Characteristics of high frequency Characteristics of high frequency Characteristics of high frequency Characteristics of high frequency gravity waves in the upper mesosphere gravity waves in the upper mesosphere b di OH i ht l l b di OH i ht l l observed in OH nightglow over low observed in OH nightglow over low latitude Indian sector during 2007 latitude Indian sector during 2007 Viswanathan Lakshmi Narayanan & Subramanian Gurubaran Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism. Contact: [email protected], [email protected]
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Characteristics of high frequencyCharacteristics of high frequencyCharacteristics of high frequency Characteristics of high frequency gravity waves in the upper mesosphere gravity waves in the upper mesosphere
b d i OH i ht l lb d i OH i ht l lobserved in OH nightglow over low observed in OH nightglow over low latitude Indian sector during 2007latitude Indian sector during 2007
AbstractSmall scale high frequency gravity waves are believed to play a vitalrole in the upper mesospheric region by means of wave breaking andtheir interactions with other waves and background mean flow. They areknown to propagate large distances from their source regions by meansof ducting which makes identification of their source distribution achallenging task. Further, their global distribution is not yet well known.In this work we have studied the characteristics of such high frequencywaves observed in OH Meinel band emissions over Tirunelveli (8.7oN,77.8oE) during the year 2007. The study reveals predominance ofmeridionally propagating waves, possibly indicating the wind filteringy p p g g , p y g geffects in the lower atmosphere. During summer period, wavespropagating towards south and south-west were observed much morefrequently. The apparent phase velocities of the waves are higher duringq y pp p g gequinox periods followed by summer and winter solstices respectively.There was no significant variation in the wavelength range of theobserved waves. Detailed discussion on the characteristics of theobserved waves. Detailed discussion on the characteristics of theobserved waves and possible source distributions around this site duringdifferent seasons are made in this study.
Imaging observations from Tirunelveli
Schematic of the instrument
• From January 2007, nightglow imaging observations are carriedout from Low latitude Indian station Tirunelveli (8.7oN; 77.8oE;-0.17o Geomagnetic) during cloudless clear sky nights whenmoon is behind the horizon.
• The all-sky imager is procured from Keo Scientific Limited andis designed for F4 optics with a back illuminated 16 bit CCD
ith 512 512 i lcamera with 512 x 512 pixels.• The CCD is not deep depleted.• Currently the instrument is operating with 6 interference filtersCurrently, the instrument is operating with 6 interference filters.• In this study, the observations of gravity waves made with a
broad-band filter covering OH Meinel band emissions in the NIRregion of 705.3 – 928.2 nm were used. The filter has a notch at ~865 nm to suppress contamination from O2 band emissions.
• At this site a collocated MF radar is measuring mesospheric• At this site, a collocated MF radar is measuring mesosphericwinds at OH emission heights.
Data Analysis• The imaging observations were made with collecting a set ofThe imaging observations were made with collecting a set of
successive images from the same filter and changing the filters insequence.Th OH i b i d i hi h i• The OH images obtained within each sequence are timedifferenced (TD) and projected in to the equi-distance grid asdiscussed in Garcia et al., (1999).( )
• From the projected images the wave parameters namelywavelength, phase velocity and propagation azimuth aree tracted hene er more than one crest or tro gh like str ct reextracted whenever more than one crest or trough like structureshows consistent motion in at least two TD images.
• The average winds between 84 – 90 km altitudes at every 20g yminutes interval were calculated to overcome data gaps in radar.Afterwards the observation period and propagation azimuth ofthe waves are noted and mean winds are calculated from this 20the waves are noted and mean winds are calculated from this 20minute averaged data.
• Imagers are capable of detecting the quasi-monochromaticImagers are capable of detecting the quasi monochromaticgravity waves, nonlinearly evolving wave systems likemesospheric bores and instability features known as ripples thataffect the mesospheric nightglow layersaffect the mesospheric nightglow layers.
• The detection of bore like events are rare and hence theobservations usually consist of quasi-monochromatic highy q gfrequency gravity waves and instability features associated withthem (ripples).
• In the c rrent st d the q asi monochromatic a es and ripple• In the current study, the quasi-monochromatic waves and ripplefeatures are not separated explicitly.
• However, reasonable estimation shows that the ripple features, ppcontribute less than about 20% of the observed events.
• Further, it appears as if most of the observed ripple features arelt f ti i t biliti i i th hresult of convective instabilities occurring in the mesosphere.
Observation of quasi-monochromatic waves on May 20, 2007on May 20, 2007
Observation of ripple features on Feb 23, 2007
Observation of a mesospheric bore on Mar 22, 20072007
• The site is at mean sea level and the sky is often cloudyThe site is at mean sea level and the sky is often cloudyrestricting the no. of. nightglow observations to 34 nights in themonths of Jan, Feb, Mar, May, Aug and Oct, 2007.
Season Months No. of. Nights
No. of. Useful
No. of.Events
No. of. Waves/h
hours ourWinter Jan &
Feb14 63 115 1.83
FebSummer May &
Aug9 31 49 1.58
AugEquinox Mar &
Oct11 32 79 2.47
Total All days 34 126 243 1.93
Distribution of wavelengthW l h W l h
Total Winter
50
60
70
80
Wavelength
ents
20
25
30
Wavelength
ents
10
20
30
40
50
N
o. o
f. ev
e
5
10
15
20
N
o. o
f. ev
e
0 10 20 30 40 500
10
Wavelength (km)0 10 20 30 40 50
0
Wavelength (km)
Summer Equinoxes
15
20
nts
Wavelength
20
25
nts
Wavelengthq
5
10 N
o. o
f. ev
en
5
10
15
N
o. o
f. ev
e n
0 10 20 30 40 500
Wavelength (km)0 10 20 30 40 50
0
Wavelength (km)
Distribution of apparent phase velocityTotal Winter
Time period (min) p ( )Time period (min)314 s 314 s
Dispersion relation used to infer vertical wavelengthwavelength
2
Neglecting coriolis effect and compressional effects,
222
22
41
)(1
)()(k
Hcuu
Hcuu
cuNm zzz −−
−+
−−
−=
Further assuming curvature and wind shear are not persistentaround 87 km region,
22
2
22
41
)( Hk
cuNm −−⎟⎟
⎠
⎞⎜⎜⎝
⎛−
=
With, N = 0.02 rad/s, H = 6km
140
52%
Wave reflection and background wind
nst
80
100
120
43% evanescentEven
ts
Evanescent Propagating
52%
30
40
50 44%46%
37%
46%
paga
tion
agai
nnd
win
d (%
)
0
20
40
60 65%
No.
of.
55%
0
10
20
Perc
enta
ge p
roba
ckgr
oun
winter summer equinox total0
140
160
Total no. of. evanescent waves No. of. evanescent waves with opposite mean windTotal no of waves with opposite background wind
100
89%100%
s with
%
)
winter summer equinox totalP80
100
120
Even
ts
Total no. of. waves with opposite background wind
60
80 74%
89% 85%
scen
t wav
esou
nd w
ind
(40
60
80 N
o. o
f.
20
40
ge o
f eva
nes
ite b
ackg
ro
winter summer equinox total0
20
winter summer equinox total0
20
Perc
enta
gop
pos
Mean parameters along different directions
40
60
800
45315
Mean observed phase velocity
ity (m
/s)
30405060
0
45315
Propagation Azimuth
23456
0
45315
Mean apparent time period
0
20
902700
20
40
aren
t pha
se v
eloc
01020
902700102030
No.
of.
even
ts
012
9027001234
Tim
e (m
in)
Tim
e (m
in)
Mean wavelength along different directions
135
180
22560
80
App
a
Mean intrinsic phase velocity
135
180
225405060
135
180
225456
Mean intrinsic time period
5
10
15
20
250
45315
Mean wavelength along different directions
(km
)
20
40
60
800
45315
ocity
(m/s
)
Mean intrinsic phase velocity
2468
1012
0
45315
in)
Mean intrinsic time period
0
5
90
135225
2700
5
10
15
20
Wav
elen
gth
0 90
135225
2700
20
40
60trins
ic p
hase
vel
o
02
90
135225
27002468
Tim
e (m
i135
180
22520
25
135
180
22560
80
Int 135
180
2251012
Average parameters of the waves3
26
20
25
30
35
vele
ngth
(km
)
18
20
22
24
avel
engt
h (k
m)
5
10
15
V
ertic
al w
a v
8
10
12
14
16
Hor
izon
tal w
100
y (m
/s)
100
120
y (m
/s)
30
50
m/s
)winter summer equinox totalwinter summer equinox total
8
40
60
80
en
t pha
se v
eloc
ity
20
40
60
80
100
si
c ph
ase
velo
city
30
-10
10
30
ackg
roun
d w
ind
(m
winter summer equinox total20
40
App
are
winter summer equinox total
0
20
Intri
ns
winter summer equinox total
-50
-30Ba
Typical mean wind profile over Gadanki (13.5oN)
Courtesy: Dr. M. Venkat Ratnam, NARL, India.
Seasonal mean temperatures and static stability profiles for 2007 over low latitudes (0 – 30o)
Probable orographic and convective sources
Summary of observationsMeridional propagation is predominant indicatingMeridional propagation is predominant indicatingprobable middle atmospheric filtering effectsTh h t t l it t tThese shortest scale gravity waves appear to getfiltered out not only by critical level interactionb t l d t fl ti lti f d lbut also due to reflection resulting from dopplershifting into frequencies above buoyancyf b f it l di t dfrequency by means of oppositely directed meanwinds
h l i iSummer shows largest asymmetry in propagationPhase velocities are relatively higher duringequinoxes
Summary of observationsVertical wavelength of propagating waves is onVertical wavelength of propagating waves is onthe average 18 kmSh f h i l l hShortest range of horizontal wavelengthsindicating probable convective sourceWave ducting might have played a major role ifwaves are convectively generated.Possibility of in-situ high frequency wavegeneration in the mesosphere by means ofg p ybreaking and interaction of low frequencywaves need to be examined.
