Gravity Wave Momentum Deposition Research Articles

Dynamical effects of internal gravity waves in the equinoctial thermosphere

Using a recently developed spectral nonlinear gravity wave (GW) parameterization implemented into a 3-D coupled general circulation model, the effects of a broad spectrum of small-scale internal GWs of lower atmospheric origin on the equinoctial thermosphere are studied for the first time. GWs propagate to F region altitudes in both hemispheres, producing appreciable drag on the mean zonal wind. Some modifications of the two-cell equinoctial mean circulation by GWs are simulated, too. The mean zonal GW drag is comparable to the ion drag up to ∼260km in the middle- and high-latitudes. While the mean dynamical effect of GWs is the deceleration of the mean flow, the instantaneous GW body force can have both signs. In the Southern Hemisphere high-latitude, GWs are found to produce large torque of more than 1000ms−1 day−1 the mechanism of which is investigated in detail. GW anisotropy plays a crucial role in offsetting and modulating wave filtering, introducing increased favourable conditions for westerly harmonics in the high-latitudes. This leads to a very large localized eastward GW drag reaching a maximum in the upper thermosphere as a consequence of enhanced molecular viscosity, thermal conduction, and ion drag. Finally, the high-latitude distribution of the GW body force is presented in the upper thermosphere along with the comparison with ion drag. It demonstrates significant interhemispheric differences and large longitudinal variations in GW momentum deposition.

Internal gravity waves in the thermosphere during low and high solar activity: Simulation study

Propagation of internal gravity waves (GWs) from the lower atmosphere into the upper thermosphere, and their dynamical and thermal effects have been studied under low and high solar activity approximated by the F10.7 parameter. It has been done by using a nonlinear spectral parameterization in systematic offline calculations with typical wind and temperature distributions from the HWM and MSISE‐90 models, and with interactive simulations using the University College London Coupled Middle Atmosphere‐Thermosphere‐2 (CMAT2) general circulation model (GCM) under solstice conditions. The estimates have been performed for relatively slow harmonics with horizontal phase velocities less than 100 m s−1, which are not affected by reflection and/or ducting. GW drag and wave‐induced heating/cooling are shown to be smaller below ∼170 km at high solar activity, and larger above. The maxima of GW momentum deposition occur much higher in the upper thermosphere, but their peaks are half as strong, 120 vs 240 m s−1 day−1 in the winter hemisphere when the insolation is large. Instead of strong net cooling in the upper thermosphere, GWs produce a weak heating at high solar activity created by fast harmonics less affected by dissipation. Molecular viscosity increases with solar activity at fixed pressure levels, but seen in Cartesian altitude grids it can either increase or decrease in the lower thermosphere, depending on the height. Therefore, in pressure coordinates, in which most GCMs operate, the influence of larger temperatures can be viewed as a competition between the enhanced dissipation and vertical expansion of the atmosphere.

Modeling the effects of gravity wave momentum deposition on the general circulation above the turbopause

A nonlinear spectral gravity wave (GW) drag parameterization systematically accounting for breaking and dissipation in the thermosphere developed by Yiğit et al. (2008) has been implemented into the University College London Coupled Middle Atmosphere‐Thermosphere‐2 (CMAT2) general circulation model (GCM). The dynamical role of GWs propagating upward from the lower atmosphere has been studied in a series of GCM tests for June solstice conditions. The results suggest that GW drag is not only nonnegligible above the turbopause, but that GWs propagate strongly into the upper thermosphere, and, upon their dissipation, deposit momentum comparable to that of ion drag, at least up to 180–200 km. The effects of thermospheric GW drag are particularly noticeable in the winter (southern) hemisphere, where weaker westerlies and stronger high‐latitude easterlies are simulated well, in agreement with the empirical Horizontal Wind Model (HWM93). The dynamic response in the F region is sensitive to the variations of the source spectrum. However, the spectra commonly employed in middle atmosphere GCMs reproduce the circulation both in the lower and upper thermosphere reasonably well.

A study into the effects of gravity wave activity on the diurnal tide and airglow emissions in the equatorial mesosphere and lower thermosphere using the Coupled Middle Atmosphere and Thermosphere (CMAT) general circulation model

Momentum deposition by gravity wave breaking is known to affect the amplitude and phase of the diurnal tide. Modelling studies of this interaction have produced some conflicting results and as yet, the exact nature of this interaction is not fully understood. In this study, the effects of parameterised gravity wave momentum deposition on the diurnal tide and subsequently on green line airglow from atomic oxygen during equinox are investigated using the Coupled Middle Atmosphere and Thermosphere (CMAT) general circulation model. The effects of gravity wave drag calculated by two different parameterisations, Meyer [1999. Gravity wave interactions with the diurnal propagating tide. Journal of Geophysical Research 104, 4223–4239] and Medvedev and Klaassen [2000. Parameterisation of gravity wave momentum deposition based on non-linear wave interactions: basic formulation and sensitivity tests. Journal of Atmospheric and Terrestrial Physics 62, 1015–1033], are compared in the low latitude MLT region between 70 and 120 km, where the amplitude of the diurnal tide and green line volume emission rates maximise. Results indicate that momentum sources from both gravity wave parameterisations act to reduce the mid-latitude zonal jets and advance the phase of the diurnal tide, such that the peak amplitude at a given height occurs at an earlier time of day. Gravity wave momentum deposition as parameterised by Meyer [1999. Gravity wave interactions with the diurnal propagating tide. Journal of Geophysical Research 104, 4223–4239] results in a reduction of the amplitude of the diurnal tide in the MLT region, whereas the tidal amplitude is increased when the Medvedev and Klaassen [2000. Parameterisation of gravity wave momentum deposition based on non-linear wave interactions: basic formulation and sensitivity tests. Journal of Atmospheric and Terrestrial Physics 62, 1015–1033] parameterisation is used. Both parameterisations affect the local time variability of the simulated airglow, and the magnitude of the peak emission rate. When using gravity wave momentum sources from the Medvedev and Klaassen [2000. Parameterisation of gravity wave momentum deposition based on non-linear wave interactions: basic formulation and sensitivity tests. Journal of Atmospheric and Terrestrial Physics 62, 1015–1033] parameterisation, the magnitude and morphology of CMAT tidal winds and O( 1S) 557.7 nm emission rates are in agreement with recent satellite observations.

Interannual variations of the diurnal tide in the mesosphere generated by the quasi‐biennial oscillation

We report results from a study with the numerical spectral model (NSM), which produces significant interannual variations in the diurnal tide. Applying Hines's Doppler spread parameterization, small‐scale gravity waves (GWs) drive the quasi‐biennial oscillation (QBO) and semiannual oscillation. With a GW source that peaks at the equator and is taken to be isotropic and independent of season, the NSM generates a QBO with variable periods around 26 months and zonal wind amplitudes close to 25 m s−1 at 30 km. As reported earlier, the NSM reproduces the observed equinoctial maxima in the diurnal tide at altitudes around 95 km. In the present paper it is shown that the QBO also modulates the tide such that the seasonal amplitude maxima can vary from one year to another by as much as 30%. To shed light on the underlying mechanisms, the relative importance of the linearized advection terms is discussed, which involve the meridional and vertical winds of the diurnal tide. These interactions generate pronounced QBO modulations of the tide at altitudes below 50 km. In the mesosphere between 50 and 80 km the signature is not well defined because of interference from the seasonal variations. At altitudes above 80 km, however, the QBO‐related interannual variations of the tide are again relatively large, but they are generated primarily by GW momentum deposition. Since the period of the QBO is variable and its phase is relative to the seasonal cycle changes, the magnitude of the QBO modulation of the tide in the upper mesosphere varies considerably as our long‐term model simulation shows. This intermittency must be attributed to nonlinear interactions, and GW drag (momentum deposition) is discussed as a potential candidate.

A time-resolved model of the mesospheric Na layer: constraints on the meteor input function

Abstract. A time-resolved model of the Na layer in the mesosphere/lower thermosphere region is described, where the continuity equations for the major sodium species Na, Na+ and NaHCO3 are solved explicity, and the other short-lived species are treated in steady-state. It is shown that the diurnal variation of the Na layer can only be modelled satisfactorily if sodium species are permanently removed below about 85 km, both through the dimerization of NaHCO3 and the uptake of sodium species on meteoric smoke particles that are assumed to have formed from the recondensation of vaporized meteoroids. When the sensitivity of the Na layer to the meteoroid input function is considered, an inconsistent picture emerges. The ratio of the column abundance of Na+ to Na is shown to increase strongly with the average meteoroid velocity, because the Na is injected at higher altitudes. Comparison with a limited set of Na+ measurements indicates that the average meteoroid velocity is probably less than about 25 km s-1, in agreement with velocity estimates from conventional meteor radars, and considerably slower than recent observations made by wide aperture incoherent scatter radars. The Na column abundance is shown to be very sensitive to the meteoroid mass input rate, and to the rate of vertical transport by eddy diffusion. Although the magnitude of the eddy diffusion coefficient in the 80–90 km region is uncertain, there is a consensus between recent models using parameterisations of gravity wave momentum deposition that the average value is less than 3×105 cm2 s-1. This requires that the global meteoric mass input rate is less than about 20 td-1, which is closest to estimates from incoherent scatter radar observations. Finally, the diurnal variation in the meteoroid input rate only slight perturbs the Na layer, because the residence time of Na in the layer is several days, and diurnal effects are effectively averaged out.

Mesosphere dynamics with gravity wave forcing: Part I. Diurnal and semi-diurnal tides

We present results from a non-linear, 3D, time dependent numerical spectral model, which extends from the ground up to the thermosphere and incorporates Hines’ Doppler spread parameterization for small-scale gravity waves (GWs). Our focal point is the mesosphere, which is dominated by wave interactions. We discuss diurnal and semi-diurnal tides in the present paper (Part I) and planetary waves (PWs) in a companion paper (Part II). To study the seasonal variations of tides, in particular with regard to GW forcing, numerical experiments are performed that lead to the following conclusions: (1) The large semi-annual variations in the diurnal tide (DT), with peak amplitudes observed around equinox, are produced to a significant extent by GW interactions that involve, in part, PWs. (2) The DT, like PWs, is amplified by GW momentum deposition, which reduces also the vertical wavelength. (3) Variations in eddy viscosity associated with GW interactions may also influence the DT. (4) The semidiurnal tide (SDT), and its phase in particular, is strongly influenced by the zonal mean circulation. (5) Without the DT present, the SDT is amplified by GWs; but the DT filters out GWs such that the wave interaction significantly reduces the amplitude of the SDT during equinox, effectively producing a strong non-linear interaction between the DT and the SDT. (6) PWs generated internally by the baroclinic instability and GW forcing produce large amplitude modulations of the DT and SDT.

Simulation of large‐scale dynamics in the mesosphere and lower thermosphere with the Doppler‐spread parameterization of gravity waves: 1. Implementation and zonal mean climatologies

Small‐scale gravity waves (GWs) originating in the lower atmosphere maintain the large‐scale structure and circulation in the mesosphere and lower thermosphere (MLT). At solstice the drag imposed by these waves supports a strong meridional circulation from summer pole to winter pole that acts as a giant “refrigerator” transporting heat from a colder summer mesosphere to a warmer winter mesosphere. The Doppler‐spread parameterization (DSP) of gravity waves developed by C. O. Hines has been implemented into the Spectral Mesosphere/Lower Thermosphere Model. Simulations for solstice and equinox conditions compare favorably with empirical models and observations. A particularly good agreement for both seasons is found with recent in situ observations of polar mesospheric temperatures. The thermal structure at high latitudes, especially at solstice, is almost entirely driven by the GW momentum deposition in the MLT. The DSP appears to provide a very realistic distribution of the GW drag imposed on the large‐scale circulation. A modification is suggested for the calculation of GW energy deposition rate that appears to be more consistent with the physics of Doppler spreading. The simulations exhibit strong seasonal variations in the energy deposition rates with a summer maximum of ∼200 mW kg−1 at middle and high latitudes in the upper mesosphere in very good agreement with recent in situ estimates.

Parameterization of gravity wave momentum deposition based on nonlinear wave interactions: basic formulation and sensitivity tests

We present a practical method for parameterization of gravity wave drag based on the Medvedev and Klaassen (1995. Journal of Geophysical Research 100, 25,841–25,853) theory of gravity wave spectral evolution and saturation. The only tuning necessary for the scheme involves assumptions about the nature of the source spectrum of subgrid-scale gravity waves, i.e. the wavenumbers, launch heights and amplitudes of the spectral components. In this paper we employ a column model with representative distributions of mean wind and temperature to examine the sensitivity of the parameterized wave drag to the source spectra. For the range of anticipated variability of source spectra in the troposphere the scheme produces plausible results consistent with observations and with theoretical estimates. Computationally, the scheme is as efficient as a multiple-wave Lindzen scheme, and suitable for use in general circulation models (GCMs) of the atmosphere.

The two‐level structure of the mesopause: A model study

Recent global‐scale observations of the mesopause thermal structure show the altitude of the mesopause to be located regularly near 100 km and in a small minority of cases near 88 km. In this paper we use a three‐dimensional nonlinear model of the middle‐upper atmosphere (0‐150 km) to explore the processes which lead to this two‐level structure of the mesopause. More specifically, we evaluate by way of sensitivity studies the influences of (1) photochemical heating processes, (2) gravity wave momentum deposition, and (3) vibrational excitation of CO2 molecules by collisions with O atoms on the global mesopause structure. We also calculate the effects which solar‐induced tides have locally on the diurnal variations of the mesopause altitude. The results of our numerical simulations suggest (1) that indeed two distinct mesopause levels exist worldwide even in terms of zonal mean states, (2) that the two‐level feature is mostly due to the photochemical heating processes in combination with gravity wave momentum deposition, and (3) that the two‐level mesopause structure is not a local phenomenon of tidal wave activity.

Lidar observations of gravity wave activity and Arctic stratospheric vortex core warming

Measurements of stratospheric thermal structure and gravity wave activity have been obtained with a Rayleigh lidar in the Canadian High Arctic at Eureka (80°N, 86°W) during five recent winters. The observations reveal that an annual late December warming of the upper stratosphere occurred in the polar vortex core and was sustained through the winter. Increased gravity wave activity was detected in the vortex jet during the warming. That these two phenomena developed in parallel suggests they are related. It is proposed that increased gravity wave momentum deposition above the jet maximum forced flow into the vortex core where it descended and warmed adiabatically.

On the role of an anisotropic gravity wave spectrum in maintaining the circulation of the middle atmosphere

The spectral parameterization of gravity wave momentum deposition proposed by Medvedev and Klaassen has been incorporated into the Canadian Middle Atmosphere Model. Numerical experiments show that an anisotropic gravity wave source significantly improves the middle atmosphere circulation compared to an isotropic source. These results suggest that, at least in midlatitudes, the gravity waves primarily responsible for the momentum deposition at mesospheric heights are launched with observed phase velocities which are primarily in the same direction as the mean wind at tropospheric (source) heights.

Simulation of tides with a spectral mesosphere/lower thermosphere model

The spectral mesosphere/lower thermosphere model (SMLTM) is an extension into the thermosphere (up to ∼220 km) of the three‐dimensional middle atmospheric (∼15–120 km) model developed by Akmaev et al. [1992]. The model incorporates realistic parameterizations of radiative transfer and dissipation processes including eddy and molecular viscosity and thermal conductivity and ion drag. A discrete multi‐component gravity wave parameterization is optimized with respect to the zonal mean temperature distribution as represented in the CIRA‐1986 empirical model. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The semi‐diurnal tide is primarily generated by ozone absorption of solar UV radiation in the stratosphere. First results of tidal simulations indicate that even without any additional tuning the simulations compare well with recent observations from the Upper Atmosphere Research Satellite (UARS). This means that the background temperature and wind distributions, gravity wave momentum deposition, turbulent diffusion, and tidal forcing all interact self‐consistently and are in agreement with observational data. It is noted that simultaneous UARS wind and temperature measurements, if they become available, will provide key information needed to distinguish the primary mechanism for damping the diurnal tide.

Observations of mesospheric wind velocities: 2. Cross sections of power spectral density for 48–8 hours, 8–1 hours, and 1 hour to 10 min over 60–110 km for 1981

Cross sections of power spectral density for 1981 between 60–110 km for the 48‐ to 8 hour and 8‐ to 1‐hour‐period bands, and of mean square velocity components in the 10‐ to 60‐min‐period band, are compared to cross sections of mean wind. Individual spectra agree well in form and amplitude to similar spectra for other sites when averaged over the entire year, but there are substantial variations in power spectral density in each band over the year. In the 10‐min to 8‐hour‐period band, minima in spectral densities follow the zero line of mean zonal velocity very closely, indicating strong gravity wave mean flow interactions. This, and the variation in gravity wave intensity in general, suggests that gravity wave momentum deposition plays an important role in balancing the heat and momentum budgets of the upper middle atmosphere.