We perform a three-dimensional simulation of a breaking internal gravity wave in a stratified, compressible, and sheared fluid to investigate the vorticity dynamics accompanying the transition from laminar to turbulent flow. Baroclinic sources contribute preferentially to eddy vorticity generation during the initial convective instability of the wave field, yielding counter-rotating vortices aligned with the external shear flow. These vortices enhance the spanwise vorticity of the shear flow via stretching and distort the spanwise vorticity via advective tilting. The resulting vortex sheets undergo a dynamical (Kelvin-Helmholtz) instability which rolls the vortex sheets into tubes which link, in turn, with the original streamwise convective rolls to produce a collection of intertwined vortex loops. Following the formation of discrete vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of adjacent vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets imposes axial vorticity variations having both axisymmetric and azimuthal wavenumber two components. Axisymmetric variations excite axisymmetric twist waves, or Kelvin vortex waves, which propagate along the tubes, drive axial flows, and deplete and fragment the tubes. Azimuthal wavenumber two variations excite m = 2 twist waves on the vortex tubes which amplify and unravel single vortex tubes into pairs of intertwined helical tubes. Other interactions, judged less fundamental to the turbulence cascade, include reconnection among tube fragments, mutual stretching of orthogonal tubes in close proximity, excitation of azimuthal wavenumber one twist waves, and the continual roll-up of weaker vortex sheets throughout the evolution. Collectively, these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

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This paper reports first steps toward a computationally inexpensive spectral gravity wave parameterization scheme whose predictions approximate those of a full three-dimensional (in spectral space) spectral model of atmospheric gravity waves. A reduction to two dimensions, as proposed by Hines, requiring the neglect of Coriolis and nonhydrostatic effects, is explored on the basis of comparisons with a full three-dimensional power-spectral model that includes Coriolis and non-hydrostatic effects. The reduction tries to be more realistic in terms of spectral shapes, though simpler in terms of wave-breaking criteria. It works remarkably well in the absence of, but less well in the presence of, background shear. The reasons for the discrepancies are under investigation, as are the implications for two-dimensional schemes, including Hines' as well as ours.

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Generation of internal gravitywaves (IGWs) by tropospheric convections and vertical propagation of the generated IGWs throughout the middle atmosphere are simulated by a non-hydrostatic compressible nonlinear two-dimensional numerical model. The present simulation demonstrates that (i) IGWs are generated by the tropospheric dry convections, (ii) zonal mean wind is decelerated by critical layer absorption of the generated IGWs in the upper tropospheric shear zone, (iii) secondary IGWs are radiated by the critical layer instability, and (iv) the secondary IGWs break down and accelerate zonal mean winds in the upper middle atmosphere. The detailed analyses show that (1) eastward propagating IGWs generated by the convection in tropospheric westerly with small wavelength of the order of 10 km and short period of the order of 10 min are dominant. It is found that the dominant waves are selected by a filtering effects of the prescribed westerly. (2) Several secondary IGWs with smaller horizontal wavelength than the primary IGW are radiated. The secondary IGWs propagate vertically in the form of wavepackets and break in the upper middle atmosphere due to local convective instability because of the exponential growth of the wave amplitudes with height. In the breaking region, the observed and theoretically predicted universal power law of the wind fluctuation, which states the m-3 dependence for the power spectra versus the vertical wavenumber m due to the wave saturation and breakdown, is also realized in the present model simulation.

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The temporal evolution of a turbulent layer is calculated in detail by solving the hydrodynamic equations. The turbulence is initiated by a Kelvin-Helmholtz instability. The field of potential-temperature fluctuations serves as a tracer for modeling entrainment of the mixing ratios of ionized constituents hypothesized to be present in the upper polar mesosphere. This entrainment modeling provides the input to a turbulence advection model capable of calculating the spectra and cospectra of ions and electrons. The turbulence advection model is used as a subgrid-scale model and is required because, given present or foreseeable computer capabilities, numerical solutions cannot span the enormous range of spatial scales from the depth of the shear layer to the smallest scales on which the most massive ions diffuse. The power spectrum of electron number-density fluctuations obtained from the turbulence advection model is compared with that measured by a rocket during the STATE (Structure and Atmospheric Turbulence Environment) experiment; agreement is found for a case of massive ions. The radar cross section for Bragg scattering is calculated from the electron number-density power spectrum and is used to calculate the signal-to-noise ratio (S/N) for the Poker Flat 50 MHz radar. The resultant S/N is then compared with the radar measurements obtained during the STATE experiment. These comparisons support the hypothesis that massive ions can cause polar mesosphere summer echoes from turbulent layers. Large-scale morphology of the turbulent layer obtained from rocket and radar measurements is reproduced by the hydrodynamic solution.

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During the last two decades many estimates of turbulence strength have been made by a variety of techniques in the mesosphere above northern Norway. We have assimilated many of these results and present them in this study, enabling the reader to note systematic differences. We concentrate on seasonal variation not only in an attempt to smooth out non-representative data, but also to identify the seasonal features themselves. We note both semi-annual and annual variations in turbulent intensity, depending on the height considered. Finally we address the aforementioned systematic differences between the methods and suggest possible causes in terms of each method's underlying assumptions.

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The study of turbulent heating and diffusion in the middle atmosphere is complicated by some subtle points relating to the application of existing theory. Incorrect interpretation of turbulent spectra can result, leading to errors in estimates of the strengths of turbulence by factors of 5 and more. In this short review, the relevant turbulent spectra and equations are considered, and their applications in middle atmosphere studies are outlined. New developments with regard to some of this theory, and especially new understandings about the dynamical parameters used in some of these applications (often referred to as the “constants” of the equations) are described. Current areas of uncertainty are also considered, both in relation to turbulent energy dissipation as well as diffusion over various scales.

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Observations of winds and gravity waves (GW) by MF radars from the Arctic to the Equator are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. The years used are mainly 1993/4; the height layer 76.88 km; and the GW bands 10 100 min. and 1-6 hrs. The high/mid-latitude locations of Tromsø, Saskatoon, London/Urbana, Yamagawa, generally demonstrate similar behaviour: the monthly spectra have slopes near -5/3 in winter months, but smaller (absolute) slopes at higher frequencies (<2 hrs.) in summer. Corresponding to this, the spectral densities (10-100 min.) are larger for conditions of higher mean background windspeed - this is related by means of a new correlation-vector technique to GW propagating anti-parallel to the mean zonal winds, and the closure of the solstitial mesospheric jets. Also consistent with this, the sizes and orientations of perturbation ovals (fitted to the wind variations), demonstrate strong semi-annual-oscillations (SAO), and generally similar monthly and latitudinal directions. This suggests strong control, especially of the high-frequency GW band, by the dominant zonal windstructures of the mesosphere. In contrast the low-latitude locations of Hawaii and Christmas Island demonstrate uniquely different behaviours, with indications of significant inter-annual variability. The frequency spectra for all months tend to have smaller slopes at higher frequencies. Also the dependence of spectral density in both GW bands, upon background wind speed, is negative rather than positive, and is shown to be generally consistent with GWpropagating parallel to the mean-global winds. This is consistent with weaker vertical shears in the zonal winds (76-88 km), and lower GW momentum depositions. The perturbation ovals reveal much weaker SAO, and more variable orientations, consistent with more dependency upon GW sources, and less control by the mean winds of the mesosphere.

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Recent satellite and radar observations suggest that the semiannual oscillation (SAO) in the mesosphere is modulated by the stratospheric quasibiennial oscillation (QBO). The modulation is only apparent during the SAO easterly phase, which is considerably stronger when QBO winds are westerly than when they are easterly. We use an equatorial beta-plane model to demonstrate how such a modulation could come about through selective damping of the equatorial wave spectrum excited by deep convection. The waves affected most strongly are easterly inertia-gravity waves of phase speeds slower than - -40 m s-1. This is close to the zonal wind speed during the easterly phase of the QBO (-30 to -35 m s-1), so the waves suffer strong thermal damping or even absorption as they propagate through the stratosphere. Because these waves are important for driving the easterly phase of the mesopause SAO in the model, that phase is weaker when the stratospheric QBO winds are easterly. A similar modulation of the westerly phase of the SAO does not occur for two reasons: (1) QBO westerlies are only half as strong as QBO easterlies, and (2) much of the driving of the westerly phase of the SAO is accomplished by Kelvin waves of phase speed -40-60 m s-1. As a consequence, the QBO winds have negligible influence on the vertical propagation of waves with westerly phase velocity and hence on the westerly phase of the modeled SAO.

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We report on a series of numerical experiments conducted with the global-scalewave model (GSWM) and designed to investigate the effects of the quasi-biennial oscillation (QBO) on the migrating diurnal tide. Our results indicate that the diurnal tidal response in the upper mesosphere and lower thermosphere (MLT) is significantly affected by the QBO in zonal mean zonal winds, but largely insensitive to the QBO in stratospheric ozone. We discuss the variable mean wind results in light of previous analytic attempts to describe the diurnal tide in the presence of mean winds and dissipation. Our calculations do not explain the interannual tidal variations observed by the High Resolution Doppler Interferometer (HRDI) on the Upper Atmosphere Research Satellite (UARS).

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The long-term behavior of atmospheric tides in the mesosphere and lower thermosphere has been observed with the meteor wind radar (MWR) in Jakarta, Indonesia (6°S, 107°E) from November 1992 to August 1997. The amplitudes and phases of the diurnal tides showsystematic seasonal variations, particularly distinct in the meridional component. In addition, substantial interannual variability is evident, characterized by a biennial periodicity of tidal parameters, and considerably small tidal amplitudes exclusively seen in 1996. The MWR results are compared with the Global ScaleWave Model (GSWM) as well as MF radar data collected in two equatorial sites in Pontianak (0.03°N, 109°E) and Christmas Island (2°N, 158°W) for November 1995-July 1997 and January 1996-October 1997, respectively. Comparison studies of these radar data have revealed the detailed latitudinal structure of the diurnal tide near the equator. The GSWM has successfully described the general characteristics of the radar results, although some discrepancies are recognized. In 1996 when radar data are available at all the three sites, the monthly mean values of tidal amplitudes at 90 km agreed very well between Jakarta and Pontianak, while significant discrepancy was found for Christmas Island, suggesting the existence of geographical effects such as non-migrating tides.

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WINDII, the Wind Imaging Interferometer on the Upper Atmospheric Research Satellite measures winds and emission rates from selected excited metastable species. Here we report on measurements of the atmospheric Rayleigh scattering from the O(1S) background filter at 553 nm wavelength used to derive temperature profiles in the upper mesosphere from 70 km to 95 km, for solstice periods from December 1992/93 and January 1993/94. The data are first zonally averaged and then combined in local time over about one month. Based on these temperatures, an analysis of planetary wave structures and tidal perturbations employing least-mean-square (LMS) fits to the data has been conducted and the results are presented. The planetary wave structures observed were well described with a quasi two-day wave (QTDW). Amplitudes of 14 K and 10 K at 85 km height for downleg (descending) and upleg (ascending) sampling respectively at latitudes from 20°S to 40°S were found to be in good agreement with QTDW temperature results from the MLS/UARS experiment assuming a vertical amplitude structure of the type described by the HRDI/UARS mesospheric wind observations. It is shown that the diurnal tide amplitudes estimated from latitudes from 25°N to 35°S using the LMS fit maximize at the equator with an amplitude of about 6 K and decrease toward tropical latitudes, consistent with the classical tidal theory and predictions from the TIME-GCM model.

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A meteor radar was operated at Amundsen-Scott Station, South Pole, from January 19, 1995 through January 26, 1996 and from November 21, 1996 through January 27, 1997. Hourly wind measurements were obtained nearly continuously over these time periods, at an approximate altitude of 95 km and at about 2. latitude from South Pole along the longitude meridians 0°, 90°E, 90°W, and 180°. The scientific advances achieved to date through analyses of these data are presented, including updates to several of our previously published works. The findings addressed herein include the following: (1) Strong divergences of zonal-mean meridional winds occasionally occur over South Pole, implying extreme vertical winds; (2) The monthly mean zonally asymmetric (zonal wavenumber s = 1) wind component varies during the year in a manner consistent with migration of the center of the polar vortex with respect to the geographic (rotational) pole; (3) Strong (>15 m/s) westward-propagating migrating diurnal (s = 1) and non-migrating semidiurnal (s = 1) oscillations exist except during winter months; (4) Long-period (-2-10 days) waves exist during winter months which are primarily eastward-propagating; (5) Intradiurnal (periods -6-11.5 hours) westward-propagating oscillations exist, which are thought to be gravitational normal modes, or “Lamb” waves.

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We present radar measurements of the 12-hourwave, a zonalwavenumber 1 westward propagatingwave that exists in the southern polar mesopause region winds (Hernandez et al., 1993; Forbes et al., 1995). MF radar measurements of the horizontal winds at McMurdo (77.8°S, 166.67°E) show that the 12-hour wave is highly seasonal, occurring during the austral summer solstice. During these seasonal occurrences, thewave is highly intermittent with amplitude peaks of ≥ 30 m s-1. The burst-like occurrences of large 12-hour wave amplitudes are highly correlated between the zonal and meridional direction. The diurnal tide over McMurdo has a more constant amplitude, but it is also an almost exclusively summertime phenomenon. Inertia-gravity wave activity is evident at periods less than 12 hr during the austral winter months. The weakening of gravity wave activity during the summer is probably due to critical layer filtering by the zonal mean wind, 12-hour wave and diurnal tide which are all strong during this season. The 12-hour wave is confined in height to the vicinity of the zero crossing in the zonal winds above the westward jet. Extreme distortion is observed in the vertical phase fronts of the 12-hour wave which could signify either refraction or in situ forcing. The distortion in the phase fronts and localization of the 12-hour wave in time and height is apparently responsible for departures in period from the nominal 12 hours. We do not find the wave period to be systematically different from 12 hours. The association of the 12-hour wave events with shear in the mean wind suggests that refractive effects could conceivably cause a dilation in wave amplitude. However, the shear is of the opposite sign to cause this dilation unless the wave originates at higher altitudes and propagates downward into the mesosphere. Investigations are made of the zonal structure of the 12-hour wave by comparing phases of the 12-hour wind component between McMurdo and the dynasonde at Halley (75.8°S, 26.4°W). The phase is found to be stable and consistent with a westward propagating zonal wavenumber 2 structure during seasons when the 12-hour wave is weak. The migrating semidiurnal tide evidently dominates during these times of the year. During seasons when the 12-hour wave amplitude is large, the zonal structure is highly unstable and there is not an obvious dominant zonal wavenumber.

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A set of numerical experiments have been conducted using the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (NCAR TIME-GCM) to understand the effects of the quasi-two-day wave (QTDW) on the middle atmosphere horizontal wind and temperature fields. A zonal wavenumber three perturbation with a period of 48 hours and a latitudinal structure identical to the (3, 0) Rossby-gravity mode has been included at the lower-boundary of the model. A response in the middle atmosphere horizontal wind fields is observed with a structure qualitatively similar to observations and other model results. There is also some evidence to suggest an increase in the lower-thermosphere QTDW response due to the interaction with gravity waves. Changes are observed in the zonal mean wind and temperature fields that are clearly related to the QTDW, however it is unclear if these changes are the direct result of wave driving due to the QTDW or are from another source. Evidence for nonlinear interactions between the QTDW and the migrating tides is presented. This includes significant (40-50%) decreases in the amplitude of the migrating tides when the QTDW is present and the generation of wave components which can be tracked back to an interaction between the QTDW and the migrating tides. Clear evidence for the existence of a westward propagating zonal wavenumber six nonmigrating diurnal tidal component which results from the nonlinear interaction between the QTDW and the migrating tides is also presented.

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This paper presents evidence for a quasi-stationary Kelvin wave of zonal wavenumber 1 that is forced in situ in the upper mesosphere and propagates upward from there. Although large scale quasi-stationary longitudinal variations are common in the mesopause equatorial winds, these variations often do not have a structure associated with a vertically propagating mode. However, during the periods of easterly winds that occur near the equinoxes in association with the mesopause semi-annual oscillation (MSAO), aKelvinwave propagates away from the presumed altitude of forcing. The momentum transport by theKelvinwave is small and does not make a significant contribution to the SAO wind evolution.

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The Communications Research Laboratory (CRL) has operated theWakkanai MF radar since September of 1996, and YamagawaMF radar since August of 1994. Recent observation results on variations of mean wind, wind spectra and diurnal wind oscillations during a strong eastward wind, comparison experiments with theMUradar and rockets, and D-region electron density measurements by MF radar are summarized briefly. The spectra comparison of two MF radar observations show a high degree of spatial and temporal variability in a winter mesosphere in Northern and Southern Japan. Differences of mean wind in 1997 between both sites are shown for the periods of solstices and equinoxes. It is suggested that the propagation of the diurnal tide from the mesosphere into the dynamo region is disturbed by the sudden enhancements of the strong eastward wind.

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Zonal and meridional winds in the equatorial mesosphere and lower thermosphere (65-98 km) measured at two sites separated by 94° in longitude are used to study the zonal structure of planetary-scale waves. The data were obtained with MF radars located at Pontianak (0°N, 109°E)and Christmas Island (2°N, 157°W). The data at Christmas Island were collected from January 1990 to December 1997 and the observations at Pontianak were made from November 1995 to July 1997. Power spectral techniques are used to study the amplitude and frequency variations of long-period oscillations as a function of height and time. A mean climatology of these variations taken from years 1990-1997 is presented. Strong peaks in zonal and meridional winds are found at tidal periods and for the quasi 2-day wave. Zonal spectra exhibit considerable power at periods of 3-10 days, with transient oscillations with periods near 3.5 day and 6.5 days being especially prominent. The 6.5-day wave is particularly strong during April and September. Examination of the phase differences obtained from cross-spectra between the two stations show that the 6.5-day wave is westward propagating with zonal wavenumber 1, while the 3.5 day wave is eastward propagating with wavenumber 1. The 6.5-day wave is identified as a manifestation of an unstable mode, while the 3.5-day wave is identified as an ultrafast Kelvin wave. There are significant longitudinal variations in the amplitudes and inferred momentum fluxes of the 3.5-day wave, amplitudes being larger in the Asian region than in the central Pacific.

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This paper is concerned with observations of the long-term behavior of Kelvin waves with the wave period ranging from 3 to 4 days, which are generally called an ultra-fast Kelvin (UFK) wave. Horizontal wind velocity at 74-110 km altitudes observed with a meteor wind radar (MWR) near Jakarta (6.4°S, 106.7°E) for five years during November 1992 and December 1997 and daily radiosonde profiles in Bandung (6.9°S, 107.6°E) collected between October 1993 and March 1996 and have been analyzed. In the mesosphere and lower thermosphere (MLT) region, the UFK wave activity, defined by the spectral density of zonal wind perturbations at the 3.0-3.8 day period, is strongly enhanced twice a year. An interaction between UFK waves and a semiannual oscillation in the mesosphere (MSAO) can be suggested, although an exact mechanism is uncertain. We also have investigated seasonal variation of 3.0-3.8 day oscillations of zonal winds in the stratosphere, excluding gravity wave components, but, we have not detected an evidence of semiannual periodicity. The UFK wave activity in the MLT region exhibited intraseasonal variations, which showed some correlation with the amplitudes of zonal wind in the troposphere.

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Mesospheric winds are derived from HF radar observations of meteor echoes at Halley (76°S, 27°W), Antarctica. These meteor echoes are observed within the first range gates of the radar (less than 500 km) and are characterised by lower spectral widths than echoes backscattered from plasma irregularities in the E region of the ionosphere. The derived winds show first order agreement with mesospheric winds measured independently over Halley. Gravity wave signatures (e.g. <2-h period) and smaller-scale structure in the winds are revealed within the spatiotemporal data formed by the 16 beams. A data base of such observations is being built up from existing radar data extending over nearly a full solar cycle from 1988 to the present day.

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We conducted meteor echo observations using the Buckland Park MF radar (35°S, 138°E) at 00:40-05:45 LT on October 22, 1997. In addition to the usual full correlation analysis (FCA) technique to measure horizontal wind velocities from 60 to 100 km MF radars have a potential to detect meteor echoes and infer winds through their Doppler frequency shifts. Because of the relatively low radio frequency employed MF radars have a great advantage of providing meteor wind well above 100 km altitude, where very few techniques can measure wind velocities. There is a limitation which should be noted as well. The observations are possible only during night time when the electron density of E-region is low enough for the radio wave to penetrate into the upper region. We detected 233 underdense meteor echoes from 80 km to 120 km with a mean height of 104.4 km. Although the transmitting antenna beams were steered toward off-zenith angles of 25°, almost all the echoes were received outside of the main lobe, indicating that conventional MF radar systems with a broad transmitting beam can work well for meteor observations. Bi-hourly wind profiles were obtained from 94 to 114 km altitudes. The profiles revealed a clear wave structure with a downward phase progression with time. FCA winds from 80 to 100 km were also estimated, and a continuous wind structure was obtained from FCA to meteor heights. Note that the present observations happened to be conducted during a major meteor shower activity. However, a majority of the underdense echoes were from non-shower meteors, and observations during non-shower periods will also yield enough information.

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The seasonal variations of the zonal winds measured by meteor/MF radars and by the wind-imaging interferometer (WINDII) on board of the UARS satellite, are analyzed and compared with the ground-based Global Empirical Wind Model (GEWM) and satellite-based WINDII prevailing wind model, which are independently derived from the observational data sets. A general consistency in the main global scale wind structures is observed. The seasonal variations of zonal wind in the both models are described by almost the same annual and semi-annual components. But systematic bias is found for the annual mean zonal winds of the two models, with the GEWM winds generally smaller than those of WINDII by factor 2-2.5. This bias is practically independent of altitude and can be described by a term of A cos 4x, where A is about 20 m/s and x is colatitude. Possible origin of this offset is discussed.

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Technique appropriate to the analysis of lower thermospheric wind data recorded by global-scale networks of ground-based instruments are discussed. The S-transform technique is shown to be effective in the analysis of the main features of travelling planetary waves and this method is applied to the time series of horizontal-velocity data obtained during the DYANA campaign (January-March, 1990). The analysis reveals strongly transient behavior of the day-to-day lower thermosphere wind variations, as well as their specific longitudinal structure. In particular, it was found that the revealed quasi-15 day and quasi-5 day wind oscillations may be described as transient westwardpropagating waves with zonal wavenumber s = 1, while an oscillation with the a period near 7 days is tentatively identified as having a wavenumber s = 0.

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Anin-situ rocket technique using foil chaff is used to observe wind fields in the mesosphere and lower thermosphere (80-98 km in altitude). We launched two micro-rockets at 1200 and 1315 UT on 14 January 1997 from Uchinoura, Japan (31°N, 131°E). The MU radar (MUR; 35°N, 136°E) and the Yamagawa MF radar (MFR; 31°N, 131°E) simultaneously observed winds at the same heights by means of a meteor scattering and partial ref°lection echo from the ionosphere received by spaced antennas, respectively. The chaff and MFR winds generally agree well at 80-88 km, while MFR data were missing at the heights>88 km. In the chaff, MFR, and the MUR winds, we have found a coherent structure likely due to a large-scale gravity wave. The chaff results suggest that wind fluctuations with a vertical scale of-2 kmat 82-85 km are quite consistent with gravitywave motions. It is noteworthy that a qualitative agreement is found between the chaff descent speed fluctuations and the wave-induced vertical component, although those vertical velocities are quantitatively inconsistent. On some special occasions chaff velocity might be affected by another process, of which tentative candidates are an apparent motion of the strong echo point in a chaff cloud, and an internal mesospheric “bore”.

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In order to clarify the horizontal structure of the wavelike oscillation frequently observed in the night time sodium density profile with a small Gaussian half-width in the middle of the night and a broad distribution at dusk, simultaneous observations with two lidars at Shigaraki (34.9°N, 136.1°E) and Hachioji (35.6°N, 139.4°E) and the MU radar at Shigaraki has been carried out. In the campaign of 35 nights, simultaneous observation was successful in four nights. On December 27-28, 1995, a large scale wave motion was observed by two lidars and the MU radar with meteor observation mode. Phase velocities were almost the same at the two sites and there were only slight differences in phase between the two sites. The wave motion was inconsistent with the component of the tidal wave and similar with the results of hodograph analyses. It is possible that the wave observed by lidars on December 27-28, 1995 was a gravity wave. The results of analyses suggest the possibility of gravity waves which were observed at fixed local time.

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Simultaneous observations with a sodium lidar in Wuhan, China (30.53°N, 114.37°E) and an MF radar in Yamagawa, Japan (31.20°N, 130.62°E) were conduced during the PREASA-2 campaign from Feb. 22 to March 15, 1996. Some observation results are given. The gravity wave induced velocities measured by both techniques are estimated to compare the gravity wave activities between the places with same latitudes and different longitudes. We found that RMS velocities in Yamagawa were larger than those inWuhan, which suggest different gravity wave activities between the two places.

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In order to evaluate material transports in the upper mesosphere and lower thermosphere (MLT) region, Lagrangian transport experiments are performed using wind fields simulated by the Kyushu University middle atmosphere general circulation model (Miyahara and Miyoshi, 1997; Miyahara et al., 1993). The Lagrangian mean meridional circulation is different from the residual mean circulation defined by the Transformed Eulerian Mean equation system, which is thought to be approximately equal to the Lagrangian mean meridional circulation in the middle atmosphere in case of weak dissipation and transience (Andrews and McIntyre, 1978; Andrews et al., 1987). It is considered that the diffusivity caused by the transience and dissipation of various wave motions in the MLT region has significant effects on the material transport in this region, and makes them different from each other.

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HRDI zonally averaged daytime temperatures are used to compute the gradient wind in the 65-105 km range. Results are compared with independently measured HRDI zonal mean zonal winds. The gradient wind captures the essential features of the observed wind field in the summertime midlatitudes, including the stratospheric easterly (westward) jet and the reversal to westerly (eastward) winds in the lower thermosphere. The consistency between HRDI and gradient winds diminishes at tropical latitudes, due to substantial tidal contamination of daytime temperatures used to compute the gradient wind.

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The behavior of the 5-day and 16-day waves in the mesopause region is examined by using a general circulation model. The results are as follows. The 5-day wave is largely unaffected by the zonal mean zonal wind distribution, and the symmetric structure about the equator is clearly seen in the mesopause region. The amplitude of the 16-day wave in the summer hemisphere of the stratosphere is small. However, above the upper mesosphere, the 16-day wave appears not only in the winter hemisphere but also in the summer hemisphere. The penetration of the 16-day wave from the winter hemisphere to the summer hemisphere occurs near the mesopause region. The 16-day wave is mainly excited by heating due to the moist convection in the troposphere, and the vertical penetration into the middle atmosphere occurs. Furthermore, a correlation between the geomagnetic variation and the wind variation associated with the 5-day and 16-day waves is discussed.

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A linearized numerical model is used to simulate the propagation of stationary planetary waves through the stratosphere and mesosphere into the lower thermosphere. Wave forcing at the lower boundary has been specified by the perturbation of the geopotential height for January. The dependence of planetary wave structure on the zonally averaged wind is investigated through the analysis of results of simulation with different background wind distributions. The global model of stationary planetary waves has been modified to simulate traveling planetary waves, and the spectrum of resonant planetary modes has been obtained by forcing the model with a periodic perturbation of the vertical velocity near the surface. Wave-activity density, Eliassen-Palm flux, and its divergence are used as a diagnostics of wave propagation and wave-mean flow interaction. It is found that planetary waves can provide substantial acceleration of the mean flow which is comparable to that one associated with gravity wave and atmospheric tide breaking and/or saturation. Results of numerical simulation are compared with the climatological model of stationary planetary waves in the stratosphere and with the preliminary results of wind observations using WINDII instrument on the UARS.

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The dynamical structure of the atmosphere around the sporadic sodium layer at mid-latitude (-35°N) below 100 km was studied by simultaneous observation with the MU radar at Shigaraki (34.9°N, 136.1°E), and two Na lidars at Shigaraki and in Hachioji (35.6°N, 139.4°E). In the lidar data, fifteen Nas (sporadic sodium layer) events were detected. Wind shear, temperature, and stability indices, at around the time and height of Nas were observed with the MU radar. Strong total wind shear correlated well with Nas, especially when sporadic Es did not accompany. However, no other clear correlations, such as correlations with temperature etc., were found. The result is similar to the report of the lidar observations in Hawaii during the ALOHA-93 campaign (Qian et al., 1998), and suggests a similar generation mechanism between 20°N and 35°N.

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We present a model based analysis of rocket borne common volume measurements of electron number densities and aerosol charge densities during the ECHO campaign in 1994. During that campaign a sounding rocket was launched into a noctilucent cloud (NLC) as detected by a ground based lidar. At NLC altitudes a particle impact detector gave strong evidence for positively charged aerosols, and an electron probe measured a significant electron enhancement. We have applied a model of aerosol charging to these measurements and find that the existence of positively charged aerosols can be explained if they mainly consist of a substance with a sufficiently low work function. The electron enhancement as well as the aerosol size and number density deduced from our model are consistent with the electron probe and lidar measurements, respectively. Considering the photoelectrical properties of various metals we conclude that only sodium and potassium have a sufficiently low work function to allow for significant photoemission. Even under very favourable conditions the maximum positive charge accumulated on the aerosols is only approximately 4 elementary charges which is much less than discussed in some of the current theories for the creation of polar mesosphere summer echoes. We note that the amount of sodium or potassium required to form these particles is far above the natural abundances at NLC altitudes. The exact abundance and composition of the aerosols need to be known at the time of the in situ measurements in order to make more sophisticated comparisons between measurements and models.

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Analysis of seven years (1990-1997) of measured temperature profiles in the mesopause region (84 to 102 km) at Fort Collins, CO (41°N, 105°W), shows that, after removing seasonal variations, there was an episoidic temperature excursion with an amplitude ranging from 7 K to 14K. Observable increases began in 1992, maximum temperatures occurred during the first half of 1993, and the excursion was over by about 1996. Since this excursion followed the Mount Pinatubo eruption by a time scale consistent with published model simulations of the effect of stratospheric aerosol on the mesopause region, we attribute the temperature excursion to that eruption. In addition the data is consistent with a background cooling of roughly 1 K per year, most of which may be attributable to variability in the solar flux. Continued observation towards the coming solar maximum promises to quantify (assess) the “long-term” change in mesopause temperatures resulting from solar variability (anthropogenic effect).

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We present a new rocket instrument which measures total atmospheric density with great precision and resolution by Rayleigh scattering of infrared light. Comparison with a ground-based lidar shows: (a) both instruments measure the same physical parameters, even though they have different integration times and volumes. (b) The observed density structures change little over the course of an hour and the horizontal distance of 14 km. The rocket instrument has a basic vertical resolution of approximately 8 m and a number density precision of 4.2 · 1019 m-3. Above 56 km we integrate over an increasing vertical range, reaching 80 m at 70 km. The measured number density profile shows remarkable alternations between very stable layering and metastable layering (adiabatic lapse rate) in the atmosphere between 52 km and 71 km. Comparison with the hodograph of the horizontal wind profile measured by a falling sphere 29 minutes later shows that the metastable height regions coincide with height regions where the hodograph deviates from an ideal spiral. The observation is tentatively interpreted as a gravity wave that is saturating (or encountering a critical level) in these height regions. The comparison of the fine-scale neutral number density observations with measurements of ion density by electrostatic skin probes on board the same vehicle shows a number of ion density enhancements in the stably-layered height regions. With one exception out of six cases, these enhancements occur where the vertical gradient of the meridional wind would collect positive ions as in sporadic E layers. This may be the first observation of such ion density enhancements in the height region 55 to 70 km.

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Routine lidar observations are in progress at the National Institute for Environmental Studies (NIES), Tsukuba, Japan (36°N, 140°E), providing vertical profiles of ozone and temperature in the stratosphere and lower mesosphere. The present study focuses the winter thermal structure over Tsukuba and validation/comparison of the lidar derived temperature profiles. Atmospheric temperatures for the altitude region 30-75 km are determined from the neutral density profiles. 33 temperature observations recorded during the winter months of 1995 and 1996 are used in the study. Winter thermal structure is characterized with its variability with an observed stratopause temperature of about 260 K. Although evidence of minor stratosphericwarming is seen on some days, there appears to be no direct linkage with major mid winter warmings at the poles. Temperature profiles show evidence of mesospheric temperature inversion layers. For the purpose of validation/comparison of the lidar temperature profiles, other datasets based on rocketsondes, National Meteorological Center (NMC) analyses, Solar Mesosphere Explorer (SME) spacecraft measurements, and CIRA 86 have been used. Encouraging agreements between the lidar and rocket profiles are evident, which confirms the potential of lidar technique for deriving the absolute temperatures in the atmosphere. Climatological comparisons also reveal satisfactory agreement between the lidar and other datasets.

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Optical observations were made at Ramfjord, Norway from January 10 to February 14, 1997. Two types of Fabry-Perot interferometers (FPIs), Doppler-imaging and scanning, were installed at the EISCAT radar site and were used to acquire data simultaneously with radio instruments. Both FPIs can observe emissions of two differentwavelengths simultaneously. We can estimate the horizontal and vertical wind in different emission layers simultaneously with high time-resolution (-1 min). The observations on February 8 and 9, 1997, show some notable characteristics: (1) large-scale perturbations (≈ ±150 m/s) are observed in the upper thermospheric wind. They seem to begin 30 min after the onset of a magnetic substorm and to stop when the next substorm begins. (2) Clear wave-like structures are found in the horizontal wind variations. Some of them can be seen over the entire sky, and one of them is found in a restricted regions. (3) A clear wave-like structure is also found in the vertical wind in the upper thermosphere. A similar structure can be seen in the lower thermosphere, but these structures are not always in phase. This phases difference starts at the same time that horizontal winds between the two layer has their phase difference. (4) The relation between the vertical wind and the divergence of horizontal wind seems to change with time. The correlation coefficient between them changes one-hours before and on-time of a substorm on-set. This sign of the coefficient is negative in most of the time, with considering about time-lag. It means the vertical morion is caused by divergent flow of horizontal wind.

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Airglow variability is a topic that has been studied for decades but an understanding of the role of the dynamical influence underlying this variability has only been achieved recently. TheUARS dynamics instruments, HRDI (High Resolution Doppler Imager) and WINDII (WIND Imaging Interferometer) have been instrumental in providing this understanding, because they measure both winds and emission rates, and so are able to determine the coupling between the two. But ground-based observations are also an essential ingredient to this understanding, which has grown through intercomparisons between dataset and models through workshops such as DYSMER. This presentation begins by describing the influence of the diurnal tide on oxygen and hydroxyl airglow emission rates, including the seasonal variation. This is followed by a description of two planetary scale disturbance phenomena, the springtime transition, and a stratospheric warming. Auroral influences are also considered. While these investigations cover a wide range of mechanisms there is an underlying thread which is that it is these large scale dynamical processes that are responsible for determining the distribution of the airglow patterns detected, and thus the distribution of concentration of atomic oxygen.

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This paper focuses on the effect of diabatic processes due to photochemical heating on long-period gravity waves in the stratosphere, mesosphere and lower thermosphere. A linear diabatic gravity wave model is established and compared to a model of pure dynamical adiabatic gravity waves. The results indicate that the photochemistry has a damping effect on gravity waves in most regions of the stratosphere and mesosphere. However, the photochemistry has a destabilizing effect on gravity waves in the mesopause region. The photochemical heating process can induce a comparatively strong enhancement of gravity waves at the mesopause for lower temperatures. In the summer polar mesopause region, this growth rate may be greater by about one order of magnitude than the growth rate of gravity waves at other seasons and locations.

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Simultaneous observations of the night airglow OH (6, 2) band emission intensity and rotational temperature, by a sky scanning airglow spectrophotometer, and meteor winds, by a middle and upper atmosphere radar (MU radar), were carried out at Shigaraki (34.9°N, 136.1°E), Japan, from October 29 to November 11, 1994, as the first phase of a campaign, and from July 25 to July 31, 1995 as the second phase. Horizontal structures in the OH emission intensity and rotational temperature were monitored optically, together with the background wind and its wave induced fluctuations, measured by MU radar. Since the MU radar makes a direct measurement of the vertical wavelength, and the OH spectrophotometer makes a direct measurement of the horizontal wavelength, the two techniques are mutually complementary to determine intrinsic wave parameters. Gravity waves with intrinsic periods of 2 to 9 hours, horizontal wavelengths of 500 to 3000 km and vertical wavelengths of 12 to 75 km were identified. Between the two different observation techniques, there is a reasonable agreement in the inferred wave characteristics.

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Results from two different instrumental techniques, an Na Wind/Temperature Lidar and an OH Mesospheric Temperature Mapper, have been combined to investigate the occurrence and properties of the mid-latitude terdiurnal (8-hr) tide at near mesopause altitudes (80-105 km). High-resolution Na lidar measurements were taken throughout the diurnal and annual cycle (1996-98) at Urbana, Illinois (40°N, 88°W) to characterize the seasonal behavior of the 24, 12, 8 and 6-hr tides. Complementary measurements using a recently developed CCD imager capable of mapping OH temperature (at -87 km altitude) were made from Bear Lake Observatory, Utah (41.9°N, 111.6°W) and Ft. Collins, Colorado (40.6°N, 105°W) within the same time period. The “mean day” lidar data for the spring and fall periods investigated here each indicate an average amplitude variation of -2-5 K over the depth of the OH layer but distinct phases of <1-hr LST and -7-hr LST, respectively, for the 8-hr component. The Temperature Mapper data are in excellent agreement with these findings but in addition have shown that the amplitude of this tidal component can vary by as much as an order of magnitude (1.5-15 K) on a night-by-night basis resulting in an apparent 8-hr dominance of the nocturnal variation during investigated portions of the spring and fall seasons with little or no diurnal and semi-diurnal variability evident. Reports of terdiurnal tidal measurements in the mid-latitude nightglow emissions are exceptionally rare and have yet to be modeled. These innovative joint measurements pave the way for new research in this important area.

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The Optical Mesosphere Thermosphere Imagers (OMTI) have been developed to investigate the dynamics of the upper atmosphere through nocturnal airglowemissions. TheOMTIconsist of an imaging Fabry-Perot interferometer, three all-sky cooled-CCD cameras, three tilting photometers, and a Spectral Airglow Temperature Imager (SATI) with two container houses to install them in. These instruments measure wind, temperature and 2-dimensional airglow patterns in the upper atmosphere at multi-wavelengths of OI (557.7 nm and 630.0 nm), OH (6-2) bands, O2 (0, 1) bands, and Na (589.3 nm), simultaneously. Examples of the data are shown for the cameras, the photometers, and the SATI based on the airglow observation at a mid-latitude station in Japan. Good correlation of the photometer and SATI observations is obtained. A comparison is shown for small- and large-scale wave structures in airglow images at four wavelengths around the mesopause region using four cooled-CCD cameras. We found an event during which large-scale bands, small-scale row-like structures, and large-scale front passage occur simultaneously.

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A wideview CCD imager for OH airglow observations was operated at the MUradar site in Shigaraki, Japan (35°N, 136°E). From the 18 months' observation, dominant gravity wave components in the OH images are extracted, and seasonal variation of the characteristics of the waves is investigated. These waves typically have short horizontal wavelengths (5 km-60 km) and short periods (5 min-30 min), with horizontal phase speeds of 0-100 m/s. All the wave events are separated into two groups by a boundary of a horizontal wavelength of 17.5 km, which is close to the boundary between ripples and bands. For the waves with larger horizontal wavelengths, the horizontal propagation direction showed clear seasonal variation with summer eastward and winter westward preferences, with a change of direction in mid-March and at the end of September. This suggests that these waves are propagated from the lower atmosphere and filtered in the middle atmosphere by the mean winds. However, the small scale waves propagate in almost all azimuths with a slight seasonal variation. Therefore, in-situ generation would be the major source of such waves although the wavelength as a physical boundary between the two groups could be smaller than 17.5 km. The seasonal variation of the wave parameters especially between summer/winter and equinoctial months is also discussed. The waves with small horizontal wavelengths (<15 km), longer periods (>10 min), and slow horizontal phase speeds (<30 m/s) are mainly seen in summer/winter.

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