Journal of Atmospheric and Solar-Terrestrial Physics
General circulation model results on migrating and nonmigrating tides in the mesosphere and lower thermosphere. Part I: comparison with observations
Introduction
Tides in the middle atmosphere have been studied comprehensively over the last three decades using model simulations and observations on the basis of different experimental methods including rocket soundings, radar and lidar measurements and space-based observations. Especially medium frequency radar (MFR) measurements have been used to study tides and their variability, e.g. by Manson and Meek (1985), Manson et al. (1999) Fritts and Isler (1994), Singer 1992, Singer 1997, and Jacobi et al. (1999). The measurements at several observational stations have been discussed in connection with migrating tides, which are waves travelling sun-synchronously westwards, and forced in the troposphere as well as in the stratosphere. These tides are able to propagate vertically up to the mesosphere and lower thermosphere (MLT).
The amplitude and phase dependences of the migrating diurnal and semi-diurnal components can be understood to a large extent on the basis of linear diurnal and semi-diurnal tidal wave propagation models as in Chapman and Lindzen (1970), Forbes (1982), Vial (1986), Hagan et al. (1995), and Wood and Andrews (1997). Linear propagation models take into account the zonal-mean zonal background wind and temperature as a function of latitude and height. The prescribed profiles of heating in the tropo- and stratosphere force the tidal oscillations. The damping of the tides in the MLT is due to eddy-diffusion processes.
It is known that the parameterization of gravity wave momentum deposition in linear models leads to difficulties in the determination of the seasonal variation of the migrating diurnal tide (Hagan et al., 1995). Furthermore, it is a common weakness of linear models that they are not able to describe the tidal variability resulting from internal interactions of different nonlinear wave processes appropriately. Fritts and Isler (1994) considered tidal variability on the basis of radar observations. Walterscheid and Vincent (1996) showed that the nonlinear interactions between the travelling two-day wave and the diurnal tide is an important process in the variability of the latter. McLandress (1997) discussed the importance of nonlinear effects for the simulation of the semi-annual amplitude variations of the diurnal tide. Besides those nonlinear processes as sources of the tidal variability, the forcing of the nonmigrating tide, due to tropospheric latent heat release for instance as discussed in Hamilton (1981) and Forbes et al. (1997), seems to be important as a source of the long-term variability, especially in the case of the diurnal tide. These nonmigrating components are not taken into account in classical 2d models at all, so the comparison between these model results and observations should only be understood as a first approach (Hagan et al., 1997).
General circulation models (GCMs) describe the full nonlinear interactions between mean fields, planetary waves and tides as well as their coupling with vertically propagating gravity waves. These small-scale waves cannot be resolved explicitly by the model, and so results in the MLT depend on the parameterization of gravity wave momemtum deposition. GCMs extending from the troposphere and stratosphere up to the MLT (e.g. Hamilton, 1995a; Hamilton et al., 1995b; Norton and Thurburn, 1997; McLandress, 1997; Miyahara and Miyoshi, 1997; Volodin and Schmitz, 2001, hereafter VS) offer the possibility to study the tidal oscillations and their dependence on nonlinear processes and nonmigrating components of tidal forcings. Miyahara et al. (1999) considered the contribution of the nonmigrating diurnal and semi-diurnal tidal components to the total tidal fields in the MLT on the basis of the middle atmosphere circulation model of the Kyushu University. Their results yield large diurnal tidal amplitudes at high latitudes where the tidal waves with a wavenumber larger than one are able to propagate vertically in a strong mean background wind field.
In this paper the tropo-strato-mesospheric GCM of the Department of Numerical Mathematics of the Russian Academy of Science (VS) is used, describing the atmosphere from tropospheric to MLT heights (upper boundary at height) to study both diurnal and semi-diurnal tides. The GCM takes into account all the physical parameterizations which are usually included in tropo-stratosphere general circulation models (Alexeev et al., 1998) and therefore it should be able to describe the migrating and nonmigrating forcings in an appropriate manner. The main aim of this paper is a detailed comparison of total model tides with radar measurements in the MLT region at different geographical locations. On the basis of the model results, the migrating and nonmigrating tidal components are shown separately at different grid points, which should give a first indication of the contribution of nonmigrating tidal components to the total tide at single observational sites. A comparison of model tidal results on nonmigrating components with observational results can be performed only qualitatively because there are, on the one hand, some modelling uncertainties, while the determination of tides on the basis of measurements is, on the other hand, affected by observational biases. The model must describe the forcings of both migrating as well nonmigrating tides. They arise due to very different physical processes. Only the forcing of migrating tides are simulated appropriately in present state-of-the-art GCMs. Further errors result from uncertainties in the simulation of the mean fields of zonal wind and temperature up to the MLT which strongly influence the vertical tidal propagation. Also inaccurate representation of planetary waves on different time scales can lead to problems because these motions interact with the thermal forced tides. Finally, momentum deposition by dissipating gravity waves must also be included in a GCM, but here none of the parameterization schemes available for this appears fully satisfactory at this stage.
This paper has been organized as follows. In the next section, the data basis and the main GCM characteristics are given. Section 3 presents comparisons between model results and observations, especially with respect to the variability of both diurnal and semi-diurnal tides. The separation into migrating and nonmigrating tidal components is given in Section 4. A summarizing discussion follows in Section 5.
Section snippets
Data
In this paper, only tidal data derived from MFR measurements at northern hemispheric sites have been used for comparison with model results. The different observation sites and their data characteristics are given in Table 1. For the comparison between the observations and the daily model output, the following vectorized averaging procedure has been applied to determine the mean fields and standard deviations.
With the amplitude Ai and phase ϕi for distinct analyses i, (1⩽i⩽N) the tidal
Diurnal and semi-diurnal tides in model and observations
Let us now compare the tides as derived from MFR measurements with the diurnal and semi-diurnal model tides. Results primarily from 1998 and 1999 are shown for the observed tides, with the exception of the two sites at HA and JU (abbreviations are given in Table 2) where data are available for nearly a decade.
Fig. 3, Fig. 4 show diurnal tidal amplitudes and phases of the eastward wind component for January and July, respectively. In January and high latitudes (AN) the model describes the
Migrating and nonmigrating tides
The total tidal variation at a distinct observational site consists of sun-synchronous travelling wave components and others which are not sun-synchronous. The procedure for separating the tides into migrating and nonmigrating parts was explained in Section 2. This separation was done only for the global set of model data because of the gaps in the global coverage of MFR sites. Assuming the model describes the tides at the observational sites in an appropriate manner, as discussed in Section 3,
Discussion and conclusion
Using the tropo-strato-mesosphere GCM of VS from the surface to about altitude, the behaviour of diurnal and semi-diurnal tidal oscillations has been analysed and compared to MFR measurements at different northern hemispheric sites for January and July conditions. In the model, the tidal oscillations are generated as an internal process due to short- and longwave radiation absorption, and large-scale condensation and convective heating in the troposphere and stratosphere. The Hines 1997a,
Acknowledgements
Evgeny Volodin would like to acknowledge grants 99-65533 and 99-05-65442 of the Russian Fund of Basic Research. Alan Manson acknowledges grants from NSERC Canada and thanks the University of Saskatchewan and ISAS for support. David Fritts received support for this research under grant ATM 9813774 from the National Science Foundation. Werner Singer was partially supported by grant SI 501/3-1 of the Deutsche Forschungsgemeinschaft for the Andenes MF radar. We thank U. Achatz for reading the first
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