ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-34-781-2016Atmospheric inertia-gravity waves retrieved from level-2 data of the satellite microwave limb sounder Aura/MLSHockeKlemensklemens.hocke@iap.unibe.chhttps://orcid.org/0000-0003-2178-9920LainerMartinhttps://orcid.org/0000-0002-1129-3406MoreiraLorenahttps://orcid.org/0000-0002-4791-8500HagenJonashttps://orcid.org/0000-0002-3484-4266Fernandez VidalSusanaSchranzFranziskaInstitute of Applied Physics, University of Bern, Bern, SwitzerlandOeschger Centre for Climate Change Research, University of Bern, Bern, SwitzerlandKlemens Hocke (klemens.hocke@iap.unibe.ch)19September20163497817888June201624August20165September2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/34/781/2016/angeo-34-781-2016.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/34/781/2016/angeo-34-781-2016.pdf
The temperature profiles of the satellite experiment Aura/MLS are
horizontally spaced by 1.5∘ or 165 km along the satellite orbit.
These level-2 data contain valuable information about horizontal fluctuations
in temperature, which are mainly induced by inertia-gravity waves. Wave
periods of 2–12 h, horizontal wavelengths of 200–1500 km, and vertical
wavelengths of 6–30 km efficiently contribute to the standard deviation of
the horizontal temperature fluctuations. The study retrieves and discusses
the global distributions of inertia-gravity waves in the stratosphere and
mesosphere during July 2015 and January 2016. We find many patterns that were
previously present in data of TIMED/SABER, Aura/HIRDLS, and ECMWF analysis.
However, it seems that Aura/MLS achieves a higher vertical resolution in the
gravity wave maps since the maps are derived from the analysis of horizontal
fluctuations along the orbit of the sounding volume. The zonal mean of the
inertia-gravity wave distribution shows vertical modulations with scales of
10–20 km. Enhanced wave amplitudes occur in regions of increased zonal wind
or in the vicinity of strong wind gradients. Further, we find a banana-like
shape of enhanced inertia-gravity waves above the Andes in the winter mesosphere.
We find areas of enhanced inertia-gravity wave activity above tropical deep
convection zones at 100 hPa (z∼ 13 km). Finally, we study the temporal
evolution of inertia-gravity wave activity at 100 hPa in the African
longitude sector from December 2015 to February 2016.
Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)Introduction
Atmospheric gravity waves transfer momentum through the atmosphere, and the
breaking of gravity waves changes the circulation of the atmosphere
. According to , gravity waves can be
divided into three classes: inertia-gravity waves (or low-frequency gravity
waves), medium-frequency gravity waves, and high-frequency gravity waves. In
the case of inertia-gravity waves, the Coriolis force has to be considered as a
restoring force in addition to the buoyancy force.
High-frequency gravity waves have wave periods less than 1 h, horizontal
wavelengths less than 150 km, and vertical wavelengths greater than 10 km.
High-frequency gravity waves are most important for transfer of momentum from
the lower to the middle and upper atmosphere .
High-frequency gravity waves can be generated by orography, jet streams and
convective activity. Medium-frequency gravity waves have periods of 1–3 h
and horizontal wavelengths between 150 and 300 km. Numerical simulations by
showed that ocean surface waves can generate medium- and
high-frequency atmospheric gravity waves propagating up to the thermosphere.
Inertia-gravity waves have periods from several hours to 24 h and horizontal
wavelengths from several hundreds of kilometres to about 2000 km. The vertical
wavelengths generally decrease with increase of the wave period. A strong
inertia-gravity wave was found in the stratosphere above the southern Andes with
a vertical wavelength of 6–7 km and a horizontal wavelength of about 400 km
. The Andes wave regularly occurs in
high-resolution meteorological analyses during austral winter
. investigated the cause of the
stratospheric gravity wave belt that extends eastward from the southern Andes
to southeast of Australia. They found that this belt of enhanced gravity wave
activity is a robust climatological feature due to non-orographic tropospheric
gravity wave sources: spontaneous emission from jets in rapidly evolving
baroclinic systems, frontogenesis, and convection.
investigated inertia-gravity wave generation around the polar vortex in the
stratosphere above the Syowa station (69∘ S, 39∘ E). They
explained that the observed inertia-gravity waves are generated by a
spontaneous adjustment around the geostrophically unbalanced polar night jet.
The spontaneous generation of inertia-gravity waves above the regions of
strong curvature of the tropospheric mid-latitude jet stream was simulated in
the study of . studied
inertia-gravity waves in the large-scale flow of a rotating, two-layer
annulus experiment. provided a review about
inertia-gravity waves from atmospheric jets and fronts.
observed inertia-gravity waves generated from tropical cyclones, while
investigated inertia-gravity waves associated with tropospheric
deep convection in Korea. derived a global morphology of
gravity wave activity in the stratosphere by using the GPS occultation data
of the GPS/MET experiment.
Analysing a meteor train, detected an inertia-gravity wave
with a vertical wavelength of 16 km and an amplitude of 30 m s-1 in the
mesopause region. Lidar and radar measurements over Antarctica gave evidence
for inertia-gravity waves in the polar mesopause region with vertical
wavelengths of about 22 km, apparent periods of 5–8 h, horizontal
wavelengths of about 1100–2200 km, and horizontal phase speeds of 68–80 m s-1. explained that the large horizontal wind
fluctuations of inertia-gravity waves can play a role in wave mixing of
atmospheric trace gases. performed atmospheric simulations
with and without inertia-gravity waves. Their results strongly suggest that
gravity wave mixing of minor constituents represents a very important
mechanism for the coupling between the dynamics and the photochemical system
of the mesosphere and lower thermosphere.
In the past, the Microwave Limb Sounder on the satellite Aura (Aura/MLS)
provided deep insight into the global distributions of high-frequency gravity
waves . The retrieved high-frequency
gravity waves have horizontal wavelengths less than 140 km and vertical
wavelengths of about 10 km. The high horizontal resolution is reached by using
the level-1 data of Aura/MLS, namely the brightness temperature of 20
microwave channels. presented advanced global and regional
distributions of high-frequency gravity wave activity. In addition, they
evaluated the high-resolution data of ECMWF meteorological reanalysis. In the
conclusions, they suggested a further study to retrieve inertia-gravity
waves from the level-2 data of Aura/MLS. Here, we follow this way and we
present global distributions of inertia-gravity wave activity in the
stratosphere and mesosphere. Observations of global maps of mesospheric
inertia-gravity waves are quite novel. We are only aware of a study by
in which they presented global maps of gravity wave momentum flux at 70 km altitude retrieved from TIMED/SABER observations.
Data analysis
We apply a data analysis which evaluates the horizontal temperature
fluctuations at a fixed pressure level and along the orbit of the sounding
volume of the Aura satellite. The advantage of evaluating the horizontal
fluctuations is that background features such as the stratopause or the
mesopause are not misinterpreted as atmospheric waves. Further, large-scale
equatorial waves, tides, or planetary waves with horizontal wavelengths of
several thousand kilometres will not be mistaken for inertia-gravity waves. In
addition the vertical resolution of the gravity wave maps is of the order of
3–6 km. Such a resolution cannot be achieved by high-pass-filtering the
vertical oscillations of temperature profiles.
Recently, removed the global-scale background temperature
distribution by a 2-D-Fourier decomposition in longitude and time. This method
is also capable of removing global-scale waves with periods as short as about
12 days, and thus global distributions of gravity-wave momentum flux absolute values
can be derived not only in the stratosphere but also in the mesosphere. This
demanding data analysis method of is beyond our capabilities,
and we prefer to derive the gravity wave-induced horizontal temperature
fluctuations with a simple method which takes advantage of the dense
horizontal sampling of Aura/MLS.
The level-2 data consist of atmospheric vertical profiles with a spacing of
165 km (1.5∘ along the satellite orbit which is sun-synchronous with
an inclination of 98∘ and a period of 98.8 min; ). In the present study, we evaluate the mean
and the standard deviation of five consecutive temperature values separately at
each pressure level from about 100 to 0.001 hPa. Before calculation of
the standard deviation the data are detrended in order to remove the effect of
large-scale, horizontal background variations. We selected five consecutive
measurement points, since over a distance of 5×165 km a straight line
fit of the horizontal background variation is a good approximation. The five
points are collected within a time interval of 2 min. In the case of more
measurement points (e.g. seven) the straight line fit approximation becomes more
invalid since the background atmosphere non-linearly varies over long
distances. Further, the horizontal resolution of the gravity wave maps would
be reduced. On the other hand, in the case of three consecutive points the
standard deviation and the mean are not well defined, and the measurement
noise may dominate. Thus, the choice of five consecutive points is the best
compromise.
The standard deviation that is calculated by the
five-consecutive-points method as a function of the horizontal wavelength
λh for artificial sine waves with an amplitude of 1. The parameter
α is the angle between the horizontal wave vector and the orbit of the
sounding volume (0∘ is wave propagation along the orbit; red line). The
apparent wavelength is given by λh/cosα. The magenta line
denotes the Nyquist length when the wavelength is 2 times the sampling
length of the near-polar orbit, which is about 165 km. Generally, our method
is sensitive to inertia-gravity waves with wavelengths between about 200
and 1500 km. Planetary waves in zonal direction have a high value of α
and are nicely suppressed by our method.
By means of artificial sine waves which are sampled with a spacing of 165 km
along the sounding volume orbit, we can estimate the response of our method.
Figure shows the result for different angles α between the
horizontal wave vector and the orbit. Waves propagating along the track
(α=0∘, red line) can have wavelengths of about 1500 km, and they
still contribute to the standard deviation. Tides and planetary waves which
have a α value of about 80∘ (in the case of the near polar orbit of
Aura) and large horizontal wavelengths are strongly suppressed by our method
as the blue line in Fig. shows. There are aliasing effects left
of the magenta line in Fig. .
Thus, our method includes some noise from high-frequency and medium-frequency gravity waves.
Fortunately, the amplitudes of these waves in nature are smaller than the amplitudes of inertia-gravity
waves so that the noise and aliasing problem should be not a serious problem for our data analysis.
In summary, the standard deviation will be a good proxy for the inertia-gravity
waves with horizontal wavelengths from 200 to 1500 km.
explained in detail that the variable angle α
between the line of sight and the wave fronts can lead to measurement
geometry biases. investigated the sensitivity of
space-based measurements of stratospheric mountain waves to the viewing
geometry. For the purpose and conclusions of the present study these biases
are not so relevant since we are mainly interested in the order of magnitude
and the rough geographic distribution of inertia-gravity wave activity.
The vertical resolution of the temperature profiles of Aura/MLS ranges from 3 km
in the stratosphere to 6 km in the mesosphere . The
present study utilizes Aura/MLS data of version 4.2. Temperature
fluctuations with vertical wavelengths of 6 to 30 km are expected to have
the strongest response. The approximated dispersion relation of
inertia-gravity waves under inclusion of the Coriolis frequency f is
ω^2=N2kh2m2+f2,
where N is the buoyancy frequency, ω^ is the intrinsic wave
frequency, kh is the horizontal wave number, and m is the vertical wave
number. For a constant buoyancy frequency N=2π/5 min and f for
45∘ latitude, the intrinsic gravity wave periods are of the order of
2–12 h for horizontal wavelengths of 200–1500 km and vertical wavelengths
of 6–30 km, which most efficiently contribute to the standard deviation of the
horizontal temperature fluctuations.
Monthly zonal mean of temperature (upper panel) and its
standard deviation (SD(T), lower panel) for July 2015 as observed by the
satellite experiment Aura/MLS. The cold summer mesopause in the Northern
Hemisphere and the cold polar vortex in the lower stratosphere of the
southern winter Northern Hemisphere are present.
Monthly zonal mean of temperature (upper panel) and its
standard deviation (SD(T), lower panel) for January 2016 as observed by the
satellite experiment Aura/MLS. Compared to the winter Southern Hemisphere in
Fig. , the winter Northern Hemisphere shows larger stratospheric
temperature variations, which are mainly due to planetary Rossby waves and the
deformation and displacement of the stratospheric polar vortex.
Figure shows the zonal mean temperature field and its standard
deviations for July 2015. This figure gives a first orientation. In the
present study we intercompare the data of July 2015 and January 2016. As
expected, the mesopause temperature has a minimum in the summer Northern
Hemisphere in Fig. . The standard deviation of the zonal
averages is around 10 K in the winter stratosphere while the summer
stratosphere is very quite (<5 K). Figure shows the zonal mean
temperature field and its standard deviations for January 2016. In the
winter Northern Hemisphere the standard deviation of the zonal means reaches
high values of 20 K. Indeed, the winter 2015/2016 showed large deformations
and displacements of the stratospheric polar vortex with zonal wave numbers 1
and 2. Generally, the mesopause region of Fig. shows higher
values of the standard deviation compared to Fig. . In summary,
the temperature profiles of Aura/MLS seem to have a high quality and they
might be usable over the range from 15 to 100 km altitude, though the
recommendation of restricts the temperature profile use
to altitudes above 16 km and below 90 km (or 100 to 0.001 hPa).
ResultsZonal mean of inertia-gravity wave activity
Figure shows the zonal mean of the inertia-gravity waves in July
2015 and January 2016. The standard deviations have been calculated for
groups of five consecutive measurement points at the same pressure level as
described in the section data analysis. Then, the standard deviations are
binned with a sliding latitude window of 5∘. Generally, the
enhancement of gravity waves over the winter hemispheres has already been
reported by , , and other studies. However, Fig. is calculated for inertia-gravity waves. Apparently their
distribution differs not so much from the distribution of high-frequency
gravity waves. Contrary to other studies, Fig. shows vertical
modulations of SD(T) with scales of 10–20 km. This modulation could be due
to interactions between the inertia-gravity waves and large-scale flows such
as tides, planetary waves, and vortices. The inertia-gravity waves of the
southern winter stratosphere are larger than those of the northern winter
stratosphere. This feature is also observed by , who found weak
gravity waves in the northern winter stratosphere compared to the southern
winter stratosphere where the Andes mountain ridge and the Antarctic
Peninsula are strong wave-makers. Interestingly there is enhanced gravity
wave activity over northern midlatitudes in the winter mesosphere at 85 km
altitude.
Monthly zonal mean of the standard deviation in
temperature (SD(T)) as retrieved from level-2 data of the satellite
experiment Aura/MLS. The standard deviation is determined from five consecutive
measurement points of Aura/MLS (details are given in the text). SD(T) can be
regarded as the mean amplitude of the inertia-gravity waves. The left-hand
side panel is the result for July 2015 while the right-hand side shows the
result for January 2016. The contour lines show the SPARC climatology of
zonal wind. Solid contour lines belong to positive eastward winds (≥ 0 m s-1)
while dashed lines are negative eastward winds. The step size is 10 m s-1.
The contour lines denote the SPARC climatology of zonal wind. The monthly
zonal wind climatology is derived from the UARS Reference Atmosphere Project
(URAP), combining results from UK Met Office (METO) analyses with winds from the UARS High
Resolution Doppler Imager (HRDI). Details of the URAP winds are described in
. Obviously, there is a relationship between waves and
zonal wind in Fig. . The wave amplitudes are strong at the
location of the polar night jets, and sometimes enhanced wave amplitudes occur
in the vicinity of vertical or horizontal wind gradients.
This is in agreement with the theory of spontaneous emission of inertia-gravity waves by shear
flows.
Figure shows that the SPARC zonal wind climatology has enhanced
eastward wind at mesospheric altitudes at northern mid-latitudes in January
where also increased amplitudes of inertia-gravity waves occur.
performed simulations of the mesospheric wind jet and
gravity waves in the winter Northern Hemisphere. They found that with
increasing altitude smaller-and-smaller-scale structures develop in a zone of
erosion near the border of the mesospheric jet. Further, they reported that
the erosion of the mesospheric jet stream may generate new gravity wave like
structures of short time periods which penetrate even higher into the lower
thermosphere. We find enhanced mesospheric gravity wave activity over
northern mid-latitudes not only in January 2016 but also in January 2015 so
that we are quite sure that this is a regular feature.
Global maps of inertia-gravity wave activity
For each pressure level, the SD(T) values are interpolated to a regular grid
with a resolution of 5×5∘ in latitude and longitude using the
Delaunay triangulation method (Matlab function TriScatteredInterp.m with the
option “nearest neighbor interpolation”). Figures and
show the vertical evolution of the inertial-gravity waves in the mesosphere
and the stratosphere, respectively. The colour scale of Fig. is
different since the SD(T) values are smaller in the stratosphere than in the
mesosphere. First at all, we find a good agreement of the stratospheric
SD(T) values and the patterns with the amplitudes of Aura/HIRDLS and ECMWF
analysis shown in Figs. 1 and 4 of . Further, in agreement
with we find a stratospheric gravity wave belt that
extends eastward from the southern Andes to southeast of Australia in our
Fig. . Going upward to mesospheric heights, the gravity wave belt
changes its shape and position. At 0.01 hPa, a banana-like pattern is around
the Andes in July 2015. The shape could be related to the mid-latitude
tropospheric jet stream. In the stratosphere, over Scandinavia, there is
enhanced gravity wave activity in the stratopause region in January 2016.
This behaviour is not found for January 2015 (not shown here), but it is found
in the long-term average of January data from 2002 to 2010 in TIMED/SABER
data in Fig. 1 of .
Global maps of inertia-gravity wave activity at various
pressure levels in the mesosphere. SD(T) can be regarded as the mean
amplitude of the inertia-gravity waves.
Global maps of inertia-gravity wave activity at various
pressure levels in the stratosphere and the stratopause region. Please note
that the temperature scale is different than in Fig. . SD(T) can
be regarded as the mean amplitude of the inertia-gravity waves.
Inertia-gravity wave activity at 100 hPa over tropical deep convection zones
Aura/MLS can also measure the temperature fluctuations in the tropopause
region. Studies on wave activity in the upper troposphere in the tropics are
rare since the separation between the atmospheric background profile and the
atmospheric perturbation is ambiguous in the region around the sharp tropical
tropopause. This could be a reason why selected the height
region of 20–30 km (well beyond the tropical tropopause) for the retrieval of
a global map of gravity wave activity from GPS radio occultation data.
Further, the retrieval of high-frequency gravity wave activity from
brightness temperature of AURA/MLS is restricted to the altitude region from
20 to 50 km where the saturation of different microwave channels occurs (Table 1 of ). Thus, the assessment of gravity wave activity in the
tropical tropopause region was a challenge in the past. The method presented
in the present paper has no problem with the derivation of inertia-gravity
wave activity at 100 hPa in the tropical tropopause region since the method
evaluates the horizontal temperature variations in the vertical profiles
recorded with a dense sampling along the orbit of the Aura satellite.
Figure a shows the result for the inertia-gravity wave activity
(SD(T)) at 100 hPa (z∼ 16 km). The global map is averaged over the time
interval from December 2015 to February 2016. The wave activity is enhanced
above the tropical convection zones, namely above the Amazon region, central
Africa, and Indonesia. Figure b shows the temporal evolution of the
wave activity over central Africa. SD(T) reaches for short time intervals
amplitudes of 2 K and more. The SD(T) time series is modulated by periods of
a few days to a month. For example, the wave amplitudes over central Africa
are increased in mid December and mid January. It is likely that such
information is valuable for validation of the inertia-gravity wave activity
in atmospheric models.
(a) Global map of inertia-gravity wave activity (SD(T))
at 100 hPa (z∼ 16km) in the upper troposphere–lower stratosphere region,
averaged for the time interval from December 2015 to February 2016.
(b) Temporal evolution of SD(T) in the African longitude sector (20
to 40∘ E).
Conclusions
The study showed that the dense horizontal sampling of the vertical
temperature profiles from Aura/MLS allows the retrieval of the global
distribution of inertia-gravity waves. Wave periods of 2–12 h, horizontal
wavelengths of 200–1500 km, and vertical wavelengths of 6–30 km efficiently
contribute to the standard deviation of the horizontal temperature
fluctuations. We found many patterns that were previously present in data of
TIMED/SABER, Aura/HIRDLS, and ECMWF analysis. However, it seems that Aura/MLS
achieves a higher vertical resolution in the gravity wave maps since the maps
are derived from the analysis of horizontal fluctuations along the orbit of
the sounding volume. The zonal mean of the inertia-gravity wave distribution
shows vertical modulations with scales of 10–20 km. Generally, there is a
close relationship between the wave field and the zonal wind climatology.
Enhanced wave amplitudes occur in regions of increased zonal wind or in the
vicinity of strong wind gradients. Further, we find a banana-like shape of
enhanced inertia-gravity waves above the Andes, particularly in the winter
mesosphere. At northern winter mid-latitudes enhanced wave activity is
present at 80–90 km altitude (Fig. ). Areas of enhanced
inertia-gravity wave activity are found above the tropical deep convection
zones at 100 hPa (z∼ 16 km). The temporal evolution of inertia-gravity
wave activity at 100 hPa in the African longitude sector from December 2015
to February 2016 showed modulations of the tropical wave activity on timescales from a few days to a month. Further studies could be performed by
analysing the horizontal fluctuations of other parameters such as ozone and
water vapour volume mixing ratio, which are also measured by Aura/MLS.
Intercomparisons of the horizontal fluctuations from Aura/MLS with coincident
temporal fluctuations from ground-based remote sensing could be interesting
too.
Data availability
The level-2 data are available at the Aura Validation Data Center (AVDC,
2016). The data of the viewgraphs and routines are available by the authors.
The data supplement shows the gravity wave distribution for a different year.
The Supplement related to this article is available online at doi:10.5194/angeo-34-781-2016-supplement.
Acknowledgements
We thank the Aura/MLS team and NASA/JPL for the microwave limb sounding
measurements and the provision of the level-2 data set at the Aura Validation
Data Center (http://avdc.gsfc.nasa.gov/). The SPARC Data Center provided the
zonal wind climatology (http://www.sparc-climate.org/data-center/). We thank
the reviewers and the editor for constructive suggestions and corrections.
The topical editor, C. Jacobi, thanks two anonymous referees for help in evaluating this paper.
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