CHAPTER 5 Estelar -...
Transcript of CHAPTER 5 Estelar -...
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CHAPTER 5
Characteristics of tidal variability in the mesosphere:
Signature of Terdiurnal tide
5.1 Introduction It is well known that gravity waves and tides are very important carrier of energy
and momentum from the lower atmosphere to the upper atmosphere and they modify the
dynamical condition of the concerned ambient atmosphere. Because of that they have got
significant importance in active research and become a interesting topic of frontline
research in recent few decades (Hines, 1960; Chapman, 1970; Reisin and Scheer et al,
1996; Fritts and Alexander, 2003; Taori et al, 2007). Tides are generated due to
absorption of incoming solar radiation by the atmospheric molecules, e.g. ozone, water
vapor, carbon dioxide etc. and also geared by the latent heat forcing in the atmosphere,
mostly in lower atmosphere and change the structure and dynamics of mesopause region.
Nature of migrating tide is well studied theoretically which shows the periodicities are
sub-harmonics of solar day, i.e. 24, 12, 8, 6, 4 hour etc. and they propagate westward due
to the relative motion of sun relative to earth (Chapman and Lindgen, 1970). Modeling
studies by several investigators (Bernard, 1981; Forbes and Vial, 1989; Hagan et al,
1995) have enriched our knowledge of tidal structure and their variability to some extent.
Interaction between tides and gravity waves is also strong enough to cause short term
tidal variability (Walterscheid, 1981; Fritts and Vincent, 1987; Lu and Fritts, 1993;
Nakamura et al, 1997; Beard et al, 1999).
Study of tides using airglow emissions was pioneered by Fukuyama, 1976;
Petitdidier and Teitelbaum, 1977. Airglow emission (OH layer ~ 87 km, O2 layer ~ 94
Km, Na layer ~ 89 km etc.) is an effective tool for performing upper atmospheric
variability study. As airglow is a passive remote sensing tool for acquiring information of
mesospheric processes, it can be used to obtain tidal characteristics of periodicity less
than 12 hour (Taylor et al, 1999) very efficiently. Thus routine measurement of
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mesospheric airglow emissions can give more insight in tracing out tidal variability and
their effect on global scale processes and associated change.
There are several reports of observation of diurnal and semi-diurnal tides in
mesospheric regions at various geographical locations with various ground based
instruments, e.g. LIDAR (Liu et al, 2007; Taylor et al, 1999), RADAR (Zhang et al,
2003), Airglow (Won et al, 2007) as well as space based instruments (Smith et al, 2000)
but still detection of terdiurnal tide lacks of measurement. Observation of terdiurnal tide
was first detected by meteor echo (Revah, 1969). After that very few investigators have
reported its occurrence in low latitude (Taori et al, 2005), mid-latitude (Wiens et al,
1995; She et al, 2002) and in high latitude (Younger et al, 2002; Wu et al, 2005b).Using
Radar, structure, characteristics of terdiurnal tide was extensively studied by Thayaparan
et al, 1997 throughout the year and the investigators got significant dominance around
spring and winter compared to the other times of the season. Taylor et al, 1999 also
obtained the large amplitude of terdiurnal tide at the same time mentioned before from
mid latitude stations (40 - 42oN) with very less or no evidence of diurnal and semi-
diurnal tidal components. Pendleton et al, 2000 have done comparative study of
terdiurnal tide in two mid latitude sites in OH nightglow emission.
The generation of terdiurnal wave is not very much understood still today.
Complex interaction mechanisms are responsible for the creation of the terdiurnal tide.
Probable reasons may be thermal excitation by solar heating (Chapman and Lindzen,
1970) or due to non-linear interaction between diurnal and semi-diurnal tides (Smith,
2000) or due to combination of two factors mentioned before (Glass and Fellous, 1975;
Teitelbaum et al, 1989) or due to interaction of tides with gravity waves (Miyahara and
Forbes, 1991).
5.2 Observation In the present study we have chosen late-winter dataset from a low latitude station
Maui, Hawaii (20.8o N, 156.2o W) to investigate the characteristics of terdiurnal tide
component in mesospheric OH and O2 airglow emission temperatures. The systematic
observation was actually a part of Maui Mesosphere and Lower Thermosphere (MALT)
program which was started since November, 2001. All the clear night data has been
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incorporated for analysis of both the OH and O2 emissions. We have used nocturnal data
of February-2003 which includes 20 clear nights with observation period greater than five
hours.
5.3 Data analysis and Results The nocturnal temperature data for the concerned month has been analyzed for
investigating long period terdiurnal tide signature. Figure 5.1 shows one typical nocturnal
temperature profile with respect to UT in hour for O2 and OH on UT Day - 40. It is clear
from the plot that the data is dominated by long period wave with short period
oscillations are superposed on it. A simple sinusoidal model has been used to fit best the
observed data.
)2(T
tSinBAYfit×
×+=π 5.1
where A is the DC level of the temperature data, which is nearly equal to the mean
temperature value, B is the observed wave amplitude, T is the period of the oscillation
and t is the time of variation of the data. Best fit result shows T ~ 7.04 ± 0.04 and 8.65 ±
0.04 hour, B ~ 5.14 ± 0.28 and 7.28 ± 0.25 K and A ~ 200 ± 0.2 and 198.5 ± 0.2 K for
OH and O2 respectively.
Figure 5.2 shows the ensemble of the temperature profiles of the whole month (20
Days) after taking half hourly average to smooth the profiles from short period
oscillations for OH and O2. O2 data exhibits larger range of variability compared to OH
data. On the other hand OH data exposes more consistent nocturnal variation for all the
nights shown here in comparison with O2 where variability is somewhat zigzag in
fashion. One should note the dominant long period oscillations are significant in both the
profiles (OH and O2).
To elucidate monthly nocturnal wave feature we have carried out average of all
the night profiles to a single temperature profile for both the OH and O2 temperature data
which is shown in Figure 5.3. Again best fit model as described before has been applied
to both the profiles and the best fit wave parameters comes out to be T ~ 7.52 ± 0.8 and
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Figure 5.1 Nocturnal Temperature patterns of O2 and OH have been shown with a
best sinusoidal fit to the data on the UT Day-40, 2003.
7.00 ± 0.11 hour, B ~ 3.13 ± 0.33 and 3.82 ± 0.55 K, A ~ 202.46 ± 0.24 and 198.41 ±
0.41 K for OH and O2 respectively. The observed phase difference between O2 and OH
temperature is ~ 1 hour which signifies the downward phase progression of upward
propagating wave.
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Figure 5.2 Ensemble of the nocturnal temperature profiles with respect to UT hour
for all the days of the month after taking half hourly average to smooth the short period
features for both the O2 and OH. More consistent pattern is observed in OH in
comparison with O2.
For better quantification of the terdiurnal tidal component on the nocturnal
variation pattern we have performed continuous wavelet analysis on the OH and O2
monthly averaged temperature data. Wavelet analysis reveals the conspicuous variability
pattern of wave amplitude with respect to time span of observation and the periodicities
(Torrence and Compo, 1998). We have used complex Morelet as a mother wavelet for
analysis which is actually a plane wave modulated by a Gaussian envelope to find out the
time-period spectrum of a non-stationary signal. The wavelet coefficient deduced by
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Figure 5.3 Same plot as defined in Figure 5.1, but here monthly average temperature
profiles have been plotted.
integrating the product of time series data and mother wavelet over all the time span is
actually signifying the concerned wave amplitude. The upper and middle panels show the
O2 and OH spectra respectively. The bottom panel represents the cross wavelet spectrum
which indeed describes the common wave packets present at the time of observation in
both the altitudes (87 and 94 km).
Both the spectra (O2 and OH) reveal a number of wave packets existing in the
nocturnal temperature. O2 wavelet spectrum indicates a dominant 6 - 8 hour wave for
most of the time of observation, whereas OH spectrum exhibits almost same wave (6 - 9
hour) characteristics and they are existing around ~ 5 - 7, 8 - 10, 11 - 14 UT hour and also
in the late night hours. Several common wave-packets are also observed in the spectra
with less magnitude. Evident from the cross wavelet spectrum, is the dominant 7 hour
periodicity wave around 5.5, 9, 12.5 UT hour.
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Figure 5.4 Top and middle panels are showing wavelet spectra of O2 and OH
average temperature profiles which is shown in Figure 5.3. Both the spectra show ~ 6 - 9
hour periodicity waves dominating over the nights. Bottom panel describes the cross
wavelet spectrum deduced from the OH and O2 wavelet spectra to figure out the common
wave features between two.
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5.4 Discussion In the present study we have investigated the terdiurnal tidal variability in the
nocturnal temperature data derived from OH and O2 airglow emission intensities in the
Figure 5.5 Same plot as described in Figure 5.2 but here temperature data is filtered
through a band pass digital filter centered at 8 hour with lower cut-off ~7 hour and
higher cut-off ~ 9 hour.
month of February, 2003 in Maui, Hawaii (20.8oN, 156.2oW ) using MTM. Results show
large day to day variability with a clear signature of terdiurnal tide for the whole month
of observation.
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To classify the terdiurnal component separately from other waves and tidal
features we have extracted the eight hour wave component with the help of a band pass
digital filter centered around 8 hour and lower cut off ~ 7 hour and higher cut off ~ 9
hour, after applying to the nocturnal temperature data profiles of the month for O2 and
OH. The ensemble of the filtered nocturnal profiles has been plotted in Figure 5.5.
Figure 5.6 This plot is same with Figure 5.3 except the fact that the profile is deduced
from the 8 hour filtered temperature profiles as shown in Figure 5.5. Amplitudes are
somewhat lesser compared to Figure 5.3 because of the omission of the other wave
components.
It is interesting to note the smoother variation of all the profiles for both OH and O2. It is
quite acceptable that the range of variability for both OH and O2 has been reduced to
some extent because of the cancellation of contribution of the wave periodicities less than
7 hour and greater than 9 hour to the concerned 8 hour wave component. Here we have
accepted only the waves with periodicities ranges between 7 and 9 hour.
Figure 5.6 depicts the monthly mean temperature profile of the filtered O2 and OH
data and also the best fit analysis is done which is same as the analysis done in Figure
5.3. The best fit model parameters are found to be T ~ 7.68 ± 0.06 and 7.01 ± 0.1 hour, B
~ 2.5 ± 0.18 and 2.72 ± 0.31 K, A ~ 202.28 ± 0.13 and 198.3 ± 0.23 K for OH and O2,
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Figure 5.7 Wave characteristics of terdiurnal tide have been shown here. Top panel
is showing amplitude variability observed in all the days. Second panel is showing phase
variation of the terdiurnal wave for OH and O2 Third panel is showing vertical
wavelengths deduced from the phase differences between OH and O2. Wave growth
factor is plotted in the bottom panel.
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respectively. It is noteworthy that almost same periods are obtained in both the unfiltered
and filtered data (OH and O2 temperature) but amplitudes are somewhat less in filtered
data with compared to the unfiltered one because of absence of the contribution from the
other wave components and it also corroborate the dominance of the 7 - 8 hour wave
features in the nocturnal variability throughout the month.
To characterize the terdiurnal tide we have calculated tidal amplitude, phase,
vertical wavelength and growth factor for all the days of the observing month (Feb, 2003)
with 8 hour filtered temperature data described before, which is shown in Figure 5.7. The
upper panel shows the 8 hour wave amplitude with respect to the corresponding days of
the month. The plot shows large variability in the amplitude for both OH and O2. O2
amplitudes show relatively larger value compared to the OH ones with a maximum ~ 19
K, whereas OH values show a maximum around ~ 10 K. Taori et al, 2005 found the
variability of tidal amplitude in the range ~ 5 - 17 K with average ~ 5.5 K which agrees
with our observed variability (~ 2 - 19 K). Using airglow imager and Na Lidar
observation in a mid latitude station during spring and fall equinox, Taylor et al, 1999
obtained variability of 8 hour component ~ 1.5 - 15 K and they concluded the dominance
of 8 hour wave is due to interplay of diurnal and semi-diurnal tidal components.
Pendleton et al, 2000 observed significant amplitude up to ~ 15 K for OH layer
observation in a mid latitude station during fall equinox, which is close to our observed
ones. Using Lidar temperature measurement She et al, 2002 showed that the variability
lies between ~ 1- 4 K which is quite low compared to other observations.
Phases of 8 hour tide have been shown in the second panel (from the top) for OH
and O2 with respect to UT days of the month. It is important to note that all the nights
show O2 phase leads ahead the OH one which validates the downward phase progression
with a mean phase shift ~ 1 hour. Taori et al, 2005 have found prominent phase shift ~ 1
hour among the two emission heights which matches exactly with our obtained value.
Vertical wavelengths (VW) have been derived with the values from phase
difference between OH (~ 87 Km) and O2 (~ 94 Km) and plotted in third panel (from the
top). Values of VW show significant day to day variability (~ 14 - 112 Km) with an
average of ~ 46 Km. Vertical wavelength obtained by several investigators match more
or less with our observed values. Thayaparan et al, 1997 observed VW ~ 30 ± 7 Km in
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February, 1995 from a mid-latitude station. Using Meteor Radar measurement Younger et
al, 2002 found VW ~ 31 ± 5.2 Km (in zonal wind) and 31.1 ± 5.8 Km (in meridional
wind) during winter season. Wu et al, 2005b have found VW for terdiurnal wave in a
high latitude station ~ 35 km. Taori et al, 2005 obtained a large variability of VW (~ 26 -
110 km) with a mean value ~ 63 km, which matches very well with our obtained VWs.
The growth factor (GF), a measure of wave amplitude growth/attenuation as the
altitude goes high, has been shown in bottom panel of Figure 5.7. From the plot it is clear
that most of the nights (GF > 1) show significant growth of 8 hour wave as it reaches
from 87 Km to 94 Km with a maximum value ~ 4.6 on UT Day-43 with a monthly
average ~ 1.6. It should be noted that some nights are dominated by significant wave
dissipation between these two layers as indicated by the plot. Taori et al, 2005 observed
GF variability ~ 0.6 - 1.9 in the same location in July, 2002 which is lesser compared to
our observed ones.
Very few investigators have studied the characteristics of terdiurnal tide so far.
Younger et al, 2002 have shown seasonal characteristics of terdiurnal tide using Meteor
Radar observation in polar mesosphere and they observed increase of tidal amplitude
with altitude. Using satellite based wind observation Smith et al, 2000 found terdiurnal
wave amplitude maximizes during spring and fall equinox in mid-latitude. Also using
radar observation Thayaparan et al, 1997 found significant contribution of 8 hour tide
during spring and winter season and less dominance during summer and fall time. Zhao
et al, 2005 found different seasonal pattern of 8 hour tide variability unlike other
investigators.
5.5 Summary Our present study in a low latitude site using best fit analysis of monthly averaged
data reveals the presence of terdiurnal tide in nocturnal temperature pattern of O2 and OH
airglow emissions with strong day to day variability in the mesosphere region during late
winter time. Wavelet analysis has been carried out on the monthly averaged temperature
data which shows quasi 8 hour wave dominance in both the layer in nocturnal
temperature pattern throughout the night. Cross wavelet analysis shows a strong quasi 8
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hour (~ 7 hour) wave, which exists in two altitudes (at 87 km and at 94 km)
simultaneously.
We have further analyzed the data by applying a band pass digital filter centered
at 8 hour and width ~ 2 hour. Monthly mean temperature profile shows ~ 1 hour phase
difference among OH and O2 layer with leading O2 phase over OH which indicates
downward phase progression. For characterization of the 8 hour wave we have derived
amplitude, phase, vertical wavelength and wave growth factor. Our observed terdiurnal
amplitude shows significant high value during the period of observation and it is well
compared with observations of other investigators performed previously. Observed
phases (in UT hour) of filtered O2 and OH temperatures show significant spread over the
nocturnal time span of the month. Vertical wavelength reveals larger range of variability
with compared to the most of the investigators mentioned before with an average value of
~ 46 ± 5 km. Growth factors as we obtained, indicate most of the nights are conducive for
large wave growth (mean ~ 1.6) for quasi 8 hour wave because of supportive dynamical
condition.
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