Short period gravity waves and ripples in the South Pole mesosphere

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi:10.1029/2011jd015882, 2011 Short period gravity waves and ripples in the South Pole mesosphere S. Suzuki, 1,2 M. Tsutsumi, 1 S. E. Palo, 3 Y. Ebihara, 4,5 M. Taguchi, 6 and M. Ejiri 1 Received 28 February 2011; revised 11 July 2011; accepted 15 July 2011; published 12 October 2011. [1] In this study, we determined the characteristics of mesospheric wave structures over South Pole Station (90 S) derived from sodium airglow imaging observations. During the winter months of 2003 to 2005 (105 nights), we extracted a total of 768 wave events and separated them into two types (band type gravity waves and ripples) according to their horizontal wavelengths. The distributions of the observed wave parameters, except for the horizontal propagation directions, were similar to those obtained by imaging observations at other latitudes. The observed gravity waves showed a preference for propagation toward 30 60 E and 210 240 E, whereas the ripples showed a preference for motion toward 90 120 E and 300 330 E. The gravity waves had a weak tendency of being observed in 0100 0700 UT, although the ripples did not show such a time dependence. We also investigated the characteristics of atmospheric instabilities from the alignment of the phase fronts of the observed ripples. Citation: Suzuki, S., M. Tsutsumi, S. E. Palo, Y. Ebihara, M. Taguchi, and M. Ejiri (2011), Short period gravity waves and ripples in the South Pole mesosphere, J. Geophys. Res., 116,, doi:10.1029/2011jd015882. 1. Introduction 1 National Institute of Polar Research, Tachikawa, Japan. 2 Now at Leibniz Institute of Atmospheric Physics, Kühlungsborn, Germany. 3 Department of Aerospace Engineering, University of Colorado at Boulder, Boulder, Colorado, USA. 4 Solar Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan. 5 Now at Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan. 6 College of Science, Rikkyo University, Toshima ku, Japan. Copyright 2011 by the American Geophysical Union. 0148 0227/11/2011JD015882 [2] Airglow imaging is a useful technique for the investigation of two dimensional horizontal characteristics of wave structures (such as gravity waves and ripples) in the mesosphere and lower thermosphere (MLT); their horizontal motions can be identified directly from sequential airglow images. [3] Atmospheric gravity waves significantly contribute to the wind/thermal balances in the MLT by vertically transporting horizontal momentum. To date, quite a few studies employing airglow imaging have reported seasonal and latitudinal variations in gravity wave characteristics [Suzuki et al., 2009, and references therein]. However, little is known about such wave characteristics in the polar MLT, especially in the southern hemisphere, owing to insufficient observations. [4] Recently, Nielsen et al. [2009] reported the climatology of short period (<1 h) gravity waves in the MLT by using an all sky airglow imager at Halley Station (76 S, 27 W) in Antarctica. They showed that the gravity waves in the mesosphere propagated mainly toward the Antarctic continent with monthly changes (westward in fall and eastward in spring). [5] Ripples are also commonly observed in the MLT airglow images as smaller wavy structures. It is known that ripples originate from local instabilities [Hecht, 2004] and their activities are correlated with those of gravity waves [Yue et al., 2010]. Though several case studies have been conducted on ripples in airglow images [e.g., Li et al., 2005; Nakamura et al., 2005], their statistics have been reported in only a few papers on the basis of midlatitude observations [Nakamura et al., 1999; Yue et al., 2010]. [6] In this paper, we show the first statistics of short period gravity waves and ripples in the South Pole mesosphere by using the all sky imager installed at Amundsen Scott South Pole Station (90 S, geomagnetic latitude 74.3 ). 2. Observations [7] We used the data set from the all sky imager at South Pole Station, which has been operated by the National Institute of Polar Research (NIPR) [Ejiri et al., 1999; Ebihara et al., 2007]. The imager began conducting observations of mesospheric gravity waves through the sodium airglow emission in 2002 as well as imaging observations of the aurora from 1997. This imager has five interference filters on a rotating wheel, a fish eye lens with a 180 field of view (Nikkor f = 6 mm, F1.4), and a cooled CCD camera with a 512 512 pixel resolution. The optical imaging observations are made automatically during the period when the Sun is below the horizon with an elevation angle of 12, and hence, these observations are limited to the austral winter months (between April and August). [8] In order to investigate the climatology of gravity waves in the mesosphere, we used sodium emission images at a 1of6

Figure 1. Summary histograms of parameters of all waves (768 events) observed in the South Pole sodium airglow images between April 2003 and August 2005: (a) horizontal wavelength, (b) observed phase speed, (c) observed period, and (d) horizontal propagation directions. The mean and standard deviation of the distribution are also shown in Figures 1a 1c. (e l) are the same as Figures 1a 1d but for showing plots of distributions of gravity waves (l h 17 km, 555 events, Figures 1e 1h) and ripples (l h < 17 km, 213 events, Figures 1i 1l). wavelength of 589.3 nm (typical emission height 90 km), which were obtained between April 2003 and April 2005 (105 clear days). Since the sodium emission is least susceptible to auroral contamination, gravity wave structures in weak airglow can be detected even at high latitudes. The above mentioned sodium emission images were mainly taken every 100 s with an exposure time of 64 s. [9] For the analysis, the images were projected onto geographical coordinates with a size of 256 256 km, and then, the time difference (TD) images between consecutive images [Swenson and Mende, 1994] were calculated to provide a better contrast in moving structure. 3. Results [10] During the 105 clear nights, 768 wave events were extracted based on a visual inspection of the sodium TD images. The left column in Figure 1 shows summary histograms of wave parameters of all the observed wave events: horizontal wavelength l h (Figure 1a), observed phase speed c (Figure 1b), observed period t (Figure 1c), and horizontal propagation direction (Figure 1d). In Figures 1a 1c, the average and standard deviation of each parameter are mentioned on the upper right side. The observed wave structures mainly had a horizontal wavelength of 10 35 km, observed phase speed of 20 60 m s 1, and observed period of less than 15 min. They preferentially propagated toward 30 60 E and 210 240 E. In Figure 1a, we can see a significant peak at 10 15 km, which is characteristic to ripples, as well as at 30 km. Ripples are considered to be generated by in situ instabilities such as convective and shear (dynamical) instabilities; therefore, they should be differentiated from band type gravity waves to discuss their characteristics [Taylor et al., 1997; Nakamura et al., 1999]. [11] Figure 2 shows examples of the images of band structure of gravity wave (Figure 2a) and ripple (Figure 2b). Band structures are often present over the entire sky from horizon to horizon, while ripples usually appear in a region which is spatially limited. 2of6

Figure 2. Examples of wave structure observed in all sky (TD) sodium airglow images obtained at the South Pole: (a) band structure of gravity wave on 19 June 2003 and (b) ripple structure on 14 May 2004. gravity waves (more than a few hours). This is consistent with a typical ripple signature (i.e., ripples usually last within a few tens of minutes) suggested in previous studies [e.g., Yue et al., 2010]. It should be also noted that 16% (34 events/ 213 events) of waves classified as ripples exhibited long lifetime greater than 1 h. This may imply that some gravity waves have horizontal wavelength less than the boundary in this analysis (17 km). [13] Because the Sun is more than 12 below the horizon for 4 months at the South Pole, we could make all day observations of optical imaging; on almost all the days we analyzed in this study, observations were made for the entire 24 h. Figure 3 shows the hourly occurrence of gravity waves (Figure 3a) and ripples (Figure 3b) for 0000 2400 UT. In general, the occurrences of both the gravity waves and the ripples did not show strong time dependence, although the gravity waves showed a weak tendency of being observed in the 0100 0700 UT interval. The occurrence of ripples over the South Pole is different from the results in the midlatitudes. Yue et al. [2010] reported that the occurrence of ripples showed strong local time dependence based on the long term observations in Colorado (41 N). [14] On the basis of hourly averaged horizontal winds measured by the South Pole meteor radar [Forbes et al., 1995], we also calculated intrinsic parameters for gravity waves. The radar receives meteor echoes from four orthogonal directions (0, 90 E, 180, and 90 W longitudes). The radial winds are projected onto a horizontal plane by assuming that their vertical components are sufficiently smaller than their horizontal components. The horizontal winds used in this analysis were Gaussian weighted averages with a height of 92 ± 5 km. Out of the 555 gravity wave events extracted, 307 with simultaneous wind measurements were available for this analysis. [12] Ripples typically have small horizontal wavelengths less than 16 km [Peterson and Adams, 1983]. Some other boundaries of horizontal wavelength between gravity waves (bands) and ripples have been suggested in previous papers (e.g., 17.5 km [Nakamura et al., 1999], 15 km [Yue et al., 2010]). In this paper, we applied the boundary of 17 km. The middle and right columns in Figure 1 show, in the same manner as the left column, the distributions of parameters of gravity waves (l h 17 km, total events: 555) and ripples (l h < 17 km, total events: 213), respectively. The gravity waves had a horizontal wavelength of 20 35 km (with average and standard deviation of 29.2 ± 8.6 km), observed phase speed of 30 60 m s 1 (49.7 ± 15.1 m s 1 ), and observed period of 5 15 min (10.3 ± 3.8min), whereas the ripples had corresponding parameters of 10 15 km (11.8 ± 2.4 km), 20 40 m s 1 (30.7 ± 10.7 m s 1 ), and less than 5 min (4.3 ± 3.8 min). The gravity waves tended to propagate toward 30 60 E and 210 240 E, which is similar to the result of whole wave events (Figure 1d). On the other hand, the ripples appeared to prefer motion toward 90 120 E and 300 330 E. It is noteworthy that the lifetime of the observed ripples (less than 45 min in most cases) was quite shorter than that of the Figure 3. (b) ripples. Hourly occurrence of (a) gravity waves and 3of6

Figure 4. Histograms of the parameters of gravity wave events calculated using the wind measured by the South Pole meteor radar: (a) horizontal intrinsic phase speed, (b) intrinsic period, and (c) vertical wavelength. [15] Figure 4 shows histograms of the intrinsic phase speed c u (where u is the horizontal wind along the wave heading obtained by the meteor radar) (Figure 4a), intrinsic period l h / c u (Figure 4b), and vertical wavelength l z (Figure 4c). [16] For the calculation of l z, we used the linear dispersion relation m 2 ¼ 2 2 ¼ N 2 2 2 1 2 z ðc uþ h 4H 2 ; where N is the Brunt Väisälä frequency and H is the density scale height; at the MLT height, N and H are typically 0.02 s 1 and 6 km, respectively. It should be noted that out of the 307events with wind measurements, 27 events ( 10% of total) showed evanescent behavior (i.e., m 2 < 0), and these events were omitted from Figure 4c. The percentage of waves with negative m 2 is comparable to results at other sites (e.g., 13% at Resolute Bay, Canada (75 N) [Suzuki et al., 2009]). The intrinsic parameters (Figures 4a and 4b) showed distributions similar to those of the observed parameters (Figures 1f and 1g), although the distribution of the intrinsic phase speed was slightly broader (20 70 m s 1 ) than that of the observed phase speed (30 60 m s 1 ). This was probably caused by the weak wind fields in the mesosphere over the South Pole. 4. Discussion [17] The observed characteristics of mesospheric waves over the South Pole were similar to those obtained by the airglow imaging measurements at other stations, except for their horizontal propagation directions. From 18 month longterm imaging observations of small scale (l h < 100 km) gravity waves at Shigaraki (35 N), Japan, Nakamura et al. [1999] determined the distribution of horizontal wavelength, which had double peaks one below 15 km and the other at around 30 km, which is similar to our result shown in Figure 1a. The characteristics (horizontal wavelength, phase speed, and period) of gravity waves and ripples at Shigaraki were also quite similar to the results at the South Pole. Note that Nakamura et al. [1999] defined their boundary of horizontal wavelength between gravity waves and ripples as 17.5 km. [18] The preferred motions of observed gravity waves toward 30 60 E and 210 240 E may suggest wave blocking in the directions perpendicular to the observed motions and/or two wave sources in the 210 240 E and ð1þ 30 60 E. These propagation directions are quite different from the results at Halley Station (76 S), Antarctica, obtained by Nielsen et al., [2009] (mainly poleward motion). [19] In the Antarctic, although few observations of mesospheric short period gravity waves have been made, statistics of gravity waves with much longer period and larger scale in the stratosphere have been studied actively by using a large data set of radiosonde observations. On the basis of 4 year balloon soundings, Pfenninger et al. [1999] showed that the gravity waves in the South Pole stratosphere had no preferred propagation direction. Considering their results, the preferred direction of the gravity waves observed in the mesospheric airglow may be attributed to the wind fields (critical level filtering) in the middle atmosphere between the stratosphere and the mesosphere. The filtering effect is one of the mechanisms for formation of horizontal anisotropy in gravity wave propagation; only those gravity waves with phase velocity greater than the peak of the underlying background wind or those whose direction is opposite to the background wind can reach the upper atmosphere. The horizontal winds over the South Pole, however, are significantly weak (HWM 93 [Hedin et al., 1996]) and are not likely to filter the gravity waves. It has been reported that, in the Antarctic, the gravity wave energies are more correlated with the winds in the lower stratosphere than with those in the troposphere; this suggests that the gravity waves are generated in the stratosphere rather than from topographic forces [Yoshiki and Sato, 2000]. In order to account for the distribution of the observed gravitywave propagation direction, additional data sets such as temperature profiles, polar night jet, and auroral electrojets are required. [20] We next would like to infer the generation mechanism of the observed ripples by roughly assuming that the ripples originated from (or were triggered by) the gravity waves observed in this study. [21] Ripples in the airglow images can be considered to be manifestations of atmospheric instabilities (convective or dynamical instabilities), which are generated by gravity waves. The instabilities are traditionally characterized by Richardson number R i (= N 2 /S 2, where S is the vertical shear in horizontal winds). The convective instability occurs under the condition of N 2 < 0 (hence, R i < 0), and induced ripples are observed to be aligned perpendicular to the incident gravity waves; that is, the wavenumber vector (k vector) of ripples points perpendicular to that of gravity waves [e.g., Hecht et al., 2000]. On the other hand, the dynamical instability occurs in the area of 0 < R i < 0.25 and k vectors of ripples and gravity waves are parallel [e.g., Yamada et al., 2001]. 4of6

mechanism causing the observed preferred propagation direction are not yet clear. In contrast, for the ripples, motions toward 90 120 E and 300 330 E were preferred. The observed ripples may be resulting from convective and dynamical instabilities (45% and 30% respectively). [25] To further investigate the reasons for preferred directionality of gravity waves in the Antarctic, it would be necessary to carry out more measurements of wind and temperature profiles in the middle atmosphere and monitor auroral activity. [26] Acknowledgments. The authors thank T. Nakamura and Y. Tomikawa of the National Institute of Polar Research for their helpful discussions and comments. S.S. was supported as a research fellow of the Japan Society for the Promotion of Science (JSPS). The South Pole imager project had been carried out under an agreement of cooperation between the U.S. National Science Foundation and the National Institute of Polar Research. This work was supported by grant in aid for JSPS fellows (21 1828), and scientific research (09041115, 11691137, and 23740368) of the Ministry of Education, Culture, Sports, Science and Technology of Japan. Figure 5. (right) Distribution of included angles between k vectors of observed ripples and gravity waves. (left) Occurrence distribution as a function of included angles. [22] On the basis of the present data set, we found that 128 ripples occurred during the gravity wave events. Figure 5 shows the scatter plot of the included angles made by the k vectors of ripples and gravity waves (Figure 5, left) and their occurrence distribution (right panel). The observed ripples tended to move almost perpendicular (90 ) or parallel (0 ) to the gravity waves k vectors. Assuming that the events with included angles between 60 and 90 (0 and 30 ) indicate convective (dynamical) instability, it can be said that the convective instabilities (45%, 58 events/128 events) occurred more frequently than dynamical instabilities (30%, 39 events/128 events). This is consistent with the observed preferential directions of gravity waves and ripples (Figures 1h and 1l); the preferred azimuth of ripple motions shifted by 90 to that of gravity wave propagations. 5. Summary and Conclusions [23] We presented, for the first time, the climatology of mesospheric waves over the South Pole (90 S) on the basis of sodium airglow imaging observations for 105 days in the winter months of 2003, 2004, and 2005. We identified 768 wave events and investigated them by dividing them into two groups according to their horizontal wavelength: gravity waves (l h 17 km, 555 events) and ripples (l h < 17 km, 213 events). 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Yoshiki, M., and K. Sato (2000), A statistical study of gravity waves in the polar regions basedon operational radiosonde data, J. Geophys. Res., 105(D14), 17,995 18,011. Yue, J., T. Nakamura, C. Y. She, M. Weber, W. Lyons, and T. Li (2010), Seasonal and local time variability of ripples from airglow imager observations in US and Japan, Ann. Geophys., 28, 1401 1408. Y. Ebihara, Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto 611 0011, Japan. M. Ejiri and M. Tsutsumi, National Institute of Polar Research, 10 3 Midoricho, Tachikawa, Tokyo 190 8518, Japan. S. E. Palo, Department of Aerospace Engineering Sciences, University of Colorado at Boulder, 431 UCB, Boulder, CO 80309 0429, USA. S. Suzuki, Leibniz Institute of Atmospheric Physics, Schloss Str. 6, 18225 Kühlungsborn, Germany. (suzuki@iap kborn.de) M. Taguchi, College of Science, Rikkyo University, 3 34 1 Nishi Ikebukuro, Toshima ku, Tokyo 171 8501, Japan. 6of6