons. This makes this region a focus of interest for global circulation and climate studies. We analyze a case study on 8 May 2006, combining observations from the Atmospheric Infrared Sounder instrument on the Aqua satellite and the High Resolution Dynamics Limb Sounder instrument of the Aura satellite to form a three-dimensional picture of the wave field. The observations show a widespread wave pattern over the southern Andes extending eastward over the south Atlantic. Simulations with the Weather Research Forecasting model clearly identify the waves as orographic in origin, but the observed wave pattern is far from the simple two-dimensional wave field forced by steady flow over a mountain ridge. The morphology of the pattern is consistent with three-dimensional linear theoretical calculations of downstream propagation and latitudinal focusing of mountain waves into the stratospheric jet. The observations confirm the importance of this process in the stratosphere, and we find the process also occurring in the global analysis and forecasts from the European Centre for Medium-Range Weather Forecasting. Our analysis evaluates some strengths and weaknesses of current orographic wave drag parameterizations in global models and the relevance of parameterization assumptions in global models with high resolution. Copyright 2011 by the American Geophysical Union.

The Statistical Characteristics of Gravity Waves over Wuhan

By analyzing the routine radiosonde data obtained at Wuhan (30.5° N, 114° E) from 2000 to 2004, the characteristics of inertial gravity waves in troposphere and lower stratosphere(TLS) were studied, and a strong correlation between the variation of gravity wave energy and jet intensity was found. Similar result of Wuhan was obtained by analyzing the routine radiosonde data obtained at Haikou (20°N, 114°E) and Beijing (40°N, 116°E). Small scale gravity waves with wavelength less than 1 km were extracted from temperature data to give a further discussion on vairation of gravity wave energy at different heights and latitudes.