Characteristics of high frequency gravity waves in the upper mesosphere observed in OH nightglow over low latitude Indian sector during 7 Viswanathan Lakshmi Narayanan & Subramanian Gurubaran Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism. Contact: narayananvlwins@gmail.com, gurubara@iigs.iigm.res.in
Abstract Small scale high frequency gravity waves are believed to play a vital role in the upper mesospheric region by means of wave breaking and their interactions with other waves and background mean flow. They are known to propagate large distances from their source regions by means of ducting which makes identification of their source distribution a challenging task. Further, their global distribution is not yet well known. In this work we have studied the characteristics of such high frequency waves observed in OH Meinel band emissions over Tirunelveli (8.7 o N, 77.8 o E) during the year 7. The study reveals predominance of meridionally propagating p g waves, possibly indicating the wind filtering effects in the lower atmosphere. During summer period, waves propagating towards south and south-west were observed much more frequently. Theapparent phase velocities of the waves are higher during equinox periods followed by summer and winter solstices respectively. There was no significant variation in the wavelength range of the observed waves. Detailed discussion on the characteristics of the observed waves and possible source distributions around this site during different seasons are made in this study.
Imaging observations from Tirunelveli
Schematic of the instrument
From January 7, nightglow imaging observations are carried out from Low latitude Indian station Tirunelveli (8.7 o N; 77.8 o E; -.17 o Geomagnetic) during cloudless clear sky nights when moon is behind the horizon. The all-sky imager is procured from Keo Scientific Limited and is designed for F4 optics with a back illuminated 16 bit CCD camera with 1 x 1 pixels. The CCD is not deep depleted. Currently, 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 NIR region of 7.3 98. nm were used. The filter has a notch at ~ 86 nm to suppress contamination from O band emissions. At this site, a collocated MF radar is measuring mesospheric winds at OH emission heights.
Data Analysis The imaging observations were made with collecting a set of successive images from the same filter and changing the filters in sequence. The OH images obtained within ihi each sequence are time differenced (TD) and projected in to the equi-distance grid as discussed in Garcia et al., (1999). From the projected images the wave parameters namely wavelength, phase velocity and propagation azimuth are extracted whenever er more than one crest or trough like structure re shows consistent motion in at least two TD images. The average winds between 84 9 km altitudes at every minutes interval were calculated to overcome data gaps in radar. Afterwards the observation period and propagation azimuth of the waves are noted and mean winds are calculated from this minute averaged data.
Imagers are capable of detecting the quasi-monochromatic gravity waves, nonlinearly evolving wave systems like mesospheric bores and instability features known as ripples that affect the mesospheric nightglow layers. The detection of bore like events are rare and hence the observations usually consist of quasi-monochromatic high frequency gravity waves and instability features associated with them (ripples). In the current study, the quasi-monochromatic waves and ripple features are not separated explicitly. However, reasonable estimation shows that the ripple features contribute less than about % of the observed events. Further, it appears as if most of the observed ripple features are result of convective instabilities occurring in the mesosphere.
Observation of quasi-monochromatic waves on May, 7
Observation of ripple features on Feb 3, 7
Observation of a mesospheric bore on Mar, 7
The site is at mean sea level and the sky is often cloudy restricting the no. of. nightglow observations to 34 nights in the months of Jan, Feb, Mar, May, Aug and Oct, 7. Season Months No. of. Nights No. of. Useful No. of. Events No. of. Waves/h hours our Winter Jan & 14 63 1 1.83 Feb Summer May & 9 31 49 1.8 Aug Equinox Mar & Oct 11 3 79.47 Total All days 34 16 43 1.93
ents 8 7 6 No. of. eve Distribution of wavelength Total Winter Wavelength Wavelength No. of. eve ents No. of. even nts Wavelength (km) Summer Wavelength No. of. even nts Wavelength (km) Equinoxes Wavelength Wavelength (km) Wavelength (km)
Distribution of apparent phase velocity Total Winter Apparent phase velocity Apparent phase velocity No. of. even nts No. of. even nts 6 8 1 1 Apparent phase velocity (m/s) Summer Apparent phase velocity 6 8 1 1 14 1 Apparent phase velocity (m/s) Equinoxes Apparent phase velocity No. of. even nts 8 6 4 6 8 1 1 Apparent phase velocity (m/s) No. of. even nts 8 6 4 6 8 1 1 Apparent phase velocity (m/s)
No. of. eve ents Distribution of intrinsic phase velocity Total Winter Iti Intrinsic i phase velocity Iti Intrinsic i phase velocity No. of. eve ents 16 14 1 8 6 4 6 8 1 1 Intrinsic phase velocity (m/s) Summer Intrinsic phase velocity 6 8 1 1 Intrinsic phase velocity (m/s) Equinoxes Intrinsic phase velocity 8 8 7 No. of. even nts 6 4 ts No. of. event 6 4 3 6 8 1 1 Intrinsic phase velocity (m/s) 1 6 8 1 1 Intrinsic phase velocity (m/s)
No. of. events No. of. events Propagation angle of the waves Total Winter Propagation Azimuth No. of. events Propagation Azimuth 6 3 4 3 4 7 9 7 13 13 6 18 18 Summer Equinoxes Propagation Azimuth Propagation Azimuth 3 4 3 4 7 9 7 13 13 18 18 No. of. events 9 9
Apparent and intrinsic time periods 6 Apparent time period 3 Intrinsic time period nts No. of. eve nts No. of. eve Time period (min) Time period (min) 314 s 314 s
Dispersion relation used to infer vertical wavelength wavelength Neglecting coriolis effect and compressional effects, 4 1 ) ( 1 ) ( ) ( k H c u u H c u u c u N m z zz + = Further assuming curvature and wind shear are not persistent around 87 km region, 4 1 ) ( H k c u N m = With, N =. rad/s, H = 6km
Wave reflection and background wind Events No. of. 1 1 8 Evanescent Propagating 43% evanescent 6 6% % % winter summer equinox total Percentage pro pagation again nst backgroun nd wind (%) 46% 46% 44% 37% winter summer equinox total Events No. of. 16 1 1 8 6 Total no. of. evanescent waves No. of. evanescent waves with opposite mean wind Total no. of. waves with opposite background wind winter summer equinox total s with %) scent waves ound wind ( ge of evanes ite backgro Percentag oppos 8 74% 6 89% % 8% winter summer equinox total
Mean parameters along different directions No. of. events 6 7 6 Propagation Azimuth 3 18 4 13 9 ity (m/s) Appa arent phase veloc 8 6 7 6 8 Mean observed phase velocity 3 4 13 18 9 Time (min) 6 4 3 1 7 1 3 4 6 Mean apparent time period 3 4 13 18 9 Wavelength (km) Mean wavelength along different directions 3 7 4 9 13 18 Int trinsic phase velo ocity (m/s) 8 6 3 Mean intrinsic phase velocity 7 9 6 13 8 18 4 Time (mi in) 1 8 6 4 7 4 6 8 1 Mean intrinsic time period 3 4 13 18 9
Average parameters of the waves 6 4 18 16 14 1 3 winter summer equinox total Horizontal wavelength (km) Vertical wavelength (km) 8 winter summer equinox total - 1 Apparent phase velocity (m/s) Background wind (m/s) 8 6 8 6 velocity Intrinsic phase y (m/s) - - winter summer equinox total winter summer equinox total winter summer equinox total
Typical mean wind profile over Gadanki (13. o N) Courtesy: Dr. M. Venkat Ratnam, NARL, India.
Seasonal mean temperatures and static stability profiles for 7 over low latitudes ( o )
Probable orographic and convective sources
Summary of observations Meridional propagation is predominant indicating probable middle atmospheric filtering effects These shortest t scale gravity waves appear to get filtered out not only by critical level interaction but also due to reflection resulting from doppler shifting into frequencies above buoyancy frequency by means of oppositely directed d mean winds Summer shows largest asymmetry in propagation Phase velocities are relatively higher during equinoxes
Summary of observations Vertical wavelength of propagating waves is on the average 18 km Shortest range of horizontal wavelengths indicating probable convective source Wave ducting might have played amajor role if waves are convectively generated. Possibility of in-situ high frequency wave generation in the mesosphere by means of breaking and interaction of low frequency waves need to be examined.