Properties of QBO and SAO generated by gravity waves

In this paper we present an extension for the 2D (zonal mean) version of our numerical spectral mode (NSM) that incorporates Hines’ Doppler spread parameterization (DSP) for small-scale gravity waves (GW). This model is applied to describe the seasonal variations and the semi-annual and quasi-biennial oscillations (SAO and QBO). Our earlier model reproduced the salient features of the mean zonal circulation in the middle atmosphere, including the QBO extension into the upper mesosphere inferred from UARS measurements. The model is extended to reproduce the upwelling at equatorial latitudes that is associated with the Brewer–Dobson circulation — which affects significantly the dynamics of the stratosphere as Dunkerton had pointed out. In the presence of GW, this upwelling is produced in our model with tropospheric heating, which generates also zonal jets outside the tropics similar to those observed. The resulting upward vertical winds increase the period of the QBO. To compensate for that, one needs to increase the eddy diffusivity and the GW momentum flux, bringing the latter closer to values recommended in the DSP. The QBO period in the model is 30 months (mo), which is conducive to synchronize this oscillation with the seasonal cycle of solar forcing. Associated with this QBO are interannual and interseasonal variations that become increasingly more important at higher altitudes — and this variability is interpreted in terms of GW filtering that effectively couples the dynamical components of the mesosphere. The computed temperature amplitudes for the SAO and QBO are in substantial agreement with observations at equatorial and extra-tropical latitudes. At high latitudes, however, the observed QBO amplitudes are significantly larger, which may be a signature of propagating planetary waves not included in the present model. The assumption of hydrostatic equilibrium not being imposed, we find that the effects from the vertical Coriolis force associated with the equatorial oscillations are large for the vertical winds and significant for the temperature variations even outside the tropics, but the effects are small for the zonal winds.     

Latitude variability of acoustic-gravity waves in the upper atmosphere based on satellite data

Based on satellite measurements, we investigated the properties of acoustic-gravity waves in different geographical areas of the Earth’s upper atmosphere. To study wave activity at high latitudes, we used the concentration of neutral particles measured by the low-altitude polar satellite Dynamic Explorer 2 and measurements from the equatorial satellite Atmosphere Explorer-E for analysis of waves at low latitudes. In the range of heights 250–400 km, there are observed latitudinal variations of amplitudes, together with variations in the morphological and spectral properties of acoustic-gravity waves. In the polar regions of thermosphere, the wave amplitudes amount to 3–10% in terms of relative variations of density and do not exceed 3% at low and middle latitudes. At low latitudes, regular fluctuations induced by the solar terminator are clearly seen with a predominant wave mode moving synchronously with terminator. Moreover, at low and middle latitudes, there are observed sporadic local wave packets of small amplitudes (1–2%) that can have origins of various natures. We also investigated the relation between some of the observed wave trains and the earthquakes.     

Midlatitude mesopause region winds and waves and comparison with stratospheric variability

We present time series of January–May mean mesosphere/lower thermosphere (MLT) mean winds and planetary wave (PW) proxies over Europe together with stratospheric stationary planetary waves (SPW) at 50°N and time series of European ozone laminae occurrence. The MLT winds are connected with stratospheric PW and laminae at time scales of several years to decades. There is a tendency for increased wave activity after 1990, together with more ozone laminae and stronger MLT zonal winds. However, possible coupling processes are not straightforward. While mean MLT winds before the 1990s show similar interannual variations than stratospheric PW at 100 hPa, later a tendency towards a connection of the MLT with the middle stratosphere SPW is registered. There is also a tendency for a change in the correlation between lower and middle stratosphere SPW, indicating that coupling processes involving the European middle atmosphere from the lower stratosphere to the mesopause region have changed.     

Model studying the effect of the solar terminator on the thermospheric parameters

The results of model studying the mechanism of formation of large-scale disturbances in the thermosphere under the action of short-period waves propagating from the middle atmosphere are presented. The region of the solar terminator is considered as a source of such waves. The calculation results for the summer conditions in the Northern Hemisphere indicate that such an energy source, local in the space and time, results in the formation of large-scale inhomogeneities at altitudes of 120–160 km. A disturbance of these inhomogeneities is caused by the nonlinear and dissipative processes accompanying HF propagation in the terminator region. Large-scale disturbances have a quasistationary character and are localized near the morning and evening terminators. Vortex motions around the pole at altitudes of 140–160 km are formed at nigh latitudes of the winter hemisphere under the polar night conditions.     

Planetary waves in coupling the lower and upper atmosphere

The purpose of the paper is to answer the question if planetary waves (PW) are capable of propagating into the thermosphere. First the simplest vertical structure equation of the classic tidal theory accounting for a realistic vertical temperature profile is considered. Analysis and simulation show that the well-known normal atmospheric modes (NM), which are trapped in the lower and middle atmosphere, exhibit a wave-like vertical structure with a large vertical wavelength in the thermosphere. Moreover, the reflection of these modes from the vertical temperature gradient in the lower thermosphere causes appearance of the wave-energy upward flux in the middle atmosphere, and in a linearized formulation this flux is constant above the source region. To investigate a possibility of the NM forcing by stratospheric vacillations and to consider the propagation of different PW up to the heights of the upper thermosphere, a set of runs with a mechanistic Middle and Upper Atmosphere Model has been performed. The results of the simulation show that quasi-stationary and longer-period PW are not able to penetrate into the thermosphere. The shorter-period NM and ultra-fast Kelvin wave propagate up to the heights of the lower thermosphere. However, above about 150 km they are strongly suppressed by dissipative processes. The role of the secondary waves (nonmigrating tides) arising from nonlinear interaction between the primary migrating tides and quasi-stationary PW is discussed. We conclude that PW are not capable of propagating directly up to the heights of the ionospheric F2 region. It is suggested that other physical processes (for instance, the electrostatic field perturbations) have to be taken into account to explain the observed PW-like structures in ionospheric parameters.     

Response of the global fields of temperature and tropospheric wind and the middle atmosphere to variations in the solar UV radiation flow during a solar activity cycle in the presence of planetary waves (3D modeling)

Using a model of the general circulation, the response of the temperature and wind in the Earth’s atmosphere to variations in solar UV radiation flows during an 11-year activity cycle is studied with respect to their dependence on the wavelength. Satellite measurement data for the 23rd cycle that were characterized by anomalously low flows of UV radiation at minimum activity are used in calculations. To implement numerical scenarios, wavelength-dependent variations in the UV radiation flow changing absorption in the bands of ozone and molecular oxygen are used in the radiation block of the model. Based on empirical data, a spatial structure of stationary disturbances having the wavenumbers S = 1, 2, and 3 are assigned at the lower boundary of the model. The calculation results demonstrating the changes in the atmospheric parameters between the solar activity maximum and minimum within the height range 0–120 km are presented. It is shown that the response of monthly average values has a wavy structure in latitude, i.e., a nonseasonal character, amounting to several degrees in the lower atmosphere. The results obtained indicate that planetary waves are an important link in the transmission of an external effect on the lower atmosphere.     

Observational evidence of the semiannual oscillation in the tropical middle atmosphere—A review

Observational studies on the semiannual oscillation in the tropical stratosphere and mesosphere are reviewed. Results of many statistics based on rocket and satellite observations reveal that the long-term behavior of the mean zonal wind exhibits two semiannual cycles which have their maximum amplitudes centered at the stratopause level and the mesopause level, each one being associated with the semiannual temperature variations predominating at levels about 10 km lower.Observational evidence obtained from recent studies of the dynamical properties of upper stratospheric waves strongly supports the theoretical consideration that the stratospheric semiannual oscillation is the manifestation of the wave-zonal flow interaction with alternating accelerations of the westerly flow by Kelvin waves and the easterly flow by planetary Rossby waves.Regarding the semiannual variation in the upper mesosphere, however, very little is known about the possible momentum source. Therefore, emphasis is placed on the need for further observations of the structure and behavior of the tropical middle atmosphere.     

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

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

Long term studies of equatorial spread F using the JULIA radar at Jicamarca

Jicamarca unattended long term investigations of the ionosphere and atmosphere radar observations of equatorial spread F (ESF) plasma irregularities made between August 1996 and April 2000 are analyzed statistically. Interpretation of the data is simplified by adopting a taxonomy of echo types which distinguishes between bottom-type, bottomside, topside, and post-midnight irregularities. The data reveal patterns in the occurrence of ESF in the Peruvian sector that are functions of season, solar flux, and geomagnetic activity. We confirm earlier work by Fejer et al. (J. Geophys. Res. 104 (1999) 19,859) showing that the quiet-time climatology of the irregularities is strongly influenced by the climatology of the zonal ionospheric electric field. Under magnetically quiet conditions, increasing solar flux implies greater pre-reversal enhancement amplitudes and, consequently, irregularity appearances at earlier times, higher initial altitudes, and higher peak altitudes. Since the post-reversal westward background electric field also grows stronger with increasing solar flux, spread F events also decay earlier in solar maximum than in solar minimum. Variation in ESF occurrence during geomagnetically active periods is consistent with systematic variations in the electric field associated with the disturbance dynamo and prompt penetration described by Fejer and Scherliess (J. Geophys. Res. 102 (1997) 24,047) and Scherliess and Fejer (J. Geophys. Res. 102 (1997) 24,037). Quiet-time variability in the zonal electric field contributes significantly to variability in ESF occurrence. However, no correlation is found between the occurrence of strong ESF and the time history of the zonal electric field prior to sunset.     

SMLTM simulations of the diurnal tide: comparison with UARS observations

Wind and temperature observations in the mesosphere and lower thermosphere (MLT) from the Upper Atmosphere Research Satellite (UARS) reveal strong seasonal variations of tides, a dominant component of the MLT dynamics. Simulations with the Spectral mesosphere/lower thermosphere model (SMLTM) for equinox and solstice conditions are presented and compared with the observations. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The model incorporates realistic parameter-izations of physical processes including various dissipation processes important for propagation of tidal waves in the MLT. A discrete multi-component gravity-wave parameterization has been modified to account for seasonal variations of the background temperature. Eddy diffusion is calculated depending on the gravitywave energy deposition rate and stability of the background flow. It is shown that seasonal variations of the diurnal-tide amplitudes are consistent with observed variations of gravity-wave sources in the lower atmosphere.     

A numerical model of the zonal mean circulation of the middle atmosphere

The annual cycle of the zonally averaged circulation in the middle atmosphere (16–96 km) is simulated using a numerical model based on the primitive equations in log pressure coordinates. The circulation is driven radiatively by heating due to solar ultraviolet absorption by ozone and infrared cooling due to carbon dioxide and ozone (parameterized as a Newtonian cooling). Since eddy fluxes due to planetary waves are neglected in the model, the computed mean meridional circulation must be interpreted as thediabatic circulation, not as the total eulerian mean. Rayleigh friction with a short (2–4 day) time constant above 70 km is included to simulate the strong mechanical dissipation which is hypothesized to exist in the vicinity of the mesopause due to turbulence associated with gravity waves and tides near the mesopause.Computed mean winds and temperatures are in general agreement with observations for both equinox and solstice conditions. In particular, the strong mechanical damping specified near the mesopause makes it possible to simulate the cold summer and warm winter mesopause temperatures without generating excessive mean zonal winds. In addition, the model exhibits a strong semiannual cycle in the mean zonal wind at the equator, with both amplitude and vertical structure in agreement with the easterly phase of the observed equatorial semiannual oscillation.Contribution No. 497, Department of Atmospheric Sciences, University of Washington, Seattle.     

On the application of a proposed global system for measuring meteor winds

Summary This paper discusses the need for a global network of meteor wind stations for determining the general circulation of the upper mesosphere and lower thermosphere. Continuous observations of horizontal motions from such a network would permit resolution of planetary scale eddy winds, tides, and gravity waves, and hypotheses that such motions propagate vertically from the lower atmosphere or are generated in situ by solar activity could be examined critically with observational data. The observed mean winds from the lower stratosphere to the meteor wind level are summarized to support the hypothesis that a standing wave pattern in the winds extends into the lower thermosphere. Data on tidal meridional momentum transports from meteor wind stations suggest that tides in the lower thermosphere are important for the maintenance of mean winds. Some of the geomagnetic and photochemical processes in the lower thermosphere that could be investigated with meteor wind data are briefly reviewed.This paper is adapted from our presentation at the 1966 Fall URSI meeting at Palo Alto, California     

Variations in the characteristics of acoustic gravity waves according to simulation data

The characteristics of different-scale acoustic gravity waves (wavelengths of 100–1200 km, periods of 10–50 min) under different geophysical conditions have been studied using a numerical model for calculating the vertical structure of these waves in a nonisothermal atmosphere in the presence of an altitudedependent background wind and in a situation when molecular dissipation is taken into account. It has been established that all considered acoustic gravity waves (AGWs) effectively reach altitudes of the thermosphere. The character of the amplitude vertical profile depends on the AGW scales. The seasonal and latitudinal differences in the AGW vertical structure depend on the background wind and temperature. A strong thermospheric wind causes the rapid damping of medium-scale AGWs propagating along the wind. Waves with long periods to a lesser degree depend on dissipation in the thermosphere and can penetrate to high altitudes. A change in the geomagnetic activity level affects the background wind vertical distribution at high latitudes, as a result of which the AGW vertical structure varies.     

Gravity wave intensity and momentum fluxes in the mesosphere over Shigaraki, Japan (35°N, 136°E) during 1986–1997

Averaged seasonal variations of wind perturbation intensities and vertical flux of horizontal momentum produced by internal gravity waves (IGWs) with periods 0.2/1 h and 1/6 h are studied at the altitudes 65/80 km using the MU radar measurement data from the middle and upper atmosphere during 1986/1997 at Shigaraki, Japan (35°N, 136°E). IGW intensity has maxima in winter and summer, winter values having substantial interannual variations. Mean wave momentum flux is directed to the west in winter and to the east in summer, opposite to the mean wind in the middle atmosphere. Major IGW momentum fluxes come to the mesosphere over Shigaraki from the Pacific direction in winter and continental Asia in summer.     

Ranges of AGW propagation in the Earth’s atmosphere

The propagation of atmospheric gravity waves (AGWs) is studied in the context of geometrical optics in the nonisothermal, viscous, and thermal-conductive atmosphere of Earth in the presence of wind shifts. Parametric diagrams are plotted, determining the regions of allowed frequencies and horizontal phase velocities of AGWs depending on the altitude. It is shown that a part of the spectrum of AGWs propagates in stationary air in an altitude range from the Earth’s surface through the ionospheric F1 layer. AGW from nearearth sources attenuate below 250 km, while waves generated at altitudes of about 300 km and higher do not reach the Earth’s surface because of the inner reflection from the thermosphere base. The pattern changes under strong thermospheric winds. AGW dissipation decreases with an adverse wind shift and, hence, a part of the wave spectrum penetrated from the lower atmosphere to the altitudes of F2 layer.     

Solar cycle changes to planetary wave propagation and their influence on the middle atmosphere circulation

Recent observations suggest that there may be a causal relationship between solar activity and the strength of the winter Northern Hemisphere circulation in the stratosphere. A three-dimensional model of the atmosphere between 10–140 km was developed to assess the influence of solar minimum and solar maximum conditions on the propagation of planetary waves and the subsequent changes to the circulation of the stratosphere. Ultraviolet heating in the middle atmosphere was kept constant in order to emphasise the importance of non-linear dynamical coupling. A realistic thermo-sphere was achieved by relaxing the upper layers to the MSIS-90 empirical temperature model. In the summer hemisphere, strong radiative damping prevents significant dynamical coupling from taking place. Within the dynamically controlled winter hemisphere, small perturbations are reinforced over long periods of time, resulting in systematic changes to the stratospheric circulation. The winter vortex was significantly weakened during solar maximum and western phase of the quasi-biennial oscillation, in accordance with reported 30 mb geopotential height and total ozone measurements.     

Modeling of nonmigrating tides in the middle atmosphere

On the basis of calculations using the general circulation model of the middle and upper atmosphere, the relative role of sources of nonmigrating tides distributed in atmosphere has been investigated. It is shown that in winter, when planetary waves in stratosphere are well developed, the main contribution to the generation of nonmigrating tides is caused by nonlinear interaction between migrating tides and a quasi-stationary planetary wave with zonal wave number 1 (SPW1). Taking into account the longitudinal ozone inhomogeneities in the model leads to the occurrence of additional sources of nonmigrating tides caused by longitudinally inhomogeneous heating of the atmosphere, the contribution of which can be comparable to that from nonlinear interaction under an attenuating amplitude of SPW1 in the stratosphere.     

Evidence for nonlinear coupling of planetary waves and tides in the lower thermosphere over Bulgaria

Analyses of hourly values of zonal and meridional wind near 95 km observed by meteor radar at Yambol (42.5°N, 26.6°E) during January 1991–June 1992 indicate the presence of planetary waves with prevailing periods of 1.5–2.5, 4–6, 9–10 and 16–18 days. About 20% of the whole power of atmospheric motions is connected with these waves, so they play an important role in the dynamics of the mesosphere-lower thermosphere (MLT) region. By dynamic spectral analysis applied to the hourly neutral wind and to the calculated hourly values of tidal amplitudes it has been demonstrated that there is considerable modulation of tidal amplitudes by planetary waves in the neutral wind, as this process is better expressed in the semidiurnal tides. The nonlinear interaction between tides and planetary waves is studied by bispectral analysis. The results of these analyses indicate again that the nonlinear interactions between semidiurnal tides and planetary waves with periods 2–20 days are stronger than those of the diurnal tides and planetary waves. A peculiar feature of dynamics in the MLT region above Bulgaria is the presence of strong oscillations with periods of 20 and 30 h, which indicate significant nonlinear coupling between them.     

Manifestation of gravitational tides and planetary waves in long-term variations in geophysical parameters

Long-term variations in the parameters of the Earth’s upper atmosphere and geophysical activity have been studied based on the current spectra. The main sources of quasiperiodic oscillations in the atmosphere (including variations in the solar radiation, geomagnetic activity, and gravity) have been considered. It was shown that the most stable quasiharmonic variations are related to tidal gravitational oscillations and Rossby planetary waves with stable spectra. These oscillatory processes substantially contribute to the dynamics of the middle and upper atmosphere and manifest themselves in ionospheric parameters.     

Tidal effects on the thermosphere

Theoretical understanding of the earth's atmosphere is not possible without accounting for tidal waves and their interactions. In the thermosphere, tides represent the dominant motion system. Current observational efforts are aimed at providing tidal morphologies, i.e., maps of the vertical profiles of the tidal fields as a function of time for various locations. These are in hand for the mesosphere and have proved extraordinarily useful in deriving appropriate inputs for numerical models of the thermosphere. The basic latitudinal, seasonal, and solar cycle variation of the tides in the thermosphere is much less well known but the advent of coordinated global multi-day campaigns promises to ameliorate the situation. Progress would be facilitated by agreement on a standard data reduction procedure. Theoretical efforts are focused on simulating the observed morphologies and understanding the day to day variations while investigating mutual interactions and feedback between the mean atmosphere and the tidal, gravity, and planetary waves.