ABSTRACT
Two comparative studies are presented in this chapter between CTIM, other numerical models and measurements. They are preceded by a general discussion o f factors influencing the simulated tidal profiles, thus adding to the findings o f Chapter IV. It is found that wind amplitude profiles are sensitive to the background temperature in the atmosphere, to the molecular viscosity coefficient and, at mid-to high latitudes, to the background wind field as well as the relative phase between forcing from below and the solar- and auroral in-situ momentum sources. Temperature amplitude profiles are found to be influenced by the same factors as the winds and, additionally, by the coefficients o f heat conduction and by the background winds also at low latitudes. The height of momentum dissipation varies with tidal amplitude, while the height o f energy dissipation hardly does. The tides’ vertical wavelength, however, is important for the height o f both the momentum- and energy dissipation. Comparisons with the TIGCM model generally show good agreement, with discrepancies being explained primarily by the differences in background temperature at low- to mid latitudes and differences in phase with the auroral forcing at mid- to high latitudes. Comparisons with the GSWM, HWM and MSISE90 show reasonable agreement o f the phases but generally lower amplitudes in CTIM. These cannot be accounted for comprehensively by the discussed influencing factors. Comparisons with measurements at low- and mid-latitude from the LTCS-9 campaign show good agreement o f meridional wind amplitudes, some similarities in the tem peratures and less agreement in the zonal winds. Comparisons with high latitude EISCAT measurements for seasonally averaged conditions at equinox show reasonable agreement. Finally, a series of plots are presented which compare tidal amplitudes and phases from CTIM at the mid latitude site of Millstone Hill (42 °N) to measurements and output from the above models as well as the TMTM model. Most of the discrepancies are attributed to ambiguity in the lower boundary forcing profile o f CTIM, due to the lack o f global tidal measurements.
V. 1. INTRODUCTION
This and following chapters present a number o f studies carried out with the new CTIM model code described in Chapter III. While the analyses presented in Chapter IV are mainly numerical
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experiments, this chapter focuses on comparisons with measured data and output from other numerical models. These comparisons are essential for validating the code. However, the results presented here are not only intended for vahdation but also complement those discussed in Chapter IV to give a broader understanding of the factors influencing the tidal propagation into the thermosphere. A discussion of these factors is presented in the first section o f this chapter and then applied to the comparisons and, where necessary, explored further. Chapters IV and V thus together form a comprehensive study into the nature o f tides.
Tidal simulations o f CTIM are first compared with output from two numerical and two empirical models. The numerical models used are the National Center for Atmospheric Research Thermosphere Ionosphere General Circulation Model (TIGCM and TIEGCM) {Roble et a l, 1988;
R ichm ond et a l , 1992] and the Global Scale Wave Model (GSWM) [Hagan et a l , 1995]. A num ber o f runs from both models are available on the CEDAR (Coupling, Energetics and Dynamics o f Atmospheric Regions) database, but since these are for different conditions they cannot be compared to the same CTIM run. For this reason it was necessary to carry out two separate simulations with CTIM which differ in their external tidal forcing and solar activity. Settings in the CTIM runs were chosen to match as closely as possible the settings used by the other models, in particular regarding the specification o f lower boundary tides. The TIGCM run uses output from the model by Forbes and Vial [1989] for tidal input at its lower boundary, while the GSWM model simulates self-consistently the formation and upwards propagation o f tides, thus requiring no external forcing.
The tidal winds are also compared with the empirical Hedin Wind Model (HWM) [Hedin et a l,
1993] and temperatures with the empirical MSISE90 [Hedin, 19 9 1] atmosphere model. Since semidiurnal amphtudes of those models at 80 km altitude were found to be roughly similar to those produced by the GSWM they are compared to the same CTIM simulation as the GSWM run.
In V.5, a combined comparison for the site o f the Millstone Hill observatory (42.6 °N, 71.5 °W) is presented between CTIM, measurements, the TIEGCM, GSWM and TMTM models. O f these, the TIEGCM is an updated version of the TIGCM (see also V.3.1) and the TMTM is the Tuned Mechanistic Tidal Model, which has been developed recently only and is still largely undocumented.
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V .2 . TID A L DA M PIN G IN T H E T H ER M O SPH ER E
Comparisons of two or more large numerical models, as carried out here, are often made difficult by the fact that not all information about other models which is necessary for an in-depth interpretation is available to an external user, but often known in detail only to the developer. Furthermore, a statement as to which of the models simulates situations more accurately is difficult if the runs are not compared to measurements at different globally distributed locations. Therefore, the factors influencing the tidal propagation are at first discussed in general terms in this and the following paragraph, and later apphed to the comparisons. This general discussion is important not only for the comparisons with models but also for those with measurements. The relevant momentum- and energy drag terms used in CTIM are reviewed to discuss what model properties could be responsible for any observed discrepancies.
Upwards propagating tides are identified by a horizontal- as well as a vertical structure. A common method o f studying tides is thus to examine the vertical profiles o f horizontal amplitudes, which is done later in this chapter. As expressed by equations (11.25) - (11.27) in Chapter II, a parameter in the presence of a propagating tide has at any fixed moment in time a periodic vertical structure. This is illustrated in Figure V.I for an idealized situation where no damping o f the tide occurs and thus the horizontal amplitude increases exponentially with altitude (dashed line).
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