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Tropical gravity waves that are generated by convection are generally too small in scale and too high in frequency to be resolved in global climate models, yet their drag forces drive the important...

Previous work has shown eastward migrating regions of enhanced temperature variance due to long-vertical wavelength stratospheric gravity waves that are in sync with intraseasonal precipitation and tropopause wind anomalies associated with the Madden-Julian Oscillation (MJO). Here the origin of these intraseasonal gravity wave variations is investigated with a set of idealized gravity wave-resolving model experiments. The experiments specifically test whether tropopause winds act to control gravity wave propagation into the stratosphere by a critical level filtering mechanism or play a role in gravity wave generation through an obstacle source effect. All experiments use identical convective latent heating variability, but the large-scale horizontal wind profile is varied to investigate relationships between stratospheric gravity waves and zonal winds at different levels. Results show that the observed long vertical wavelength gravity waves are primarily sensitive to stratospheric zonal wind variations, while tropopause wind variations have only a very small effect. Thus, neither the critical level filter mechanism nor the obstacle source play much of a role in the observed intraseasonal gravity wave variations. Instead, the results suggest that the stratospheric waves follow the MJO precipitation sources, and tropopause wind anomalies follow the same sources. We further find evidence of intraseasonal wave drag effects on the stratospheric circulation in reanalyzed winds. The results suggest that waves drive intraseasonal stratospheric zonal wind anomalies that descend in altitude with increasing MJO phases 3 through 7. Eastward anomalies descend farther than westward, suggesting that MJO-related stratospheric waves cause larger eastward drag forces.

This paper describes a new data set from the Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) instrument, from which gravity waves (GWs) at an altitude of 50–55 km can be inferred. CIPS is sensitive to GWs with horizontal wavelengths from ~15 to 600 km and vertical wavelengths longer than 15 km. Several examples of GWs in CIPS observations are shown, including waves associated with the Andes Mountains, island topography, convection, the polar night jet, and the tropospheric jet stream. GW signatures in the CIPS data are shown to agree well with near-coincident but lower altitude measurements from the Atmospheric Infrared Sounder (AIRS) in June of 2016. Results suggest the power of combining CIPS measurements with those from other instruments to investigate GW filtering and propagation. The CIPS data set opens new areas of inquiry, enabling comprehensive investigations of GWs in the middle atmosphere on a near-global scale.

A new measure of subseasonal variability is introduced that provides a scale‐dependent estimation of vertically and meridionally integrated atmospheric variability in terms of the normal modes of linearized primitive equations. Applied to the ERA‐Interim data, the new measure shows that subseasonal variability decreases for larger zonal wave numbers. Most of variability is due to balanced (Rossby mode) dynamics but the portion associated with the inertio‐gravity (IG) modes increases as the scale reduces. Time series of globally integrated variability anomalies in ERA‐Interim show an increase in variability after year 2000. In recent years the anomalies have been about 2% above the 1981–2010 average. The relative increase in variability projecting on the IG modes is larger and more persistent than for the Rossby modes. Although the IG part is a small component of the subseasonal variability, it is an important effect likely reflecting the observed increase in the tropical precipitation variability., Key Points A new measure of global subseasonal variability provides scale and dynamics dependent variability estimatesAn increase in variability in recent years for about 2% relative to 1981 to 2010 is found in ERA‐Interim dataThe increase is larger in variability associated with inertio‐gravity modes than with Rossby modes

In the Stratéole 2 program, set to launch in November 2018, instruments will ride balloons into the stratosphere and circle the world, observing properties of the air and winds in fine detail.

Mountain lee waves have been previously observed in data from the Moderate Resolution Imaging Spectroradiometer (MODIS) "water vapor" 6.7 micrometers channel which has a typical peak sensitivity at 550 hPa in the free troposphere. This paper reports the first observation of mountain waves generated by the Appalachian Mountains in the MODIS total column water vapor (CWV) product derived from near-infrared (NIR) (0.94 micrometers) measurements, which indicate perturbations very close to the surface. The CWV waves are usually observed during spring and late fall or some summer days with low to moderate CWV (below is approx. 2 cm). The observed lee waves display wavelengths from3-4 to 15kmwith an amplitude of variation often comparable to is approx. 50-70% of the total CWV. Since the bulk of atmospheric water vapor is confined to the boundary layer, this indicates that the impact of thesewaves extends deep into the boundary layer, and these may be the lowest level signatures of mountain lee waves presently detected by remote sensing over the land.

[1] The Tropical Warm Pool International Cloud Experiment campaign centered on Darwin (12°S, 131°E) in northern Australia in January–February 2006 provided an opportunity to study gravity wave generation by convection and the associated wave propagation and momentum transport. In this study, we discuss wave generation by a single mesoscale convective system (MCS) that occurred on 23 January. The project used a variety of radars to study the spatial and temporal variability of rainfall and the associated latent heat release during the storm. A high-resolution numerical model utilized the latent heat release derived from radar rainfall measurements to compute the spatial and geographic variation of gravity wave generation and propagation into the lower stratosphere. Gravity wave ray-tracing techniques were then used to estimate the wave energy flux penetrating to heights near 90 km, where the results were compared with direct measurements made with a meteor wind radar. This comparison is used to calibrate the momentum fluxes derived from the model and the ray-tracing results using an iterative technique. The momentum was deposited in a relatively compact region. Body forces computed from the flux divergences had their maximum values at heights near 98 km with a peak values of about 400 m s−1h−1. The effects of secondary gravity wave generation are discussed, as is the overall contribution of gravity waves generated by MCSs to the momentum budget of the tropical middle atmosphere.

[1] The main aim of this study is to find and classify hotspots of stratospheric gravity waves on a global scale. The analysis is based on a 9 year record (2003 to 2011) of radiance measurements by the Atmospheric Infrared Sounder (AIRS) aboard NASA's Aqua satellite. We detect gravity waves based on 4.3 µm brightness temperature variances. Our method focuses on peak events, i.e., strong gravity wave events for which the local variance considerably exceeds background levels. We estimate the occurrence frequencies of these peak events for different seasons and time of day and use the results to find local maxima or “hotspots.” In addition, we use AIRS radiances at 8.1 µm to simultaneously detect convective events, including deep convection in the tropics and mesoscale convective systems at middle latitudes. We classify the gravity wave sources based on seasonal occurrence frequencies for convection, but also by means of time series analyses and topographic data. Our study reproduces well-known hotspots of gravity waves, e.g., the Andes and the Antarctic Peninsula. However, the high horizontal resolution of the AIRS observations also allows us to locate numerous mesoscale hotspots, which are partly unknown or poorly studied so far. Most of these mesoscale hotspots are found near orographic features like mountain ranges, coasts, lakes, deserts, or isolated islands. This study will help to select promising regions and seasons for future case studies of gravity waves.

[1] The gravity wave-drag parameterization of Alexander and Dunkerton (1999) was implemented into a Venus Thermosphere General Circulation Model (VTGCM) to investigate breaking gravity waves as a source of momentum deposition in Venus' thermosphere. Previously, deceleration of zonal jets on the morning and evening terminators in models was accomplished via Rayleigh friction, a linear drag law that is not directly linked to any physical mechanism. The Alexander and Dunkerton (1999) parameterization deposits all of the momentum of a breaking wave at the breaking altitude and features a spectrum of wave phase speeds whose amplitudes are distributed as a Gaussian about a center phase speed. We did not find a combination of wave parameters (namely, center phase speed, amplitude at center phase speed, and distribution width) to produce sufficient drag in the jet cores that would bring VTGCM density and nightglow emissions into agreement with Venus Express observations. The zonal wind shear from 100 to 120 km altitude is very strong. Gravity waves launched below 100 km either break in the strong shear zones below 115 km or are reflected and do not propagate into the jet core regions where drag is needed. The results we present demonstrate that parameterizations developed for the middle atmosphere do not work in the thermosphere and that appropriate damping mechanisms other than nonlinear breaking/saturation dominate and should be accounted for at these heights.

Observational analyses of gravity waves in the stratosphere have revealed various climatological patterns in gravity wave activity. Seasonal, geographical, and vertical variations have all been observed. In this work, a linear model of gravity wave propagation is applied to investigate the underlying causes of some of the observed patterns. A collection of monochromatic gravity waves that represent a broad spectrum of wavenumbers and frequencies is input at 6-km altitude in the model. Propagation of the waves through realistic background atmospheric wind and stability fields is treated with linear ray theory and a simple saturation condition to limit amplitudes to stable values. The wave spectrum at the 6-km source height is specified to be constant at all latitudes, longitudes, and times, so the variability that appears at higher altitudes is due entirely to background atmosphere variations. Before the model results are compared to the observations, the spectrum of waves is filtered in a way that mimics the limitations of each of the observation techniques. The filtering is described in terms of vertical wavelength and is referred to as the “observational filter.” In a vertically varying background wind, gravity waves are Doppler-shifted in intrinsic frequency and refracted to different vertical wavelengths as they propagate vertically through the atmosphere. The observational filter and the wave refraction effects can thus couple in interesting ways that have not been explicitly considered in previous work. The model shows that this coupling can give rise to geographical, seasonal, and vertical variations in gravity wave observations without any variations in the spectrum or amplitude of gravity wave sources in the troposphere. Thus careful consideration of both the background wind profile and observational filter can greatly affect the interpretation of the observed climatological patterns in gravity wave activity.

The terrestrial Fritts and Lu [1993] gravity wave parameterization, based on the theory of wave saturation, has been adopted within the Venus thermospheric general circulation model (VTGCM) to examine the impact of upward propagating gravity waves on the Venus thermospheric subsolar-to-antisolar (SS-AS) circulation and the resultant O2 IR nightglow. It is shown that the drag force generated by the breakdown of gravity waves can effectively slow down the winds, which results in reproducing the observed day/night contrasts in temperatures and densities. With stronger drag force on the dawn terminator than on the dusk terminator, the apparently superrotating zonal wind can be forced by the dissipation of gravity waves. The equivalent superrotating zonal wind varies both with local time and altitude. The simulated distribution of the O2 IR nightglow is strongly sensitive to small changes of the characteristic intrinsic phase speeds at the model lower boundary. This sensitivity implies that minor variations in gravity wave momentum fluxes to the thermosphere can give rise to large O2 IR emission variations with time and space, in agreement with ground-based observations.

The role of gravity wave forcing in the zonal mean circulation of the stratosphere is discussed. Starting from some very simple assumptions about the momentum flux spectrum of nonstationary (non-zero phase speed) waves at forcing levels in the troposphere, a linear model is used to calculate wave propagation through climatological zonal mean winds at solstice seasons. As the wave amplitudes exceed their stable limits, a saturation criterion is imposed to account for nonlinear wave breakdown effects, and the resulting vertical gradient in the wave momentum flux is then used to estimate the mean flow forcing per unit mass. Evidence from global, assimilated data sets are used to constrain these forcing estimates. The results suggest the gravity-wave-driven force is accelerative (has the same sign as the mean wind) throughout most of the stratosphere above 20 km. The sense of the gravity wave forcing in the stratosphere is thus opposite to that in the mesosphere, where gravity wave drag is widely believed to play a principal role in decelerating the mesospheric jets. The forcing estimates are further compared to existing gravity wave parameterizations for the same climatological zonal mean conditions. Substantial disagreement is evident in the stratosphere, and we discuss the reasons for the disagreement. The results suggest limits on typical gravity wave amplitudes near source levels in the troposphere at solstice seasons. The gravity wave forcing in the stratosphere appears to have a substantial effect on lower stratospheric temperatures during southern hemisphere summer and thus may be relevant to climate.

[1] Temperature observations during January and February 2006 from the High Resolution Dynamics Limb Sounder (HIRDLS), the Microwave Limb Sounder (MLS), and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instruments are compared to illustrate the vertical range over which version 6 HIRDLS temperatures are scientifically useful. In order to determine the quality of HIRDLS temperatures in the middle atmosphere, we compare the height and temperature of the HIRDLS stratopause with MLS and SABER before, during, and after the 2006 major stratospheric sudden warming. Results show that HIRDLS observes the elevated stratopause at 78 km two days later than MLS and five days after SABER. We compare the geographical temperature structure of these data sets at 0.01 hPa during this period. Though HIRDLS temperatures are consistently 5–10 K lower in the mesosphere, this is the first study to show that the horizontal temperature distribution is in good spatial and temporal agreement with MLS and SABER up to ∼80 km. Gravity wave momentum flux and planetary wave 1 amplitudes are derived from HIRDLS and shown to be in agreement with previous studies. We use HIRDLS to show a ∼30 K increase in stratopause temperature following enhanced gravity wave momentum flux in the lower mesosphere.

[1] We examine equatorial wave structure in temperature measurements from the High Resolution Dynamics Limb Sounder (HIRDLS) on the Aura satellite. Waves with periods longer than 1 day and zonal wave numbers up to 8 (depending on frequency) are derived from an asynoptic Fourier transform analysis. HIRDLS measurement sampling and resolution afford unprecedented views of the latitude-height structure of equatorial Rossby wave, mixed Rossby-gravity wave, and Kelvin wave modes at altitudes above cloud tops in the tropical upper troposphere and stratosphere. Wave modes with vertical wavelength as short as 4 km can be clearly seen in the data. Kelvin waves comprise a dominant signal throughout the 3 years of HIRDLS measurements, and we further examine time, height, and longitude variations observed in the Kelvin waves. An annual cycle of Kelvin wave temperature amplitudes near the tropopause is observed that may have implications for annual variations in wave-cirrus formation in the tropics. This annual variation can be largely explained by effects of the background wind and stability on Kelvin wave propagation and potential energy in the tropical tropopause layer. At altitudes 20 km and above, the annual cycle gives way to an interannual cycle in Kelvin wave amplitudes that is related to the quasi-biennial oscillation in stratospheric winds. This interannual variation is a signature of Kelvin wave forcing of the descent of westerly winds in the oscillation, and we compute the Kelvin wave fractional contribution to the forcing.

[1] Gravity waves (GWs) play critical roles in the global circulation and the temperature and constituent structures in the middle atmosphere. They also play significant roles in the dynamics and transport and mixing processes in the upper troposphere and lower stratosphere and can affect tropospheric weather. Despite significant advances in our understanding of GWS and their effects in different regions of the atmosphere in the past few decades, observational constraints on GW parameters including momentum flux and propagation direction are still sorely lacking. Global Positioning System (GPS) radio occultation (RO) technique provides global, all-weather, high vertical resolution temperature profiles in the stratosphere and troposphere. The unprecedentedly large number of combined temperature soundings from the Constellation Observing System for Meteorology, Ionosphere, and Climate and Challenging Minisatellite Payload GPS RO missions allows us to obtain GW perturbations by removing the gravest zonal modes using the wavelet method for each day. We extended the GW analysis method of Alexander et al. (2008) to three dimensions to estimate the complete set of GW parameters (including momentum flux and horizontal propagation direction) from the GW temperature perturbations thus derived. To demonstrate the effectiveness of the analysis, we showed global estimates of GW temperature amplitudes, vertical and horizontal wavelengths, intrinsic frequency, and vertical flux of horizontal momentum in the altitude range of 17.5–22.5 km during December 2006 to February 2007. Consistent with many previous studies, GW temperature amplitudes are a maximum in the tropics and are generally larger over land, likely reflecting convection and topography as main GW sources. GW vertical wavelengths are a minimum at equator, likely due to wave refraction, whereas GW horizontal wavelengths are generally longer in the tropics. Most of the waves captured in the analysis of the GPS data are low-intrinsic frequency inertia-GWs, and the estimated intrinsic frequencies scaled by the Coriolis parameter also show a strong maximum at equator. Enhanced wave fluxes are linked to convection, topography, and storm tracks, among others. As preliminary tests of the analysis in deriving horizontal propagation directions, we compared the GPS estimates with the corresponding estimates from the U.S. high vertical resolution radiosonde data using the conventional Stokes parameters method and we also conducted a separate analysis of the GPS data over the southern Andes in South America. We also showed the first global estimates of GW propagation directions from the GPS data. Finally, the sensitivity of the analysis to the temporal and spatial dimensions of the longitude × latitude × time cells and the uncertainties of the analysis and possible ways to reduce these uncertainties are discussed.

[1] Convective gravity waves are an important driver of the equator-to-pole circulation in the stratospheric summer hemisphere, but their nature is not well known. Previous studies showing tight relationships between deep convection and convective waves mainly focus on tropical latitudes. For midlatitudes most analyses are based on case studies. Here we present a new multiyear occurrence frequency analysis of convective waves at midlatitudes. The study is based on radiance measurements made by the Atmospheric Infrared Sounder (AIRS) satellite experiment during the North American thunderstorm season, May to August, in the years 2003–2008. For this study we optimized an existing algorithm to detect deep convection in AIRS data to be applicable at midlatitudes. We also present a new detection algorithm for gravity waves in AIRS data based on a variance filter approach for 4.3 μm brightness temperatures. The new algorithm can detect plane wave perturbations in the altitude range from 20 to 65 km with vertical wavelengths larger than 15 km and horizontal wavelengths from 50 to 1000 km. By analyzing spatial and temporal correlations of the individual AIRS observations, it can be shown that more than 95% of the observed gravity waves in a core region over the North American Great Plains are related to deep convective clouds, i.e., are likely being classified appropriately as convective waves. We conclude that the core region is a good location to observe and characterize the properties of convective waves at midlatitudes. The statistical analyses presented here are also valuable to validate parameterization schemes for convective gravity waves. For completeness, it should be mentioned that our analyses cover not only the U.S. Midwest but the North American continent as well as the surrounding ocean regions in general. Our analysis also reveals interesting details about tropical convection and related gravity wave activity, as well as the capability of the AIRS instrument to observe these.

[1] During the Tropical Warm Pool International Cloud Experiment (TWPICE) an intense tropical low was situated between Darwin and Alice Springs, Australia. Observations made on 31 January 2006 by the Atmospheric Infrared Sounder instrument on the NASA Aqua satellite imaged the presence of atmospheric gravity waves (AGWs), at approximately 40 km altitude, with horizontal wavelengths between 200 and 400 km that were originating from the region of the storm. Airglow images obtained from Alice Springs (about 600 km from the center of the low) showed the presence of similar waves with observed periods of 1 to 2 h. The images also revealed the presence of 30- to 45-km-horizontal-wavelength AGWs with shorter observed periods of near 15 to 25 min. Ray tracing calculations show that (1) some of the long wavelength waves traveled on rays, without ducting, to the altitudes where the observations were obtained, and (2) shorter-period waves rapidly reached 85 km altitude at a horizontal distance close to the storm, thus occurring over Alice Springs only if they were trapped or ducted. The mesospheric inversion layer seen in the measured temperature data almost forms such a trapped region. The winds therefore critically control the formation of the trapped region. Wind profiles deduced from the available data show the plausibility for the formation of such a trapped region. Variations in the wind, however, would make ideal trapped region conditions short-lived, and this may account for the sporadic nature of the short-period wave observations.

[1] The Atmospheric Infrared Sounder (AIRS) on board the National Aeronautics and Space Administration's (NASA's) Aqua satellite has been continuously measuring mid-infrared nadir and sub-limb radiance spectra since summer of 2002. These measurements are utilized to retrieve three-dimensional stratospheric temperature distributions by applying a new fast forward model for AIRS and an accompanying optimal estimation retrieval processor. The retrieval scheme presented in this article does not require simultaneous observations of microwave instruments like the AIRS operational analyses. Instead, independent retrievals are carried out at the full horizontal sampling capacity of the instrument. Horizontal resolution is enhanced by a factor 3 in along- and across-track directions compared with the AIRS operational data. The total retrieval error of the individual temperature measurements is 1.6 to 3.0 K in the altitude range from 20 to 60 km. Retrieval noise is 1.4 to 2.1 K in the same vertical range. Contribution of a priori information to the retrieval results is less than 1% to 2% and the vertical resolution of the observations is about 7 to 15 km. The temperature measurements are successfully compared with ECMWF operational analyses and AIRS operational Level 2 data. The new temperature data set is well suited for studies of stratospheric gravity waves. We present AIRS observations of small-scale gravity waves induced by deep convection near Darwin, Australia, in January 2003. A strong mountain wave event over the Andes in June 2005 is analyzed in detail. Temperature perturbations derived from the new data set are compared with results from the AIRS operational Level 2 data and coincident measurements of the High Resolution Dynamics Limb Sounder (HIRDLS). The new retrieval does not show response to wave perturbations if the vertical wavelength is below 10 km. For 15 km vertical wavelength, the amplitudes are damped by a factor of two. For vertical wavelengths of greater than 20 km, AIRS shows very similar wave structure to HIRDLS and also has the advantage of providing horizontal phase front information. Data from the new full-resolution retrieval are far more suitable for gravity wave studies than results from the AIRS operational analysis.

Satellite observations of polar stratospheric clouds (PSCs) over Antarctica in June 2003 revealed small nitric acid trihydrate (NAT) particles forming suddenly along the vortex edge. Models suggest the trigger was mountain waves over the Antarctic Peninsula (AP) forming ice for NAT nucleation. We test this hypothesis by analyzing perturbations in stratospheric radiances from the Atmospheric Infrared Sounder (AIRS). AIRS data show mountain waves over the AP on 10-14 June, with no resolved wave activity before or after. Peak wave temperature amplitudes derived from independent 40 hPa channels all return values of 10-12 K, in agreement with values used to model this NAT event. These observations support a NAT wake from a small region of mountain wave activity over the AP as the source of this circumpolar NAT outbreak.

[1] We describe a 2-day wave event observed in high-resolution radiosonde soundings of horizontal wind and temperature taken during the TWP-ICE experiment in Darwin area. The vertical profiles of temperature, zonal and meridional wind speeds are analyzed using the S-transform wavelet analysis. Results of the analysis reveal the presence of 2-day inertia gravity waves in the stratosphere between 20 and 27 km. The wave presents vertical and horizontal wavelengths of around 6 km and 7220 km respectively. The wave was observed to propagate southeastward during the end of the easterly phase of the QBO. The total vertical momentum flux associated with the waves is estimated to be 1 to 2.2 × 10−3 m2s−2. This is of the same order of magnitude as previous observations of 4–10 day Kelvin waves in the lower stratosphere.

[1] We present the first evaluation of the HNO3 data product (version 2.04.09) from the High Resolution Dynamics Limb Sounder (HIRDLS) on the Earth Observing System (EOS) Aura satellite. The HIRDLS instrument obtains between 5000 and 7000 HNO3 profiles per day. HIRDLS HNO3 data are generally good over the latitude range of 64°S to 80°N and pressure range 100 to 10 hPa, with some profiles, depending on latitude, having useful information between 100 to 161 hPa. The individual profile “measured” precision is between 10 and 15%, but can be much larger if the HNO3 abundance is low or outside the 100 hPa to 10 hPa range. Global results are compared with the HNO3 observations from version 2.2 of the EOS Aura Microwave Limb Sounder (MLS), and it is found that large-scale features are consistent between the two instruments. HIRDLS HNO3 is biased 0–20% low relative to Aura MLS in the mid-to-high latitudes and biased high in the tropical stratosphere. HIRDLS HNO3 is also compared with Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). In these mostly high-latitude comparisons the HIRDLS HNO3 data are biased 10–30% low, depending on altitude. Finally, the HIRDLS HNO3 is compared to in situ data taken by the NOAA Chemical Ionization Mass Spectrometer (CIMS) instrument flown during the 2005 NASA Houston Aura Validation Experiment (AVE) and the ability of HIRDLS to measure HNO3 in the UTLS region is examined.

analyzed to derive global properties of gravity waves. We describe a wavelet analysis technique that determines covarying wave temperature amplitude in adjacent temperature profile pairs, the wave vertical wavelength as a function of height, and the horizontal wave number along the line joining each profile pair. The analysis allows a local estimate of the magnitude of gravity wave momentum flux as a function of geographic location and height on a daily basis. We examine global distributions of these gravity wave properties in the monthly mean and on an individual day, and we also show sample instantaneous wave events observed by HIRDLS. The results are discussed in terms of previous satellite and radiosonde observational analyses and middle atmosphere general circulation model studies that parameterize gravity wave effects on the mean flow. The high vertical and horizontal resolution afforded by the HIRDLS measurements allows the analysis of a wider range of wave vertical and horizontal wavelengths than previous studies and begins to show individual wave events associated with mountains and convection in high detail. Mountain wave observations show clear propagation to altitudes in the mesosphere.

[1] Orographic gravity waves generated by flow over the topography of small islands in the southern oceans have been observed from orbit with the Atmospheric Infrared Sounder on the Aqua satellite. We examine the occurrence frequencies of these waves in the stratosphere at ∼40km above 14 islands and examine geographical and seasonal changes. Our results show that these small island mountain waves occur commonly in the stratosphere in the May–September season, though not every day. Differing seasonal variations are evident at different islands, and the seasonal variations are closely related to latitude and prevailing wind patterns. We also examine interannual variability in 2years of data and the relationships between occurrence frequencies, momentum fluxes, and stratospheric and surface winds. The results suggest that stratospheric winds have a first-order limiting effect on the observations of these island mountain waves in Atmospheric Infrared Sounder (AIRS) data. Surface wind direction and island orographic relief have an additional but secondary influence on the island mountain wave occurrence frequencies in AIRS data. The implications are that these wave events are extremely common and that on many days when the waves are not observed in AIRS data they have likely dissipated and induced a drag force on the atmosphere below the 40km observation level. Observations of momentum flux during these wave events also permit a first estimate of their importance to the general circulation of the Southern Hemisphere.

[1] A VHF atmospheric radar (wind profiler) was used to study tropospheric winds during the Darwin Area Wave Experiment (DAWEX). The profiler, which operated at a frequency of 54.1 MHz, was located at Pirlangimpi (Garden Point) (11.4°S, 130.5°E) on the Tiwi Islands. Observations were made regularly up to heights near 8 km, with maximum heights occurring when convective activity was strongest. Mean winds observed between October and December 2001 are in good agreement with conditions that prevailed across northern Australia during this period. During the first two intensive observation periods (IOP) during October and November, the zonal and meridional wind components were westward and northward, respectively, with stronger values in November. By the time of IOP3 in mid-December, the zonal flow was eastward, a pattern that is typical of the Australian monsoon. Fluctuations in the three wind components for periods less than 3 hours are analyzed for IOP2 in November, when strong convective storms (“Hectors”) occurred on all afternoons over the Tiwi Islands. The fluctuations, which are ascribed to convectively generated gravity waves, show a correspondingly strong diurnal cycle, with horizontal wind variances peaking between 8 and 12 m2s−2 in the early afternoon in the lower troposphere. Variances are only ∼2 m2s−2 in the early morning hours. A power spectral analysis shows that oscillations with ground-based periods between 8 and 17 min are especially prominent during Hector events. The profiler observations are compared with a numerical model study of gravity wave generation by convection on 17 November 2001. There is a satisfactory degree of agreement between the behavior of the model and profiler oscillations, both as a function of height and time.

A comparison is presented of the 130-m images taken in the Venus thermosphere by the Pioneer Venus Orbiter Ultraviolet Spectrometer (PVOUVS) to predictions by a model which incorporates current understanding of the global structure of the thermosphere, the mechanisms which excite the 130-nm transition in O, and the radiative transport of the 130-nm triplet in the thermosphere. The features identified in the data/model comparison appear as a local time asymmetry in B(130) and O at altitudes poleward of 30 deg. Oxygen densities at the evening terminator are typically a factor of 2 higher than those at the morning terminator. This asymmetry in O has never before been observed or predicted in the global thermospheric models.

Modern models of thermospheric composition and temperature and of excitation and radiative transfer processes are used to simulate the O I 130-nm emission from Mars measured by the Mariner 9 ultraviolet spectrometer. This paper uses the Mars thermospheric general circulation model calculations (MTGCM) of Bougher et al. (1988) and the Monte Carlo partial frequency redistribution multiple scattering code of Meier and Lee (1982). It is found that the decline in atomic oxygen through the daylight hours predicted by the MTGCM cannot be reconciled with the excess afternoon brightness seen in the data. Oxygen concentrations inferred from the data show a positive gradient through the day, in agreement with the original analysis by Strickland et al. (1973). In addition, the data suggest that the oxygen abundance increases toward high southerly latitudes, in contrast with the MTGCM prediction of high values in the Northern Hemisphere. It appears that solar forcing alone cannot account for the observed characteristics of the Martian thermosphere and that wave and tidal effects may profoundly affect the structure, winds, and composition.