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Observations of wave-driven fluctuations in emissions from the OH Meinel (OHM) and O2 Atmospheric band were made with a narrow-band airglow imager located at Adelaide, Australia (35S, 138E) during the period April 1995 to January 1996. Simultaneous wind measurements in the 80–100 km region were made with a co-located MF radar. The directionality of quasi-monochromatic (QM) waves in the mesopause region is found to be highly anisotropic, especially during the solstices. During the summer, small-scale QM waves in the airglow are predominately poleward propagating, while during winter they are predominately equatorward. The directionality inferred from a Stokes analysis applied to the radar data also indicates a strong N–S anisotropy in summer and winter, but whether propagation is from the north or south cannot be determined from the analysis. The directionality of the total wave field (which contains incoherent as well as coherent features) derived from a spectral analysis of the images shows a strong E–W component, whereas, an E–W component is essentially absent for QM waves. The prevalence of QM waves is also strongly seasonally dependent. The prevalence is greatest in the summer and the least in winter and correlates with the height of the mesopause; whether it is above or below the airglow layers. The height of the mesopause is significant because for nominal thermal structures it is associated with a steep gradient in the Brunt-Vaisala frequency that causes the base of a lower thermospheric thermal duct to be located in the vicinity of the mesopause. We interpret the QM waves as waves trapped in the lower thermosphere thermal duct or between the ground and the layer of evanescence above the duct. Zonal winds can deplete the thermal duct by limiting access to the duct or by negating the thermal trapping. Radar measurements of the prevailing zonal wind are consistent with depletion of zonally propagating waves. During winter, meridional winds in the upper mesophere and lower thermosphere are weak and have no significant effect on meridionally propagating waves. However, during summer the winds in the duct region can significantly enhance ducting of southward propagating waves. The observed directionality of the waves can be explained in terms of the prevailing wind at mesopause altitudes and the seasonal variation of distant sources.

From April, 1995 to January, 1996 a nightglow imager and an airglow photometer were colocated near Adelaide, Australia. The data obtained on more than 50 clear nights revealed seasonal changes in the airglow intensities and temperatures as well as in the gravity wave activity. These temperature data are the first seasonal results from the mid-latitude southern hemisphere mesopause region. The OH Meinel band was observed to have a rotational temperature that was warmer than the O2 Atmospheric band in the winter. There were also summer solstice maxima and winter solstice minima in the O2 Atmospheric band and OI(557.7' airglow intensities. The gravity wave activity, seen in the 50 to 80 km horizontal wavelength waves, was generally greatest in the OH Meinel layer and showed a semiannual variation with a strong summer solstice maximum. The relationship between gravity wave activity and airglow intensities disagrees somewhat with models. Compared to previous studies, the data suggest that there may be a difference in the seasonal variability of short and long period gravity waves.

[1] The quasi 2 day wave, with a nominal mean period just above 50 h, is a significant feature of the 80–100 km altitude region in both hemispheres. It becomes particularly prominent in the Southern Hemisphere summer at midlatitudes where, a short time after summer solstice, its amplitude rapidly increases and its mean period is found to be approximately 48 h, producing an oscillation phase locked in local time. This lasts for a few weeks. Presented here are observations of the meridional winds and airglow over two sites in Australia, for 4 years during the austral summers of 2003–2006. We show that during those times when the large-amplitude phase-locked 2 day wave (PL-TDW) is present the diurnal tide greatly decreases. This is consistent with the Walterscheid and Vincent (1996) model in which the PL-TDW derives its energy from a parametric excitation by the diurnal tide. These data also show that the diurnal tide is more suppressed and the PL-TDW amplitude is larger in odd-numbered years, suggesting a biannual effect. The airglow data indicated that, for the PL-TDW, the winds and temperature are nearly out of phase. When the PL-TDW is present airglow amplitudes can become quite large, a result dependent on the local time of the PL-TDW maximum. The airglow intensity response was, in general, much larger than what would be expected from the airglow temperature response, suggesting that the PL-TDW is causing a significant composition change possibly due to minor constituent transport.

[1] During the Tropical Warm Pool International Cloud Experiment (TWPICE) an intense tropical low was situated between Darwin and Alice Springs, Australia. Observations made on 31 January 2006 by the Atmospheric Infrared Sounder instrument on the NASA Aqua satellite imaged the presence of atmospheric gravity waves (AGWs), at approximately 40 km altitude, with horizontal wavelengths between 200 and 400 km that were originating from the region of the storm. Airglow images obtained from Alice Springs (about 600 km from the center of the low) showed the presence of similar waves with observed periods of 1 to 2 h. The images also revealed the presence of 30- to 45-km-horizontal-wavelength AGWs with shorter observed periods of near 15 to 25 min. Ray tracing calculations show that (1) some of the long wavelength waves traveled on rays, without ducting, to the altitudes where the observations were obtained, and (2) shorter-period waves rapidly reached 85 km altitude at a horizontal distance close to the storm, thus occurring over Alice Springs only if they were trapped or ducted. The mesospheric inversion layer seen in the measured temperature data almost forms such a trapped region. The winds therefore critically control the formation of the trapped region. Wind profiles deduced from the available data show the plausibility for the formation of such a trapped region. Variations in the wind, however, would make ideal trapped region conditions short-lived, and this may account for the sporadic nature of the short-period wave observations.

[1] Aerospace imagers operating at Alice Springs (23°42′S, 133°53′E) and Adelaide (34°55′S, 138°36′E) have collected more than 4-year of OH and O2 atmospheric emission data. Images taken over the course of each moonless night at 5-min intervals were used to determine OH Meinel (6, 2) and O2 Atmospheric (0, 1) band emission intensities and temperatures, as well as atmospheric gravity wave parameters. The NCAR general circulation model TIME-GCM was run for years 2002–2005 for comparison with these data. The data presented here show the interannual variability of OH and O2A emissions at two sites, Alice Springs and Adelaide, over a 4-a period. It was found that the TIME-GCM successfully reproduces many observed features of the data, including equinoctial maxima associated with the diurnal tide, a 6-h phase shift between OH temperature and intensity maxima, and springtime OH intensity enhancements at Alice Springs. However, the model tends to underestimate the depth of the summertime temperature minimum at both sites, possibly due to inadequate specification of the seasonal variation of gravity waves in the model. The model does, however, successfully describe many of the mesospheric changes observed during the 2002 stratospheric warming event.

[1] A three-field photometer (3FP) for making observations of airglow in the Mesosphere Lower Thermosphere (MLT) region has been operated at the center of the large MF aerial array located near Adelaide (138°E, 35°S) since 1993. Observations of the 557.7 nm airglow intensity emitted by atomic oxygen (OI) at heights near 97 km and the 730.0 nm airglow intensity emitted by hydroxyl (OH) near 87 km have been made simultaneously with MF spaced antenna observations of wind velocities in the 80–100 km height region. The 3FP measures the intensity of the 557.7 nm airglow in three fields separated by about 13 km at heights near 97 km, and then the intensity of the 730 nm airglow in three fields separated by about 12 km at heights near 87 km, on a 60 s cycle. These data have been analyzed to yield gravity wave observed phase speeds and horizontal wavelengths. Simultaneous MF radar data have been analyzed to yield measurements of the neutral wind velocity in 2 km height steps in the 80–100 km height interval every 2 min, and these have been used together with the observed wave parameters to calculate intrinsic gravity wave parameters. This paper presents a very brief overview of the system, the motivation for the work, and preliminary analysis of the data from 1995 until 2000.

[1] The Darwin Area Wave Experiment occurred in Australia from October to December 2001. An objective was to characterize the atmospheric gravity wave field produced from intense convective activity that is routinely observed around Darwin during November and December. Two airglow imagers were sited at Adelaide and at Alice Springs, each located over 1000 km south of Darwin. Waves were observed at the mesopause region propagating predominantly toward the southeast, with some going to the northwest but with none observed going from east to west. The lack of waves propagating toward the west suggests some wind filtering mechanism below 80 km altitude. Waves observed over Alice Springs were analyzed in detail on three nights. On 16 November they were seen propagating toward the northwest. It is proposed that they were generated by dynamical events associated with a cutoff low-pressure system present over southwest Australia. On 17 and 19 November the observations are consistent with wave generation by convective activity present in the Darwin area. Thus as proposed by Walterscheid et al. [1999] and Hecht et al. [2001a], the ducting of waves from distant sources is shown to be a viable explanation for the quasi-monochromatic waves frequently observed in airglow observations. Walterscheid et al. [1999] suggested that ducting of waves from the extensive region of deep cumulus convection over northern Australia explained the strong poleward directionality seen in the summer months. The present study suggests that propagation from northern Australia is selective, and ducted waves from this region may not be the primary source of waves over Adelaide when convection is occurring over central Australia.

[1] This paper presents a 7 year climatology describing medium-scale gravity waves observed in the menopause region covering the years from 1995 to 2001. The data comes from the OI and OH airglow observations of the three-field photometer employed at the University of Adelaide's Buckland Park, Australia (34.5°S, 138.5°E). About 1300 gravity wave events (AGW) are identified during the years 1995–2001. These AGW events usually persist for between 40 min and 4 hours. The magnitudes range from 1% to 14% of the background intensities and peak at 2% for OI observations and at 3% for OH observations. The observed periods range from 10 to 30 min, and the horizontal phase speeds range from 20 to 250 m s−1, with dominant wave scales of 17 min, 70 m s−1 for OI observations and 20 min, 40 m s−1 for OH observations. The intrinsic parameters are obtained by using medium-frequency (MF) wind data observed at the same place. The occurrence frequency of AGW events peaks at 13 min, 40 m s−1 for both OI and OH observations. The occurrence rate of gravity waves has a major peak in summer and a minor peak in winter. There is an obvious dominating southeastward direction for gravity waves, with azimuths of 160° in summer and 130° in winter. Studies for gravity waves observed in various locations show a similar tendency of propagating meridionally toward the summer pole. This implies that the tendency of propagating toward the summer pole may be a global trend for medium-scale gravity waves observed in the mesopause region. During summer, gravity waves propagate against winds measured by MF radar in their dominating direction. Using the ray tracing method, we found that the seasonal variation of winds limits the access of gravity waves to the observation height through reflection and critical coupling, which is one of the causes leading to the seasonal behavior of gravity waves observed over Adelaide.