ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-36-825-2018Seasonal variability of atmospheric tides in the mesosphere and lower thermosphere: meteor radar data and simulationsVariability of tidesPokhotelovDimitrypokhotelov@iap-kborn.dehttps://orcid.org/0000-0002-3712-0597BeckerErichStoberGunterhttps://orcid.org/0000-0002-7909-6345ChauJorge L.Leibniz-Institute of Atmospheric Physics at the University of Rostock, Kühlungsborn, GermanyDimitry Pokhotelov (pokhotelov@iap-kborn.de)6June201836382583014February201816February201811May201814May2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://angeo.copernicus.org/articles/36/825/2018/angeo-36-825-2018.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/36/825/2018/angeo-36-825-2018.pdf
Thermal tides play an important
role in the global atmospheric dynamics and provide a key mechanism for the
forcing of thermosphere–ionosphere dynamics from below. A method for
extracting tidal contributions, based on the adaptive filtering, is applied
to analyse multi-year observations of mesospheric winds from ground-based
meteor radars located in northern Germany and Norway. The observed seasonal
variability of tides is compared to simulations with the Kühlungsborn
Mechanistic Circulation Model (KMCM). It is demonstrated that the model
provides reasonable representation of the tidal amplitudes, though
substantial differences from observations are also noticed. The limitations
of applying a conventionally coarse-resolution model in combination with
parametrisation of gravity waves are discussed. The work is aimed towards the
development of an ionospheric model driven by the dynamics of the KMCM.
Introduction
The region of the mesosphere and lower thermosphere (MLT) is characterised by
a variety of waves including atmospheric gravity waves (GWs), tides, and
planetary waves (PWs). In the MLT region these waves reach large amplitudes
such that the velocity perturbations become comparable to velocities of the
mean flow. While GWs generally break in the MLT region, the tides propagate
directly to higher altitudes and impact the dynamics of the thermosphere and
ionosphere. The tides thus play an important role in the forcing of the
coupled ionosphere–thermosphere system from below
e.g.. Pronounced features of the low-latitude
ionospheric dynamics, such as the wave-4 longitudinal structure observed in
sub-equatorial ionospheric electric fields and plasma densities, have been
attributed to the forcing from atmospheric tides . The current work is motivated by the need to simulate the tidal
dynamics in the MLT with a computationally inexpensive general
circulation model (GCM), and to drive an ionospheric model with the simulated
dynamical fields in order to analyse the impact of tides on the ionosphere.
Multi-year observations of tides with ground-based meteor radars are used
here as a benchmark for the GCM results.
The thermal tides observed in the MLT region represent an interference of the
sun-synchronous (migrating) tides generated by the absorption of infra-red
and ultra-violet solar radiation in the troposphere and stratosphere, and the
non-sun-synchronous (non-migrating) tides generated by the longitudinal
irregularities in radiative heating and latent heat release in the
troposphere and/or by nonlinear interactions between PWs and migrating tides
e.g.. The most prominent spectral components are
24 h (diurnal), 12 h (semidiurnal), and 8 h (terdiurnal) tides. A number
of observational studies using ground-based very high frequency (VHF) meteor
radars have been dedicated to the seasonal variability of atmospheric tides
in the MLT region. At low latitudes, the diurnal tide dominates the spectrum.
It's annual cycle shows minimum amplitudes around the solstices and maximum
amplitudes around the equinoxes e.g.. At
middle and high latitudes, the diurnal tides cannot effectively propagate
into the MLT region , and the spectrum is dominated by
the semidiurnal tide, with the highest amplitudes in winter months and during
the autumn transition in September e.g..
Comprehensive whole atmosphere GCMs such as the Canadian Middle Atmosphere
Model (CMAM), the Hamburg Model of the Neutral and Ionized Atmosphere
(HAMMONIA), or the Whole Atmosphere Community Climate Model (WACCM)
reproduce, to some extent, the climatology of the diurnal tide as observed by
satellites .
A substantial work on the modelling of tides was also done using the Global
Scale Wave Model (GSWM; ). In GSWM, however, the
nonlinear tidal dynamics and interactions with PWs and GWs are neglected.
Comparisons of model results focused mainly on satellite observations
yielding tidal amplitudes that are averaged over typically 2 months
. presented
comparisons with model simulations using GSWM and meteor radar observations
of tides at high latitudes in northern Sweden. presented
comparisons with WACCM and CMAM simulations and meteor radar observations at
low latitudes over Ascension Island. In these studies the comparison was done
including the observed monthly variabilities of tidal amplitudes making the
model comparison somewhat inconclusive, as the observed monthly variabilities
are comparable to the absolute values of tidal amplitudes.
The Kühlungsborn Mechanistic Circulation Model (KMCM;
) is a simplified mechanistic model that is
computationally inexpensive and suitable for numerical experiments due to its
mechanistic character. This study addresses the applicability of the KMCM for the
studies of ionospheric forcing from below. In the current article we present
a comparison between the tidal amplitudes observed with meteor radars at
middle and high latitudes, extracted using the adaptive filtering algorithm,
and the simulated tidal amplitudes extracted from the KMCM using the same
filtering algorithm. This allows for a direct comparison of the observed tidal
amplitudes with the modelled results, without results being contaminated by
the monthly variabilities of the tides.
Amplitudes of semidiurnal tides at high latitudes extracted from
meteor radar observations over Andenes. Panels (a, b) correspond,
respectively, to the zonal and meridional components.
Amplitudes of semidiurnal tides at middle latitudes extracted from
meteor radar observations over Juliusruh. Panels (a, b) correspond,
respectively, to the zonal and meridional components.
Radar observations and data analysis
VHF meteor radars provide neutral wind dynamics in the range of altitude
between about 75 and 110 km using backscatter from meteor ionisation
traces. The Radar Remote Sensing Department at the Leibniz-Institute of
Atmospheric Physics have been continuously operating meteor radars for over
a decade at high- and mid-latitude locations in Andenes, Norway
(69∘ N, 16∘ E) and in Juliusruh, Germany (54∘ N,
13∘ E). In this study the composite tidal climatologies are derived
from the datasets of years 2003–2016 for Andenes and November 2007–2016 for
Juliusruh.
Amplitudes of semidiurnal tides at high latitudes (corresponding to
Andenes) extracted from the KMCM simulation. Panels (a, b)
correspond, respectively, to the zonal and meridional components.
Amplitudes of semidiurnal tides at middle latitudes (corresponding
to Juliusruh) extracted from the KMCM
simulation. Panels (a, b) correspond, respectively, to the zonal and meridional components.
In order to separate contributions from diurnal, semidiurnal, and terdiurnal
tidal components, the 1 h time resolution meteor radar data are processed
using an adaptive spectral filter. This filter uses a sliding window of
a predefined length (3 days in this study) and fits the amplitudes and phases
for each tidal component accounting for the number of wave cycles within the
window . The fitting procedure also fits and subtracts
the linear trend and eliminates the contribution of PWs. The GW contribution
is then defined by the residuum and contains all fluctuations different from
the tides or PWs. Figures and
present tidal climatologies of semidiurnal tides for Andenes and Juliusruh,
respectively.
Zonal component of the mean flow observed with meteor radar at
Juliusruh (a) and simulated with the KMCM over the
same location (b). White bins in the top panel reflect insufficient statistics of the observed meteor echoes at high
altitudes.
Numerical simulations
The KMCM is a mechanistic GCM from the surface to the lower thermosphere with
uppermost level around 8×10-7hPa, corresponding to about
200 km height. Here we use the same model version as in
. This model simulates the dynamics of the whole
atmosphere like a comprehensive GCM. The mechanistic character is due to
simplified computations of radiative transfer and moist convection, as well
as due to the neglect of chemical processes in the middle atmosphere. This
mechanistic approach allows for the easy adjustment of model parametrisations in
order to perform sensitivity experiments. The only ionospheric process
considered is a simple parametrisation of ion drag .
Since the model employs a conventionally coarse spatial resolution (spectral
truncation at a total horizontal wave number 32 and 80 vertical layers), both
orographic and non-orographic GWs need to be parametrised.
At the locations corresponding to Andenes and Juliusruh, the model time
series are extracted and converted from pressure levels to geometric heights.
The same tidal amplitude analysis as for the meteor radar data is applied to
the model data. The resulting semidiurnal tidal amplitudes of the zonal and
meridional winds simulated with the model are shown in
Figs. and , for Andenes and
Juliusruh, respectively. In the following we compare these results with the
tidal climatology from the radar winds.
Discussion and summary
As expected from the linear tidal theory , as well as
from earlier observational and modelling studies, the MLT tidal spectra at
middle and high latitudes are dominated by the semidiurnal tide (the diurnal
and terdiurnal tide are much weaker, not shown here). The annual cycle of the
semidiurnal tide, both at Andenes and Juliusruh, shows maximum amplitudes in
winter months (December–February) and during the fall transition in
September, while minimum amplitudes are seen ∼1 month prior to the
summer and winter solstices, i.e. in May and in November. The tidal
amplitudes are ∼30 % stronger at middle latitudes (Juliusruh) than
at high latitudes (Andenes).
The simulated tides show similar behaviour as the radar-observed tides. In
particular, the highest amplitudes occur in winter and during the fall
transition. Stronger tidal amplitudes at middle than at high latitudes, as
well as stronger tidal amplitudes of the meridional than the zonal component,
are also reproduced in the simulation. We also notice substantial differences
between the observed and simulated behaviour of tides which require further
examination and could be due to the deficiencies of simulated mean flow, as
discussed below. The main difference is that the model predicts strong
amplitudes of tides around 80–85 km in the summer months, which is
not seen in the observations.
The tides are strongly affected by the interactions with mean winds and GWs
. A comparison between the observed
and simulated mean zonal winds, obtained by 21-day time averaging (see
Fig. ) shows that the mesopause wind reversal reproduced in
the model is too low in altitude by ∼5km and that the eastward
winds higher up are strongly overestimated compared to the observational
result. The slope of the mesopause wind reversal boundary from May to August
is also less steep in the simulations. This model deficiency might contribute
to the simulated amplification of the tides through summer, as well as to the
simulated amplification of the tides during the spring transition that is not
pronounced in observations. Further numerical studies of the interaction of
tides with mean flow at different latitudes are needed. The effects of GWs on
both the mean flow and on the amplitudes of tides could play an important
role, but the details are currently difficult to assess. While the GW
climatologies can be derived from the radar observations (see
Sect. ), the same analysis cannot be directly applied to the
model, where GWs are parametrised. A conventional coarse-resolution GCM (like
the current KMCM) will always produce some resolved inertial GW
activity at MLT altitudes e.g., and these GWs are strongly resolution-dependent. An
approximately realistic representation of GWs in a GCM would require
effective horizontal and vertical resolutions of less than ∼100 and
1 km, respectively. A new version of the KMCM allows us to perform
such simulations with realistic GW effects in the middle atmosphere that are
solely due to resolved GWs . However, a comparison of
these model results with observations is beyond the scope of the present
study.
We have demonstrated that the KMCM used with a conventional model setup
provides a reasonable representation of the annual cycle of the semidiurnal
tide in MLT region at middle and high latitudes, though substantial
differences from radar observations are also noticed. This opens a pathway
for the simulation of tidal influence on the thermosphere and ionosphere by
coupling the KMCM dynamics to a dedicated model of ionospheric dynamics,
specifically the Thermosphere-Ionosphere-Electrodynamics General Circulation
Model (TIEGCM; ). In this setup the ionospheric model
would be forced at its lower boundary located at ∼97km
altitude by the GCM dynamical fields at that altitude. Therefore, the
presented validation of model dynamics, and tides in particular, with meteor
radars in this altitude range is of particular interest. In this respect, the
current work represents a first step towards the analysis of tidal forcing of
the ionosphere from below.
The meteor radar data are available upon request to
Gunter Stober (stober@iap-kborn.de). The KMCM simulated data are available
upon request to Erich Becker (becker@iap-kborn.de).
The authors declare that they have no conflict of
interest.
This article is part of the special issue “Dynamics and
interaction of processes in the Earth and its space environment: the
perspective from low Earth orbiting satellites and beyond”. It is not
associated with a conference.
Acknowledgements
This work is partially supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under the SPP 1788
(DynamicEarth) Project DYNAMITE (CH 1482/1-1) and by the WATILA Project (SAW-2015-IAP-1 383). We thank the colleagues of the tidal
matrix group at IAP for helpful discussions. The topical editor, Hermann Lühr, thanks Chris Meek
and one anonymous referee for help in evaluating this paper.
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