Introduction
The middle atmosphere is the part of Earth's atmosphere that
extends from about 10 to 110 km of altitude. The upper part
(60–110 km) is referred to as the MLT (mesosphere lower
thermosphere), which is dominated by the interplay of atmospheric waves,
including tides and gravity and planetary waves. Important source regions for
atmospheric waves seen in the MLT are often found lower in the atmosphere.
With decreasing pressure and air density, upward-propagating waves
are forced to increase their amplitudes . This increase in
amplitudes can lead to wave breaking and the deposition of momentum, which in
turn supplies the driving force for large-scale residual circulations like
the Brewer–Dobson circulation . Besides the
diurnal and semi-diurnal waves, the quasi 2-day wave (Q2DW) is among the
strongest wave phenomena within the middle atmosphere. Quasi 2-day waves
originate primarily from baroclinic instabilities, which can be found in the
vicinity of jet streams such as the summertime mesospheric easterly jet. Many
studies indicate that these atmospheric regions produce fast-emerging
instabilities coupling to the zonal wave number 3 global Rossby gravity mode
. Q2DW structures in middle
atmospheric temperature observations were first discovered by
. Before that time quasi 2-day oscillations were only
found in wind data at meteor heights . Q2DWs manifest not
only in wind or temperature fields. analyzed
one of the first observations of 2-day planetary wave signatures in
atmospheric airglow. A recent numerical GCM (general circulation model)
investigation by brought new insights on the
planetary-wave-induced airglow variability in the mesosphere and lower
thermosphere. In regard to the 2-day variability, prominent oscillations were
found in this simulation during summer at a northern hemispheric midlatitude
(43∘ N, 143∘ E). Usually the Q2DW
gets amplified in temporal proximity to the solstices . For the
Northern Hemisphere (NH) the months July and August (after summer solstice)
are favored to build up strong Q2DW signs in the MLT. One reason is likely
associated with a strengthening of the summer easterly jet in the
extratropical upper mesosphere, favoring a nonlinear interaction with the
migrating diurnal tide . The mesospheric easterly jet
itself undergoes a not insignificant variability throughout the years, mainly
due to the variation of gravity wave activity as reported in .
These circumstances imply the overall complex interactions related to Q2DW
activity.
The Q2DW has been studied for decades via ground-based and space-borne
observations e.g.,.
All of these techniques have their individual advantages and disadvantages.
Analysis from satellites is required to get a global view of Q2DW
activity. Compared with ground-based techniques the temporal resolution of
local observations is poor for satellites. To perform long-term studies of,
e.g., the interannual variability of the Q2DW, ground-based measurement sites
can provide an excellent source of data. Moreover a high temporal resolution
offers the possibility of investigating nonlinear wave–wave interactions
between Q2DW and atmospheric waves with even shorter periods, like diurnal or
semi-diurnal tides . Both
observation types, global and local, complement each other and are required
to study the Q2DW in all its facets within the Earth's atmosphere.
One main temporal feature of the quasi 2-day wave is its appearance in
burst-like events, meaning that the amplitude strength is highly
discontinuous in time. As shown in other studies
and in our presented results
(Sect. ), the Q2DW signatures can manifest in a high degree of
interannual as well as intra-seasonal variability.
Apart from wind measurements as a proxy for dynamical patterns in the middle
atmosphere, it is common to use dynamical tracer observations such as water
vapor. In the mesosphere H2O is photochemically stable for weeks
and this circumstance is used to investigate middle
atmospheric wave dynamics from ground-based observations
. In this study we
present quite continuous observations of the Q2DW signature in middle
atmospheric water vapor for 7 years (84 months) by the middle
atmospheric water vapor radiometer MIAWARA at Bern–Zimmerwald
(46.88∘ N, 7.46∘ E). Such
investigations, especially from passive remote sensing observations in the
midlatitudes, are rare and will provide new insights on Q2DW variability
at mesospheric altitudes. Section is dedicated to water vapor
radiometric measurements in the middle atmosphere and the corresponding
millimeter wave radiometer MIAWARA. Further, the H2O data set of
MIAWARA underlaying this study is presented. Section focuses on
the most important results and observed features of the Q2DW above the
location of Bern. In particular, we put the focus on three subareas that
include climatological features such as averaged monthly mean Q2DW
amplitudes, temporal evolution and observed variability, and some explored
features indicating nonlinear wave–wave interactions based on an autobicoherence analysis. A conclusion is given in Sect. .
Data from ground-based water vapor radiometry
Ground-based microwave radiometry offers a technique to continuously measure
the amount of atmospheric trace gases, such as water vapor, at altitudes
between roughly 30 and 80 km under most environmental conditions.
Measurements are possible during day, night and under cloudy conditions. As
demonstrated by , microwave radiometry is widely used to
study the middle atmosphere.
Water vapor volume mixing ratio (ppm) time series as measured by the
MIAWARA microwave radiometer between October 2010 and September 2017. The
horizontal white lines indicate at which pressure levels the measurement
response drops below 50%. Clearly seen is the annual cycle in the
mesosphere with an H2O maximum in summer. The measurement response is
affected by tropospheric opacity, which is higher in summer and leads to the
observed variation in time.
The middle atmospheric water vapor radiometer MIAWARA was built in 2002 at
the University of Bern . The front end of the radiometer
receives emissions from the pressure-broadened rotational transition line of
the H2O molecule at the center frequency of 22.235 GHz. The
retrieval of water vapor from the integrated raw spectra is based on the
optimal estimation method (OEM) as presented in . We use
the ARTS/QPACK software , with which OEM is
used to perform the inversion of the atmospheric radiative transfer model
ARTS. A fast Fourier transform (FFT) spectrometer analyses the received
microwave signals. The FFT has a spectral resolution of 60 kHz and
the retrieval makes use of an overall spectrum bandwidth of 80 MHz
around the center frequency. A monthly mean zonal mean Aura MLS climatology
provides the a priori water vapor profile and additionally Aura MLS is used
to set the pressure, temperature and geopotential height in the retrieval
part. MIAWARA is part of NDACC (Network for the Detection of Atmospheric
Composition Change) and has been continuously probing middle atmospheric
H2O from the Atmospheric Remote Sensing observatory in Zimmerwald
(46.88∘ N, 7.46∘ E;
907 m a.s.l.) close to Bern since 2006. In the stratosphere the
vertical resolution of the water vapor profiles is 11 km and
degrades to about 14 km in the mesosphere . Due
to the mediocre vertical resolution of the MIAWARA radiometer quantitative
conclusions on the vertical Q2DW structure are avoided.
A recent validation against the Aura MLS v4.2 water vapor product
revealed that for most months and altitudes the relative
differences between MIAWARA and Aura MLS are below 5 %
. The MIAWARA water vapor data set used during this study
has a temporal resolution of 6 h. This is useful to study not only the
Q2DW but also possible interactions with waves of shorter periods like tides.
Compared to an even higher temporally resolved H2O data set like the
one used in with a 3 h time interval, the
6 h
interval ensures usability also during summer when the measurement sensitivity is
lower.
The MIAWARA H2O time series between October 2010 and September 2017
is shown in Fig. . The corresponding measurement response of
50 % is marked by the white horizontal lines and represents a
typical value up to which the data can be considered as reliable in regard to
the sensitivity to the a priori profile. The measurement response can be
obtained from the averaging kernel matrix of the retrieval .
Based on the variability of the measurement response, we consider different
upper measurement limits between the pressure level range of
0.02 and 0.05 hPa dependent on the actual month for the whole
H2O data set of 7 years. The approach of the numerical data analyses
is explained in the upcoming section.
Quasi 2-day wave activity
The spectral decomposition of the water vapor measurement time series uses a
wavelet-like approach as explained in . In particular, a
digital band-pass filter (non-recursive finite impulse response) with a
Hamming window with a size 3 times the central period setting
(35–65 h) is applied to the data time series. The H2O
measurements of each retrieval pressure level are handled as distinct data
time series. The application of a windowing method to individual measurement
time series ensures that the data end points fit together. Thus a smoothing
out of short-term data fluctuations is characteristic and ensures a good
mapping of oscillations with longer periods. Overall the spectral leakage can
be reduced by using numerical windowing methods . We define
the absolute amplitude of the wave as the peak-to-peak filtered signal and
the relative amplitude as relative to the time-averaged amount of water vapor
measured at the respective pressure level.
A long-term investigation of Q2DWs was done by , who
analyzed the behavior of summertime quasi 2-day waves between 2002 and 2011
in the upper mesosphere in the temperature data set of the TIMED/SABER
satellite instrument. A Q2DW lifetime evolution by different stages (growing,
maturation and attenuation phase) has been proposed. Key results indicated
that the average Q2DW amplitudes are almost twice as large in the Southern
Hemisphere than in the Northern Hemisphere. The predominant modes are
westward propagating with zonal wave numbers 3 and 4. However, no wintertime
analyses of quasi 2-day wave activity have been shown by .
Beyond observations of summertime Q2DWs, high Q2DW activity near winter
solstices has occasionally been reported from high-latitude observations
. A recent study by
analyzed COSMIC (Constellation Observing System for
Meteorology, Ionosphere, and Climate) GPS RO (Radio Occultation) measurements
at middle and high latitudes in regard to the seasonal, latitudinal and
interannual variability of the westward-propagating Q2DW in temperature
fields. They found pronounced oscillations with monthly mean amplitudes up to
about 8 K during NH fall and winter in the altitude range
20–60 km. It is of particular interest to investigate Q2DWs
observations through the whole year. Scientific reports about Q2DW activity
at midlatitudes in both winter and summer mesospheric conditions are sparse
and our study contributes with such observations.
Evolution of absolute (a) and relative (b) Q2DW amplitude in water
vapor data from the middle atmospheric water vapor radiometer MIAWARA in the
time period from 1 October 2010 to 30 September 2017. The data product is
shown in the altitude region where it can be regarded as reliable according
to Fig. .
The amplitude analysis of the Q2DW in the period range 43–53 h
of our H2O data set is shown in Fig. . The plot is only
drawn for the H2O data considered as reliable by means of
measurement response values greater than 50 %. Both the absolute
and relative Q2DW amplitudes are shown. From the overall view, the Q2DW
activity is stronger above 52 km (0.5 hPa) of altitude than
below and shows a highly developed temporal variability. Nevertheless, a
regular yearly cycle of the 2-day oscillation signatures in water vapor,
which is a recurring feature over the 7 investigated years, can be
identified. For the location of Bern there is a clear observable enhancement of Q2DW
activity during winter. During the summer months we find also
Q2DWs but not as pronounced as during winter. This might be related to the
lower measurement limit of the H2O radiometer in summer (see
Fig. ). The typical altitudes at which the W3 or W4 Q2DW summer
activity maximizes at midlatitudes (like Bern) are above
80 km , at which our instrument is not capable of
retrieving information.
Monthly averaged water vapor wave amplitude spectra with periods
between 35 and 65 h in units of ppm. Presented are the
months June (a), July (b) and August (c)
for the years from 2011 to 2017.
Same representation as in Fig. , but for the months
December (a), January (b) and February (c)
between 2010 and 2016 and between 2011 and 2017.
The appearance of the Q2DW can be described as burst-like events that rapidly
emerge. The highest amplitude in our data set reached 0.8 ppm
(14.4 %) in late January 2015 (around 25 January 2015), peaking at
around 0.1hPa, and could be related to a minor sudden stratospheric
warming (SSW) event in early January. reported about this
event, which had a large impact on transport and the chemical composition of
the lower stratosphere in the following weeks and months. Another recurring
feature of the wintertime Q2DW over Bern is not only the prevailing high
amplitudes in the upper mesosphere but also an activity across all altitude
levels down to the stratopause level (1–2 hPa). Another event
with Q2DW amplitudes as high as 0.77 ppm (14.6 %) took
place at the end of November 2016 at pressure levels above 0.1 hPa. As it
can be seen in Fig. , Q2DWs are not very persistent in time and
single burst-like events only last for a couple of days to 2 weeks at most.
From local profile observations of water vapor alone, the direction of wave
propagation, horizontal and vertical wavelengths, and zonal wave numbers
cannot be derived. Therefore, additional simultaneous measurements of at least
meridional and zonal wind would be required. An excellent possibility in
regard to deriving such information is global reanalysis models.
Monthly climatology (January to December, a–l) of wave
amplitude spectra for periods between 35 and 65 h over a period of
7 years derived from MIAWARA H2O data. The covered altitude range
in terms of pressure levels goes from 2 to 0.01hPa.
looked into the global distribution and variability of
the Q2DW in the NOGAPS–ALPHA (Navy Operational Global Atmospheric Prediction
System–Advanced Level Physics with High Altitude) reanalysis model. At middle and high latitudes two different
types of waves could be identified: (1) eastward-traveling waves with zonal
wave numbers 2 and 3 (E2, E3) during winter time and (2) westward-traveling
waves with zonal wave numbers 2, 3 and 4 (W2, W3, W4) predominantly during
the summer months. The same model system was recently used to study a
nonlinear interaction between the migrating diurnal tide and the W2–W3 waves
. The outcome of this interplay of wave forces is split
into a westward-traveling wave component W4 with a period of around
16 h and an eastward-traveling wave component E2 with a period of 2 days.
The W4 wave shows the largest amplitudes in the midlatitude winter
mesosphere and reminds one of an inertia-gravity wave in its behavior. We
reported about possible W4 wave observations with a period close to
18 h in one of our previous papers . In
Sect. we show four examples of autobicoherence spectra calculated
from 2 months of MIAWARA H2O data. With this approach we intend to
reveal nonlinear wave–wave couplings and show the complexity of middle
atmospheric water vapor dynamics.
Monthly climatological overview
The spectral decomposition of 7 years of mesospheric H2O offers a
climatological view of Q2DW activity. Overall 84 months are available to
calculate monthly mean wave spectra. Some of those are presented in
Figs. and . For simplicity, we only present 21
plots per figure focusing on three winter months (December, January,
February) and summer months (June, July, August), which gives in
total 42 monthly mean wave spectra of Q2DW amplitudes within the period range
35–65 h. By comparing Figs. and it is
important to take note of the different color bar scales. During the summer
months the monthly mean water vapor amplitude maxima do not exceed
0.2–0.25 ppm, but during the winter months these values can be
higher by 0.1 ppm (∼50%).
Overall, a high variability of Q2DW periods from one month to another and
from year to year is found for the three summer and winter months. By
comparing December, January and February a preference for stronger quasi 2-day
wave amplitudes can be attributed to January and February, except for the
year 2017. The selected summer months (June, July and August) show an
indifferent situation with no obvious preference for stronger Q2DW activity.
Relatively strong events occurred in July and August 2011, June 2013 and June 2017.
The H2O amplitudes exceeded 0.2ppm and the central
periods of maximal Q2DW mean amplitudes are found between 38 and 50 h.
In several Januaries and Februaries between 2010–2011 and 2016–2017
mean Q2DW amplitudes manifest in much higher values above 0.3ppm
(February 2012, January 2013, February 2013 and February 2016) with periods between 40 and 52 h.
The altitude region where the highest Q2DW amplitudes can be found in
all investigated months is somewhere above the stratopause level
(1hPa). Some monthly averaged H2O Q2DW spectra have an
interesting feature. At a certain altitude range two different period modes
of Q2DWs with rather low (close to 36 h) and high (higher than
60 h) periods are present. Examples in Fig. include
August 2013, July 2014 and June 2016. In Fig. such a feature is
observable (on a monthly perspective) in January 2011, December 2012 and
February 2017. Wave periods close to 36 h (harmonics of the
semi-diurnal tide) are not considered to be within the Q2DW spectrum. The
same pertains to wave periods beyond 64 h, for which an influence of
ultrafast Kelvin waves with periods in the range 3–5 days
cannot be excluded. In our data analysis a clear 3-day
wave signature is seen, for example in November 2010 and 2011.
Averaging Q2DW spectra over all seven Januaries (Fig. a), for
instance, leads to a similar signature of high amplitudes at the lower and
upper branch of Q2DW periods at 0.03–0.04hPa.
Figure clearly shows independently on a certain period band
like for Fig. , in which the Q2DW was constraint to 43–53 h,
that for a typical midlatitude observation site as Bern strong quasi
2-day oscillations preferably develop during winter months (October to March)
rather then summer months (April to September). The most sharp and distinct
Q2DW periods are found during February, October and to some extent also
December (Fig. b, j and l), meaning
that the frequency variability of the wave oscillations is much lower than,
for example, during January, March or November when a horizontal amplitude
band indicates a quite high variability (Fig. a,
c and k). The climatology for December
(Fig. l) reveals it as the only month with two peaks of quasi 2-day wave
activity at different altitude regions (0.02–0.03 and
0.1–0.2hPa) with periods near 38 h. December 2016
has an especially pronounced Q2DW signature as seen in the first subplot of the
last row in Fig. . The vertical distance between the two-wave
maxima is about 11km and the structure could be related to the
vertical propagation of planetary waves, which is what Q2DWs are. The derivation of
wave propagation characteristics would require additional observations of
wind or the study of model data that could represent the dynamics of water
vapor as we observe it with our instrument. From Fig. we get
the core message of when it is most likely to see strong Q2DW activity up to
altitudes of 70km and 0.05hpa (summer) and
75km and 0.02hPa (winter), and this could be
relevant to other measurement campaigns aiming at measuring quasi 2-day wave
oscillations in the midlatitude MLT.
Histogram of Q2DW periods observed with the MIAWARA water vapor
radiometer. Shown are the number of cases versus period bins with
3h width in which Q2DW events could be identified. The selected
criterion of a Q2DW event was a localized maximum in a monthly averaged
H2O wave spectrum exceeding 0.15ppm. The bar plots are
stacked, which divides them into winter (blue) and summer (red) groups.
A view from a different perspective can be obtained with the histogram plot
provided in Fig. . There the periods of localized primary and
secondary Q2DW events (observed in a monthly mean wave spectrum, as in
Figs. and ) are binned and color separated by
season. Summer is shown in red and winter in blue bars. A primary
Q2DW is characterized in our definition as the wave with the strongest
amplitude in the altitude versus period wave spectrum. In addition, one or
more secondary Q2DWs can be present with different periods and/or occurrence
at other pressure levels. Both primary and secondary Q2DWs have to exceed
0.15ppm to enter the histogram statistics. The pressure range
at which the amplitude peaks are valid is set between 0.02 and
2 hPa. The classical 2-day wave periods (50–52 h)
occur in 18 cases out of 110 and show a predominance during winter. The
largest amount of Q2DWs have periods in the range 38–40 h (15
in winter, 10 in summer). In total about 20 % of all 110
identified Q2DWs fall into the first bin. Regarding the normal Rossby wave
mode W3 with central periods between 50 and 52 h
we find a corresponding local maximum of events. The
remaining wave periods are ambiguously spread between summer and winter
months.
Matrix plots of the temporal evolution of monthly mean Q2DW
amplitudes in units of ppm with a dependency on the period
(38–64h). The top plot shows the pressure-layer-averaged wave
amplitudes between 0.05 and 0.2hPa, while the bottom plot shows
those
between 0.2 and 1hPa.
analyzed the Q2DW behavior (for January and July in 16 years) in the
zonal and meridional wind obtained from the medium-frequency radar at Kauai
(Hawaii; 22∘ N, 160∘ W). For
January they find most Q2DW periods at 48 h in the case of the
meridional wind or 48 and 51 h in the case of the zonal
wind. Below 42 and above 54 h no periods were detected that could
be attributed to a Q2DW. A slight displacement towards shorter periods in
July is recognizable in their histogram data. Most wind oscillations have
either 46 h (meridional) or 43 h (zonal) periods.
Afterwards (Sect. ) we restrict the analyses to pressure-layer-averaged data products and focus more specifically on the monthly mean
temporal development of Q2DWs for the whole studied time period of 84 months
(7 years) in the resolved period spectrum between 38 and 64 h.
Monthly breakdown of Q2DW (43–53h) amplitude
development over the 7 investigated years. The H2O pressure-layer-averaged amplitudes are
0.05–0.2hPa (a) and
0.2–1hPa (b). The years range depending on the month
from either 2010 to 2016 (October to December) or from 2011 to 2017 (January
to September).
Temporal evolution and variability
From the histogram plot (Fig. ) we got an overview of the
distribution of Q2DW periods in which the H2O amplitudes peaked.
However,
it is not less interesting to see how the Q2DW periods evolve in time. In
order to emphasize the temporal development we came up with an amplitude
matrix plot (Fig. ) presenting Q2DW period versus time on
monthly steps. For both sub-figures different pressure layers are defined
in which the monthly mean Q2DW amplitudes are further averaged.
Figure a represents the pressure layer from
0.05–0.2hPa, whereas Fig. b covers the data from
0.2–1hPa. The layer depths in terms of spatial dimensions are
9.6km and 11.2km. In both pressure layers a
yearly cycle of enhanced quasi 2-day oscillations of various periods is
apparent. The two plots complement the analysis provided with
Fig. in which only the mean Q2DW (43–53h) evolution
is shown. Now we focus on the hourly resolved monthly mean amplitude of the
respective Q2DW frequency. Higher amplitudes are found towards shorter
periods in the summer months in the upper pressure layer, which is consistent
with Fig. . In general the upper pressure layer is the one in
which
the Q2DW oscillations are more pronounced. Occasionally, the lower pressure
layer shows monthly mean H2O amplitudes slightly higher than
0.2ppm (December 2012, February 2015), but values as high as
0.3ppm never arise as they do at the higher investigated
pressure layer. The wintertime maxima of mean amplitudes has a quite high-frequency variability.
The strongest events exhibit periods above
50 h
(January and February 2015) in the upper mesosphere (Fig. a).
The two blue columns (May 2011 and May 2017) in each plot are the consequence of
larger measurement gaps in the MIAWARA instrument.
The last graphical display (Fig. ) of this section highlights
the temporal development of monthly averaged Q2DW amplitudes
(43–53h) separately for each month of the year. This results in
seven data points per month according to the length of the data set, which is 7 years.
The months are distinguished by the given color code. A pressure layer
averaging is applied in agreement with the data presented in
Fig. . January and February 2015 reveal the highest amplitudes
of the 43–53h quasi 2-day wave within our water vapor data set.
The amplitudes reach values around 0.27ppm in the upper
mesospheric pressure layer. These high monthly means could be related to SSW
dynamics and an enhanced gravity wave activity. Another possibility could be
a signature of the maximum phase of the 11-year solar cycle 24. For example,
showed that the January Q2DW in zonal and meridional wind has
an in-phase behavior related to the solar irradiance with a leading solar maximum of
about 1 year. In the region above the stratopause (lower mesosphere) only
February 2015 shows a significant peak related to the surrounding years
(Fig. b). We find no clear trend in the temporal evolution of
Q2DW activity within the two pressure layers. As was the outcome before,
the winter months tend to have the highest monthly mean quasi 2-day wave
H2O amplitudes and all months indicate higher Q2DW activity in the
upper investigated pressure layer between 0.05 and 0.2hPa.
Wavelet-based autobicoherence spectrum from pressure-layer-averaged
MIAWARA water vapor time series with individual lengths of 2 months. The
chosen pressure layers are 0.05–0.2 and
0.2–1hPa. Thick contours enclose regions of 80%
point-wise confidence after controlling the FDR (false detection rate). The
diagonal line separates the two-dimensional spectrum into two symmetric
regions. Interesting features on the plots are labeled with capital letters
(A–H).
Autobicoherence analysis
With a bicoherence analysis a wave coupling between two or three waves can be
determined. The degree of local quadratic nonlinearity gets high when the
phase between the waves at periods s1 and s2 (two-wave example) is
nearly constant over a significant number of realizations. A two-wave
bicoherence analysis is used to estimate the contribution of second-order
nonlinearities to the power of the two frequencies (bifrequencies) and
periods.
In a two-dimensional bicoherence graph as presented in Fig. one
usually finds two types of structures: localized point-like or elongated
line-like areas stretching over a bunch of frequencies. The first ones indicate
sharply defined and locked frequencies, while the latter are likely due to a
single frequency mode interacting with a broader range of different
frequencies . The peaks in general represent the phase
coupling between different wave periods. A significant peak located near the
diagonal slice of the spectrum indicates a phase coupling of the primary
frequency mode with its harmonic. Monte Carlo simulations are used to find
regions of normalized wavelet power in the autobicoherence spectrum that are
significant with respect to a selected confidence interval. In our case a
confidence interval of 80% is applied with a total number of 100
iterations within the Monte Carlo simulations. An in-depth view on the
methodical and computational details of the autobicoherence analysis is given
in and .
Figure presents four autobicoherence spectra from 2 months of
pressure-layer-averaged MIAWARA water vapor time series.
Figure a and b focus on January and February 2016, while Fig. c and d show results for
November and December 2016. In the case of January and February 2016 significant
phase coupling can be found between a quasi 18h
(16–18h) wave and the Q2DW with a period slightly below
48h (Fig. a, label B) in the lower pressure
layer and a coupling of 18h oscillations to diurnal periodicities
in the upper pressure layer (Fig. b, label D). Between
0.2 and 1hPa the diurnal tide is to a high degree (power: 0.8)
phase coupled to the semi-diurnal tide (12h period), as the red
area at coordinate point (24, 24) shows (Fig. a, label C).
In the upper mesosphere this tidal wave behavior is lost, but
here a tidal period s1 manifests in a line-like area across s2
periods (not significant within the 80% confidence interval) in
the Q2DW period range below 48 to above 64 h (Fig. b,
label E). In Fig. a the highest wavelet power (label A)
is found at coordinates (48, >64) and could be related to an
interference of the Q2DW with the quasi 18h wave, which itself is
likely to originate from a nonlinear wave–wave coupling between the diurnal
tide and the westward-traveling quasi 2-day wave (W2) .
A recent study by revealed dominant oscillations in
mesospheric water vapor profiles with a period close to 18h in
Northern Hemispheric winter months. However, such oscillations within
a sub-diurnal period spectrum in the MLT can also be related to low-frequency
inertia-gravity waves, as shown by with measurements from a
sodium lidar system over Fort Collins, Colorado (41∘ N,
105∘ W).
The MIAWARA autobicoherence spectra for November and December show for both
altitude regions similar quadratic phase coupling signatures. High common
wavelet power is found between 18 and 32 h (Fig. c and
d, labels F and I). The red spot (label H) at
coordinate (32, 32) also indicates a coupling between the harmonic of the
32h oscillations and the primary frequency. At s1 Q2DW
periods have a significant phase coupling to even longer periods (up to
80h) as can be seen in Fig. c near label G.
Even though we only made use of a single mesospheric H2O
data set, atmospheric wave patterns and interactions can be studied.
Evidence was found that wave–wave interactions between Q2DWs, diurnal tides
and quasi 18h waves occur in the winter midlatitude mesosphere
shown by high nonlinear phase couplings in the autobicoherence spectra of
MIAWARA H2O data.
Conclusions
The study of quasi 2-day planetary waves in
the MLT is of importance to improve the understanding of the Earth's
atmosphere. The dissipation of atmospheric waves in the MLT induces rapid
changes to the background dynamics, which in turn affects the composition of
the atmosphere through turbulent mixing or the general alternating of the
circulation. This Q2DW-driven variability can be seen in long-living trace
gases like water vapor.
The MIAWARA radiometer has provided reliable, long-term observations of middle
atmospheric water vapor since 2007. Here we made use of data since October
2010 right after the instrument was essentially improved by a hardware update
resulting in shorter integration times of the 22GHz H2O
spectra and thus a higher temporal resolution. A temporal data resolution of
6 h was the starting point for the long-term analyses of Q2DW activity
above the stratopause up to an altitude of 75km
(0.02hPa) during winter and 70km (0.05hPa)
during summer months when the increase in atmospheric opacity reduces the
upper measurement limit.
Our key results regarding the long-term Q2DW behavior above the midlatitude observation site at Bern are briefly
summarized.
Q2DW (43–53h) activity as observed by MIAWARA H2O
profiles is strongest in the upper mesosphere and during winter months; it
emerges in burst-like events. We note the altitude limitation of the MIAWARA
instrument during summer, which is limited to about 70km.
The highest individual Q2DW amplitudes reach 0.8ppm and are likely related to SSW activity.
Monthly mean Q2DW amplitude spectra show a broad variability of periods
between 38 and 64h.
A monthly climatological overview for 7 years indicates that in January,
February and November the amplitude peaks of Q2DWs are highest (up to 0.3ppm) in the observed altitude region.
A significant fraction (about 20%) of observed Q2DW events in
summer and winter manifest periods between 38 and 40h.
The evolution of different Q2DW periods (monthly average) over 84 months
revealed a yearly signature of enhanced wave activity during winter months.
Nonlinear quadratic phase coupling is detected between Q2DW, diurnal and quasi 18h H2O oscillations.
In this study we refrained from a comparison between our results and model
data like ECMWF because there is a well-known large dry model bias within the
stratosphere and mesosphere. For instance, during the LAUTLOS campaign in the
arctic a relative bias between the ECMWF analysis and FLASH-B Lyman–Alpha
hygrometer measurements of up to 20% was detected in the lower
stratosphere . However, a future study could use other
model parameters like temperature to analyze Q2DW behavior in regard to our
H2O-based results or other results from ground-based observation
methods like radar observations .
We showed that measurements from ground-based microwave radiometers can be
used to assess the quasi 2-day wave activity at local observation sites. Even
if data sets from satellite measurement platforms like Aura MLS (operational
since July 2004) can provide a global perspective of Q2DWs
, observations from the ground can be used for validation
purposes and more importantly for long-term monitoring of wave activity. In
the case of Q2DWs they can capture the interaction with shorter periodical waves
like tides or semi-diurnal oscillations, which cannot be resolved by Aura MLS
because it is a sun-synchronized satellite.