Propagation and sources of the short period gravity waves observed over Syowa (69S, 40E) and Davis(69S, 78E) studied by airglow imaging in 2016
Abstract
Gravity waves (GWs) transport momentum and energy from the lower atmosphere to the upper atmosphere and drive the general circulation, which significantly changes the temperature in the middle atmosphere [Fritts and Alexander, 2003]. Understanding this role quantitatively will improve the modern general circulation models [Garcia et al., 2017]. However, the local variation of GW characteristics (e.g., phase speed) is poorly understood, especially the GWs with small horizontal wavelengths because most satellite observations hardly detect them. In particular, their sources and propagation are only poorly understood because of the lack of observations. To understand those, our group has observed the gravity waves over Syowa (69$^\circ$S, 40$^\circ$E) using various instruments (e.g., lidar, OH imager, and MF radar). We recently compared the gravity waves over Syowa and Davis (69$^\circ$S, 79$^\circ$E), which have similar terrain and meteorological conditions, to show their horizontal variation over the East Antarctic. We found, from the lidar temperature observations, that vertical profile of gravity wave potential energy is similar between Syowa and Davis, except for a clear enhancement around 30-40 km over Davis [Kogure et al., 2017]. Horizontal propagation characteristics are more clearly observed by airglow imaging measurements of 90 km altitude. The comparison of four imagers' results between April-May 2013 have indicated that the major propagation directions were westward at three stations (Syowa, McMurdo, Halley), but at Davis, GWs seems to propagate in all the directions, which is different from the other three. [Matsuda et al., 2017]. It seems like the GWs over Davis did not suffer wind filtering in the middle atmosphere. The goal of this study is to reveal what causes the difference in the mesospheric gravity wave characteristic over Syowa and Davis. In this study, we will show the ground-based horizontal phase speed spectrum at 87 km altitude over the two stations derived from OH imagers in more detail. OH airglow images obtained for eight months (from March to October in 2016) over the two stations have been analyzed with M-transform [Matsuda et al., 2014]. The data sets of continuous images for longer than one hour without clouds and aurora contaminations were collected at both stations, which include 40 sets at Syowa and 55 sets at Davis. The mean spectra were then calculated in winter (May to August) and fall (September) at each station and were compared. We found that the winter means spectra or directionality are similar. The comparison with transmission diagrams [Tomikawa, 2015] showed that the phase velocity regions of turning and critical level filtering effects are almost same at both stations, which is similar to the spectral shape. The variance of I'/I, calculated by the phase velocity spectra in 0-150 m/s, was very similar both in magnitude and seasonal variations (maximum in winter), except for September/October. In September, directionalities are similar at both stations but power in Syowa is much larger than that in Davis. Moreover, the total powers at Syowa in September and October were 3 times larger than those at Davis. The phase velocity spectra are also calculated for 6 different period band in 5 - 60 min. We found that, in both stations, peaks of the phase velocity spectra for period bands smaller than 11 min were located at prohibited propagation areas by turning level reflections. This suggests that such high-frequency gravity waves were generated in the stratosphere or mesosphere, and not below tropopause. On ten nights in 2016, clear sky and aurora free data were obtained at both Davis and Syowa during the night. The phase velocity spectrum obtained on each night at two stations were very similar on only one night out of ten. On five nights, the spectra were quite different. On the other four nights, the spectral peaks with slow westward phase velocity (> 50 m/s) were commonly observed, but additional spectral peaks were found over Davis and not over Syowa. We investigated, using raytracing method, where those gravity waves propagated from. This investigation suggested that source activities of the gravity waves cause the difference in the spectra. The raytracing showed that most of the peaks in horizontal phase velocity spectrum correspond to the gravity waves were generated below the tropopause. Also, notable is that the majority of the gravity waves were generated above the southern ocean and those from the Antarctic continent were minority.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E.717K