Investigating seasonal gravity wave activity in the summer polar mesosphere

Journal of Atmospheric and Solar-Terrestrial Physics ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Investigating seasonal gravity wave activity in the summer polar mesosphere Y. Zhao a,n, M.J. Taylor a, C.E. Randall b, J.D. Lumpe c, D.E. Siskind d, S.M. Bailey e, J.M. Russell IIIf a

Center for Atmospheric and Space Sciences, Utah State University, Logan, UT, USA Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA c Computational Physics Inc., Boulder, CO, USA d Naval Research Laboratory, Washington, DC, USA e Bradley Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, VA, USA f Center for Atmospheric Sciences, Hampton University, Hampton, VA, USA b

art ic l e i nf o

a b s t r a c t

Article history: Received 5 September 2014 Received in revised form 15 February 2015 Accepted 23 March 2015

The NASA Aeronomy of Ice in the Mesosphere (AIM) satellite is the first spaceborne mission dedicated to studying high-altitude (
83 km) Polar Mesospheric Clouds (PMCs). Since its launch in 2007, the Cloud Imaging and Particle Size (CIPS) instrument onboard AIM has obtained large-field, high resolution (25 km2/pixel) images of the PMCs, enabling a unique investigation of mesospheric gravity wave activity in the summer polar mesosphere where previous measurements have been sparse. In this study, we have analyzed 12 consecutive seasons of AIM/CIPS PMC albedo data to determine the statistical properties of medium and large horizontal scale ( 4100 km) gravity waves present in the PMC data. Over 60,000 wave events with horizontal scale-sizes ranging up to 42000 km have been identified and measured, revealing a wealth of wave events particularly in the
300–800 km range where our analysis sensitivity is largest. These data are ideal for investigating the intra-seasonal, inter-annual and hemispheric variability of these waves as observed over the whole summer polar cap regions. Throughout this 6 year study, the wave activity in the southern hemisphere was found to be consistently 10–15% higher than in the northern hemisphere and both the northern and southern hemisphere wave activity was determined to decrease systematically (by
15%) during the course of each summer season. This decrease agrees well with previous seasonal stratospheric studies of variations in the wave energy, suggesting a direct influence of the lower atmospheric sources on polar mesospheric dynamics. Very similar and consistent results were also found from season to season in both hemispheres providing new information for gravity wave modeling and dynamical studies of the high-latitude summer-time mesosphere. & 2015 Elsevier Ltd. All rights reserved.

1. Introduction Gravity waves (GWs) are ubiquitous at all latitudes and seasons (e.g., Reid et al., 1988; Taylor et al., 1997; Rauthe et al., 2008; Wu and Eckermann, 2008; Shiokawa et al., 2009; Hoffmann et al., 2010; Suzuki et al., 2011). Through their ability to transport energy and momentum upwards from copious tropospheric sources, gravity waves are now known to be key drivers of the general circulation and temperature structure of the middle and upper neutral atmosphere and ionosphere (see review by Fritts and Alexander, 2003). Gravity wave influences have been shown to be strongest in the upper mesosphere and lower thermosphere (MLT) n

Corresponding author. E-mail address: [email protected] (Y. Zhao).

region (
80–100 km), due to the onset of wave breaking and dissipation effects associated with their rapid growth in amplitude with altitude. Gravity waves are therefore an essential component in current General Circulation Models (GCMs), such as the Whole Atmosphere Community Climate Model (WACCM) and Whole Atmosphere Model (WAM) (e.g., Liu et al., 2010; Akmaev, 2011). Associated theory and modeling studies have shown that the global deposition of gravity wave momentum flux forces closure of the mesospheric jets in summer and winter, and drives a residual meridional inter-hemispheric circulation (e.g., Lindzen, 1973; Holton, 1983; Garcia and Solomon, 1985; McIntyre, 2001). The resultant strong upwelling in the summer polar region creates a remarkably cold summer mesopause region at polar latitudes due to strong adiabatic cooling (Fritts and Alexander, 2003; Lübken, 1999).

http://dx.doi.org/10.1016/j.jastp.2015.03.008 1364-6826/& 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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The anomalously cold summer mesopause temperatures (typically 130–150 K) provide the environment for the formation of Noctilucent Clouds (NLC). At these temperatures, microscopic ice crystals can nucleate and grow to observable sizes, over periods of several hours to a few days, when they become visible to the naked eye as NLC (e.g. Gadsden, 1982; Jensen and Thomas, 1988; Rapp and Thomas, 2006). Initial reports of these high-latitude clouds extend back in time to well over 100 years (Backhouse, 1885; Leslie, 1885). From the ground, NLC are seen by strong forward scattering of sunlight from the ice particles, often appearing as an extensive cloud layer, characterized by a variety of spatially periodic gravity wave structures (e.g. Witt, 1962; Fogle and Haurwitz, 1969; Taylor et al., 1984; Gadsden and Schröder, 1989). Due to their tenuous nature, NLC can only be seen by eye during the hours of twilight when the observer and the atmosphere below are in darkness, while the clouds themselves are sunlit (this situation occurs for solar depression angles,
6–16°) limiting their observations over the past
100 years to the latitude range
50– 65°. However, lower latitude sightings are becoming more common in recent years (e.g., Taylor et al., 2002; Gerding et al., 2007). Ground-based, rocket-borne, and more recently lidar measurements have determined the remarkably consistent altitude of NLC during the summer period to be at a mean height of
83 km in the upper mesosphere (e.g. Jesse, 1896; Störmer, 1933; Paton, 1964; Witt et al., 1971; Taylor et al., 1984; Gadsden and Taylor, 1994; von Zahn et al., 1998; Chu et al., 2001; Collins et al., 2009; Thayer et al., 2003; Fiedler et al., 2009; Gerding et al., 2007). Ground-based observations, mainly from the northern hemisphere, have provided important information on the common wave structures evident in the mesopause region during the summer-time at high latitudes enabling investigations of the spatial properties of the waves, their dynamics and associated instability processes (e.g., Fogle and Haurwitz, 1969; Gadsden and Schröder, 1989; Taylor et al., 2011; Fritts et al., 1993; Dalin et al., 2004; Pautet et al., 2011; Baumgarten and Fritts, 2014). However, these measurements are most often from a single site and are limited by the prevailing weather conditions (Gadsden and Schröder, 1989). Satellite observations of these unusual clouds, aptly termed Polar Mesospheric Clouds (PMC), have revolutionized our understanding of their spatial and temporal variability. From the earliest measurements by the OGO-6 satellite (Donahue et al., 1972), and by the Solar Mesosphere Explorer (SME) satellite (Thomas, 1984), they have established the near continuous presence of a PMC layer in the perpetually sunlit summer polar regions over the Arctic (May–August) and Antarctic (November–February) (e.g., Jensen et al., 1988; Thomas et al., 1991; Evans et al., 1995; Carbary et al., 2000; von Savigny et al., 2004; Bailey et al., 2005; Russell et al., 2009). The first satellite image measurements of the PMC layer were obtained by the WINDII limb viewing interferometer aboard the Upper Atmosphere Research Satellite (UARS) which recorded patchy structures at visible wavelengths in the cloud layer as observed in the Earth's limb at
83 km (Evans et al., 1995). However, the instrument resolution was insufficient to identify any wave properties. The Ultraviolet and Visible Imaging and Spectrographic Imaging (UVISI) instrument on the Mid-Course Space Experiment (MSX) satellite provided the first evidence of large-scale wave structures in PMC over the polar regions during summers of 1997, 1998 and 1999. Video images were obtained on a limited number of transpolar orbits (26), showing extensive “transverse wave structures” in the PMCs imaged in the Earth's sub-limb. Importantly, these limited results suggested a predominance for medium to large horizontal-scale waves of
500–1000 km (Carbary et al., 2000) over both the northern and southern summer polar regions. Subsequent analyses of these image data also revealed smaller-scale structures with horizontal wavelengths less

than 100 km, typical of those frequently evident in ground-based NLC data (Carbary et al., 2003; Gadsden and Schröder, 1989; Dalin et al., 2004; Pautet et al., 2011). The NASA Aeronomy of Ice in the Mesosphere (AIM) satellite was launched in April, 2007. It is the first satellite mission dedicated to studying the phenomenon of PMCs (Russell et al., 2009). In particular, the Cloud Imaging and Particle Size (CIPS) instrument onboard AIM is a nadir-pointing, high-resolution UV imager that takes large-field snapshots of PMCs as the satellite traverses over the summer polar region (Rusch et al., 2009; McClintock et al., 2009; Lumpe et al., 2013). These data enable novel investigations of the broad spectrum of gravity waves present in the PMC layer over the summer polar regions. Initial measurements of a variety of structures evident in the high-quality CIPS PMC image data were reported by Rusch et al. (2009) and Russell et al. (2009). Chandran et al. (2009) utilized the first season of CIPS data in 2007 to investigate the characteristics of small-scale gravity waves in the summer polar cap region, while Taylor et al. (2011) subsequently used 2-D spectral analysis of the CIPS data from the same season to compare the wave properties (wavelength, location and orientation) to those generally observed in NLCs. This paper presents results of an extensive survey of mesospheric gravity waves imaged by CIPS over 6 consecutive northern and 6 southern hemisphere PMC seasons (2007–2013). Our study has naturally focused on the larger-scale wave structures (4100 km) present in the CIPS orbital imagery as it utilizes the same analysis method developed by Carbary et al. (2000) to investigate transpolar structures in the MSX satellite dataset. Our analysis reveals a rich and extensive spectrum of gravity waves that is ubiquitous in the summer polar mesosphere. We present new results on their spatial properties, intra-seasonal, and interannual variability as well as their inter-hemispheric differences.

2. The CIPS instrument and example data The CIPS instrument on the AIM satellite is a panoramic UV imager (centered at 265 nm, bandwidth 15 nm) designed to measure radiance and morphology of PMC over a wide range of scattering angles (Russell et al., 2009; McClintock et al., 2009; Lumpe et al., 2013). The CIPS instrument consists of an array of four CCD cameras arranged in a “cross pattern” with overlapping fields resulting in a wide angle view of 120° (along orbit track 1140 km) by 80° (across orbit track, 960 km) centered at the subsatellite point, with a pixel spatial resolution of
2 km in the nadir and about 5 km at the edges of the forward and aft cameras. CIPS records cloud radiance in the summer hemisphere from the terminator to
40° latitude along the sunlit portion of each orbit by taking 27 sets of overlapping “snapshot” images (
1 s integration) of PMCs every 43 s. These data are later combined to create a
1000 km wide by
8000 km long mesospheric image swath, re-binned to a uniform pixel resolution of 25 km2, and providing the PMC field for each orbital pass. This is a standard CIPS level 2 data product, which is available at http://lasp.colorado. edu/aim/index.html and provides measurements of the cloud presence, spatial morphology and microphysical parameters (albedo, particle mean radius, and ice water content) on an orbit-byorbit basis. AIM's near-polar orbit and the wide cross-track field of view of CIPS create 15 PMC swaths per day that overlap at latitudes higher than
70°, and nearly the entire summer polar cap region (up to
85° latitude) is mapped daily. While the data were obtained over a range of scattering and view angles for each location along the orbit track, the resultant cloud albedo has been normalized to 90° scattering angle and nadir (0°) viewing angle for uniformity and to enable direct comparison with other data sets. The CIPS level 2 retrievals are performed over a solar zenith angle

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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Fig. 1. An example of CIPS PMC data (orbit 1359) (upper plot) showing large-scale periodic wave structures imaged on July 25th, 2007, over the northern polar regions during the middle of the first AIM PMC season. The picture displays the cloud albedo over the full orbital swath; and only PMCs brighter than 2x10  6 sr  1 are plotted. The yellow circles in the enlargement (lower plot) identify two regions of smaller scale wave activities. The latitude circles extend from 80°N (most inner circle) to 40°N in 10° intervals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(SZA) range from 40° to 95° on each orbit (Lumpe et al., 2013). The CIPS albedo and cloud frequencies have been found to agree very well with coincident measurements from the Solar Back Scatter Ultraviolet (SBUV-2) instruments (Benze et al., 2009, 2011). Baumgarten et al. (2012) have also analyzed CIPS data obtained in close coincidence with ground-based lidar measurements and determined good agreement in the cloud brightness observed by these two very different methods. In summary, the exceptionally high spatial resolution of the CIPS data and the massive sample area (8  106 km2) per orbit provide an important new capability for investigating gravity waves in the summer polar mesosphere during multiple PMC seasons. Further details of the CIPS instrument, its operation and data processing are given in McClintock et al. (2009) and Lumpe et al. (2013). In this study, we have utilized the individual orbit CIPS level 2 albedo data products to determine the occurrence and characteristics of periodic wave structures present in the PMC field. Fig. 1 presents an example CIPS swath (orbit 1359) recorded during the middle of the first northern hemisphere (NH) AIM PMC season on July 25th, 2007. The figure shows the cloud albedo over the full orbital swath, only PMCs brighter than 2  10  6 sr  1 are displayed. (Note, for convenience, we will use the abbreviation 1 G ¼ 1  10  6 sr  1 in later figures.) Latitude circles are shown at 10° increments from 80° down to the lowest latitude observed (
40°), while longitude lines are in 45° increments. An extensive cloud field was detected, extending
3000 km from Iceland (left side) across northern Canada into Alaska (right side). The data were characterized by several large bands of clouds separated by low brightness (o2 G) regions, indicative of a large horizontal scale (
500 km) quasi-periodic perturbation in the PMC albedo over this region. A very similar wave pattern was also observed on prior and consecutive overlapping CIPS swaths that were
1.5 h apart in time, suggesting that it was a long duration (several hours) event. As with previous NLC/PMC studies in the literature, we interpret these periodic structures as signatures of mediumand large-scale gravity waves passing through the cloud layer. These structures are further enhanced in the associated enlargement of the figure, which also reveals much smaller-scale, gravity wave features, within each bright band (as indicated by the yellow circles).

Fig. 2. (a) Single pixel data scan along the center of orbit 1359 (Fig. 1) plotting the albedo variation as a function of range, and (b) the Lomb periodogram analysis results. The horizontal line in (b) marks the 1% probability of peaks caused by white noise.

3. Data analysis To identify periodic structures in the PMC albedo field, a single pixel wide scan along the center of the swath (Fig. 1) was made to determine the albedo variation as a function of range, and is plotted in Fig. 2a. This method closely follows that used by Carbary et al. (2000) to quantify transpolar structures observed in the MSX satellite data. The resulting “data scan” reveals a horizontally extended periodic structure with 4 prominent consecutive peaks. The amplitude of these structures ranged from 15 to 35 G while the associated albedo minima indicated no cloud detections, as plotted in Fig. 1. As the clouds were not continuous, a Lomb periodogram analysis was utilized to determine the horizontal scale of this wave event. The Lomb periodogram analysis is a standard tool for identifying periodic structures in irregularly sampled datasets (Press et al., 1992). The results of this analysis are shown in Fig. 2b, which plots the horizontal wavelength up to 2000 km versus the normalized power of the wave structures. The horizontal line depicts the 99% confidence level for this analysis, so peaks above this line have less than 1% chance of being caused by white noise (Press et al., 1992). A total of 5 significant periodic structures were identified, comprising a dominant horizontal scale

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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Fig. 3. (a) An example of CIPS albedo data from the SH summer (orbit 9335) observed on January 10th, 2009 in the middle of the second SH PMC season. (b) The central albedo data scan identifying multiple periodic structures, and (c) the corresponding Lomb periodogram results. The horizontal line in (c) marks the 1% probability of peaks caused by white noise. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. A typical example of the CIPS PMC data, obtained on July 14th, 2008 (orbit 6638) during AIM's second NH PMC season. The PMCs exhibited nearly continuous cloud coverage extending over the high Arctic during the middle of the northern 2008 summer season. The white lines mark the positions of the three data scans as shown in Fig. 5, which are separated by 200 km. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of
500 km (corresponding to the prominent large-scale PMC features identified by eye in Fig. 1) and 4 other peaks at
300 km, 380 km, 800 km and 1620 km, respectively. In addition, the analysis indicates smaller scale structures that are associated with the periodic oscillation that can be also seen in the albedo scan (Fig. 2a). However, as discussed by Carbary et al. (2000), they usually fall below the level of significance in the Lomb analysis, due to their much smaller amplitudes. This is especially true for gravity waves which typically exhibit smaller wave amplitudes at shorter wavelengths (e.g., Fritts et al., 1989). Thus, our analysis focuses on quantifying the dominant larger-scale quasi-periodic wave structures (horizontal wavelengths 4 100 km) detected in the PMC field. It is also important to note here that the horizontal scales of the waves detected by this method are “apparent wavelengths along the track” which may be larger than the true horizontal wavelength depending on the orientation of the waves relative to orbit (e.g., by a factor of
1.4 for 45° orientation, Carbary et al., 2000). An examination of the individual CIPS images and our spectral analysis results suggests that many of the waves that we are most sensitive to tend to be orientated more normal to the orbital scan rather than near parallel to it (e.g., Figs. 1, 3a and 4). Fig. 3a shows an example image of gravity waves from the southern hemisphere (SH) summer (orbit 9335) observed on January 10th, 2009, during the middle of the second SH PMC season. This example was chosen to illustrate the existence of

large-scale periodic structures similar to those observed in the NH (Fig. 1). In this case, the PMC structures were observed to extend from coast to coast across the Antarctic continent. Fig. 3b shows the corresponding albedo scan along the center of the
8000 km orbital track. The PMCs were found to extend over 5500 km along the swath. Compared to the northern PMC data shown in Fig. 2a, the PMCs were generally fainter, exhibiting a maximum brightness of
20 G, but they still exhibited high contrast ratio as the associated minima were also o2 G. On this occasion, the albedo scan revealed multiple quasi-periodic structures over Antarctica. The Lomb analysis of these data (Fig. 3c) identified eight significant peaks, including a prominent peak at
300 km and four peaks with wavelengths less than 500 km, revealing considerably more smaller-scale wave structures than in the NH example of Fig. 1. However, the analysis also identified larger-scale periodic features of similar magnitude to the NH waves structures detected in Fig. 2b. The examples in Figs. 1 and 3 were selected to show medium and large-scale wave patterns in the PMC field. In general, the CIPS level 2 data reveal more complex patterns containing a variety of different structures as well as multiple embedded wave patterns. Fig. 4 presents a more typical example of the CIPS PMC data, which was obtained on July 14th, 2008 (orbit 6638) during AIM's second NH PMC season. This example exhibited nearly continuous cloud coverage extending over the high Arctic during the middle of the PMC season. The swath also shows other types of PMC structures,

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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Fig. 5. (a) PMC albedo for the three data scans identified in Fig. 4, and (b) the Lomb periodogram results for each of the three scans. The horizontal line marks the 1% probability of peaks caused by white noise. The upper, middle and lower panels in (a) and (b) are corresponding to the upper, middle and lower scans in Fig. 4 (white lines).

such as voids (holes in the clouds, Thurairajah et al., 2013a, b) as well as limited regions containing visually less prominent periodic structures compared with those evident in Figs. 1 and 3. To investigate the statistics of transpolar structures in these more complex data, three scans along the orbital track were selected; one along the center of the orbit and one on either side, separated by 200 km from the central scan (later referred to as central, upper and lower scans). This enabled us to sample the various wave structures over a broader region within the CIPS 2D field of view. Fig. 5a plots the corresponding PMC albedo along the three orbit tracks delineated in Fig. 4. While the peak albedos were somewhat higher than the PMCs evident in Figs. 1 and 3, the contrast of the structures was lower, particularly in the upper scan,

as the minimum albedo levels were typically 10–15 G. Nevertheless, all three tracks represent well the information on the PMC structures and their spatial variability across the polar cap region as captured by CIPS. The Lomb analysis technique was applied to each of these scans and the results are plotted in Fig. 5b. Several well-defined peaks were evident in all three plots (each having peaks well above the 99% confident level), quantifying the dominant periodic features in the PMC field. For example, the central track identified a prominent peak around 350 km as well as broader (less well resolved in wavelength) peaks at longer wavelengths, up to the measurement limit of
2000 km (imposed by the PMC data length
4000 km, see Fig. 5a). Similar large-scale periodic structures were identified in the accompanying two off-

Fig. 6. Summary histograms showing the number of periodic events detected versus their apparent horizontal wavelengths (summed in 50 km wide bins) as obtained during July of the NH 2007 PMC season. The numbers in each plot indicate the total wave events detected for each of the three data scans.

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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nadir scans, but not at exactly the same wavelengths, and not always present in all three scans. These results are typical in our extensive multi-season analysis. Although the peaks identified in these three scans show measureable differences in a single orbit, when data from multiple orbits were analyzed, we found no significant differences in the resultant statistical horizontal wavelength distributions. This is illustrated in Fig. 6, which plots histograms of the number of periodic events detected versus their horizontal scale sizes (summed in 50 km wide bins) obtained during one month (July) of the NH 2007 PMC season. All three distributions are similar in shape and each contains a similar total number of wave detections (around
1900–2000). As we are primarily interested in the statistical properties of the waves present in the PMCs, for our seasonal analyses presented below, we have utilized only the results from the central scan to investigate medium and large-scale gravity waves and their variability during the 12 consecutive northern and southern hemisphere PMC seasons.

4. Results A total of 12 seasons of PMC measurements have been analyzed, starting from NH summer 2007, and comprising 6 successive seasons in each hemisphere. For this analysis, orbital scans containing less than 100 pixels of PMC detections were not used. (For comparison, a typical mid-season orbital scan contains
1000 pixels of PMC.) In addition, we have imposed a threshold criterion that the albedo of the brightest cloud within each data scan must exceed 5 G. These basic limits have minimized false detections and ensured the identification of significant peaks in the Lomb analysis. In addition, the spectral analysis results were inspected to remove any spectra that contained possible false detections (i.e., side lobes that can result in the analysis due to the limited horizontal extent of the waves and the finite PMC data length). For each season, this constituted
8–10% of the total number of spectra. In total, more than 11,000 data scans (
900 orbits on average for each season) qualified and have been analyzed to determine the statistical distribution of periodic structures in the PMC field for both the northern and southern summer polar regions. 4.1. Horizontal wavelength spectrum The results for the horizontal wavelength distributions are

plotted in Fig. 7a, and b, and are binned at 100 km resolution for presentation. The 6 NH datasets (Fig. 7a) compare results from a similar number of qualified data scans,
1000 for each year, and reveal near identical distributions in the number of periodic events detected versus their horizontal scale sizes. All 6 plots rise sharply at shorter scales o 200 km, and for each season, most of the waves detected occurred in the
300–800 km range with a similar maximum number of events (4 700). The distributions then decrease rapidly to half of their peak value by
800 km. Thereafter, the number of events steadily decreased to o50 by 2000 km, the nominal limit of the wave measurements. The maximum horizontal scale is limited by the length of the PMC coverage in each orbit, which was typically 4000 km for the majority of orbits in each season. Note that occasionally credible periodic structures with wavelengths 42000 km were detected, but the total number of events was very small compared to the
5000 events observed each season; these events are therefore not included in this analysis. The corresponding results for the 6 SH PMC seasons are plotted in Fig. 7b. The distributions are almost identical in shape to the NH spectrum and any difference could be attributed to the reduced PMC spatial coverage (shorter data length) for the transpolar scans recorded in the SH. However, less waves were detected in the SH around the peak range (
600 events), except for the 2009/10 and 2012/13 seasons, which were anomalous and exhibited 4800 wave detections around the peak. The total number of waves identified for the 6 NH seasons (
34,000 events) was significantly larger than that in the SH (
28,000 events). This was mainly caused by differences in the length of the PMC seasons in the northern and southern hemispheres. In particular, the number of SH orbits that satisfied our selection criteria (
650 orbits/ per season) was about 70% of that in the NH, except for the 2009/ 10 and 2012/13 seasons, which started
20 days earlier than the other four southern PMC seasons, and exhibited similar durations and wave properties to the NH seasons. In summary, the abundance of waves detected over a broad spectral range provides an important data set for investigating intra-seasonal and hemispheric variability. 4.2. Seasonal and hemispheric variability The PMC measurements by CIPS provide new insight on the intra-seasonal, inter-annual and hemispheric variability of wave occurrence over an exceptionally large region, almost the whole polar cap, during the summer seasons. Fig. 8 shows the daily total

Fig. 7. Summary of horizontal wavelength distributions for the 12 seasons binned at 100 km resolution: (a) 6 NH seasons and (b) 6 SH seasons. Note: the two extended SH seasons (2009/10 and 2012/13). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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Fig. 8. Daily total number of wave detections (black) together with the observed PMC occurrence frequency (red) versus days from solstice (DFS) for the 6 NH (left) and 6 SH (right) seasons. Note the apparent early ending of the SH 2007/08 PMC season was due to a satellite “safe hold” problem that occurred in February, 2008. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

number of wave detections (black) together with the observed PMC occurrence (red) versus days from solstice (DFS) for the 6 NH (left) and 6 SH (right) seasons. The cloud occurrence frequency is also a CIPS level 2 data product (Lumpe et al., 2013) and is given by the ratio of the number of pixels with cloud detections to the total number of pixels in that orbit. The daily cloud occurrence frequency plotted in Fig. 8 was obtained by averaging the 15 orbits of each day. All 12 plots were made using the same scales for the number of waves and percentage of cloud cover versus DFS to facilitate direct comparisons between different seasons and hemispheres. The northern PMC seasons started in late May and ended around 60–65 days after summer solstice (late August), lasting
90 days. The beginning of each season was characterized by a rapid increase in cloud occurrence from a few percent to
20% PMC cover in less than 10–15 days. Thereafter, the PMC occurrence steadily increased to a maximum of
40% during the middle of the season, followed by gradual decrease starting around early

August (40 DFS). This evolution in PMC activity is well documented in the literature, and is clearly evident in all six northern seasons (e.g. Benze et al., 2009). In contrast, the SH PMC season usually started about 20 days later, typically lasted closer to 70 days, ending at about the same time after summer solstice as the NH PMC season. However, the conclusion of the SH PMC seasons were often more abrupt than in the NH, due to a sudden decrease in cloud cover. Comparison of the southern and northern PMC coverage clearly shows that the average PMC cloud cover in the SH (
20–30%) was significantly lower and much more variable than the steady (
30–40%) coverages observed in the NH. As noted earlier, the 2009/10 and 2012/13 PMC SH seasons differed significantly from this pattern, as they both started almost a month before summer solstice (even earlier than the start of the NH seasons), and subsequently exhibited an increased percentage in cloud cover, emulating a NH season. Karlsson et al. (2011) have shown that the early start of the 2009/10 PMC season was related to the early breakdown of the Antarctic stratospheric polar vortex.

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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Fig. 9. Daily number of waves per scan versus DFS for the southern 2008/9 (open circles) and the following northern 2009 (solid circles) seasons.

The black lines in each plot of Fig. 8 depict the corresponding total number of waves detected for each day during the 12 PMC seasons. At the beginning and end of each NH and SH PMC season, when the cloud cover was less than
20%, the number of waves detected was variable, with fewer waves identified due to the reduced number of orbits with sufficient PMC detections. However, once the season became established, the daily number of wave detections was no longer dependent on changes in the PMC occurrence, for both the NH and SH, with only limited day-to-day variability. This situation occurred for each season once the cloud cover reached
20%. For example, during the first PMC season (NH 2007), the cloud cover doubled from
20% to 40% prior to solstice, and then temporarily dipped to
30%, around midsummer, but the number of waves detected did not vary with the changes in the cloud occurrence. Close inspection of the NH 2011 and SH 2007/8, 2009/10 and 2012/13 seasons provides additional evidence in support of this finding. To better quantify the variability in the number of waves detected during each PMC season, and to facilitate a clearer

comparison between the NH and SH seasons, we have divided the total number of waves detected each day by the number of qualified data scans for that day. By normalizing the number of waves detected in this manner, the daily average number of waves per data scan is no longer dependent on the cloud cover in each scan, nor on the number of qualified data scans. The results of this analysis are illustrated in Fig. 9, which plots the daily number of waves (per data scan) versus DFS for the southern 2008/9 (open circles) and the following northern 2009 (solid circles) seasons. One prominent feature now evident is that the detected number of waves systematically decreased throughout these two northern and southern PMC seasons. At the beginning of the NH PMC season (at –20 DFS) there were
7 waves detected per data scan. Eighty days later, near the end of the season this number had steadily decreased to
5.5 waves/data scan. Similarly, the number of waves measured during the southern 2008/9 season started at
8 waves/ data scan and ended at
5.5 waves/data scan. Furthermore, on average, the daily number of waves (per data scan) in the SH was somewhat higher than that in the NH summer, and SH waves exhibited relatively larger day-to-day variability. The corresponding results for the 12 PMC seasons are plotted in Fig. 10. In both plots, the data have been smoothed using a 20 day running average to investigate the main variability in the datasets. A general trend is now evident in all 12 of the PMC seasons demonstrating a steady decrease in wave activity beginning shortly before solstice and persisting through much of the season. In the NH summary plot (Fig. 10a), the number of waves per data scan detected around solstice was
5.5–6.5 waves on average and decreased to
4.5–5.5 waves over the next 4–6 weeks, indicating a decrease of
15% throughout the season. After
40 DFS, the trends became less coherent, especially for 2007, 2008 and 2012, where an increase in gravity wave activity was observed at the end of the PMC season, possibly due to an increase in planetary wave activity during the same period of time (e.g. Merkel et al., 2009; Nielsen et al., 2010). A similar decreasing trend in wave activity was found for the SH, as shown in Fig. 10b. At the beginning of the PMC season, the number of waves ranged from 6 to 8.5 waves/data scan. After solstice, the number of waves systematically decreased also by about 15% (note: an exception to this is the 2007/8 season, which exhibited a slight increase during the course of the season).

Fig. 10. Summary results of the daily number of waves per scan versus DFS for each of the 12 PMC seasons as illustrated in Fig. 9. The data for each hemisphere have been smoothed using a 20 day running average to identify the main variability in these datasets, and are plotted on the same scale to facilitate direct comparisons. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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5. Discussion In this investigation, we have utilized 12 seasons of AIM PMC albedo measurements to determine the statistical properties of periodic transpolar wave structures evident in the PMC field in the upper mesosphere. In particular, the high-quality and large fieldof-view of the CIPS data have provided an exceptional capability for investigating gravity wave signatures (horizontal wavelengths 4100 km) in the summer polar mesosphere where previous measurements are sparse (e. g., Carbary et al., 2000; Thayer et al., 2003; Chandran et al., 2009, 2010; Dalin et al., 2010; Pautet et al., 2011; Taylor et al., 2011; Kaifler et al., 2013). Additionally, the continuous nature of the CIPS observations over the past 6 years has enabled new investigations of the inter-annual and interhemispheric (north versus south) properties of these waves and their variability. Over 60,000 medium and large-scale periodic structures have been identified and measured, exhibiting horizontal wavelengths that extend from
100 km to 4 2000 km and revealing an abundance of waves in the wavelength range of 300– 800 km in both the NH and SH PMC fields, with less waves detected at shorter and longer horizontal wavelengths. In interpreting these results, the following two factors are important. (1) As previously mentioned, this analysis method is not sensitive to shorter wavelength gravity waves ( o100 km), due to their usually smaller amplitudes which will limit the shorter wavelength region of the observed spectrum. (2) The reduced spectral resolution in the longer wavelength range may limit the number of detection of the waves with large horizontal scales. Together, these two factors contribute to the overall characteristics of our wave spectrum. Further studies are needed to investigate the true spectrum shape over this extended spectral range. In this regard, Chandran et al. (2009) and Taylor et al. (2011) have used CIPS imagery from the first CIPS NH season in 2007 to investigate the small-scale wave spectrum. Their results revealed a preponderance for small-scale waves with dominant scale sizes of
50 km, or less. These results are typical for short-period gravity waves observed in high-latitude NLCs and in the MLT nightglow emissions, as reported from a number of equatorial, mid- and high-latitude stations (e.g., Taylor et al., 1997, 2009, 2011; Medeiros et al., 2003; Pautet et al., 2005, 2011; Suzuki et al., 2011). Subsequently, Chandran et al. (2010) utilized the first 4 seasons of CIPS data to investigate gravity wave longitudinal variability. Their wavelet analysis focused on wave in the PMC data with horizontal scales up to 300 km and they identified a spectral peak centered

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around 250–300 km, close to the limit of their measurements. Our analysis extends the spectral coverage to much longer wavelength (up to
2000 km). A key result of this analysis has been the detection of a systematic decrease in wave activity (
15%) over the summer seasons in both the NH and SH at PMC altitudes (see Fig. 10). A very recent investigation of gravity wave variations in the polar stratosphere and mesosphere has been reported by Liu et al. (2014) using temperature observations from the AIM satellite by the Solar Occultation For Ice Experiment (SOFIE) (Gordley et al., 2009). A prime focus of their investigation was seasonal variations of gravity waves. They determined that the monthly mean gravity wave potential energy (PE) was much stronger in the winter months than during the summer, and that the PE exhibited large annual variations in both the stratosphere and mesosphere. The data were filtered to include gravity waves with vertical wavelengths of 2–15 km, and although no horizontal wave measurements were possible using this technique, their vertical wave scales correspond well with the broad range of horizontal wavelengths detected by CIPS. Of particular interest, their Fig. 4 plots gravity wave PE versus month of the year, averaged over 6 years for both the NH and SH. Detailed examination of the summertime results reveal a decrease in wave PE at the highest altitude range (80–88 km) encompassing the PMC layer. While not discussed in their paper, the mesospheric results also show a decrease in wave PE during the summer season. Importantly, by observing the same mesospheric region over the same period (2007–2013) using a different measurement technique, they provide independent evidence reinforcing our identification of the systematic decrease in wave activity during the summer months at PMC altitudes. One possible cause of this steady decrease in MLT wave activity could be the gradual changes in the filtering of the waves by the background wind field, termed critical level filtering, as waves propagate upwards to the PMC layer from copious sources in the lower atmosphere. The phenomenon of critical level wind filtering occurs whenever the observed horizontal phase speed of the gravity wave becomes equal to the background wind in the direction of wave propagation at that altitude (Hines and Reddy, 1967; Taylor et al., 1993; Medeiros et al., 2003; Ejiri et al., 2009). Only waves with phase speeds larger than the background wind can continue to propagate upwards. This causes the upward propagating wave spectrum to be filtered by the background wind field with increasing altitude. There have been a number of reports of the effects of critical level filtering of short-period gravity waves

Fig. 11. Zonal mean zonal wind (NOGAPS-ALPHA) at 70°N (a) and 70°S (b). The data are plotted for 00 UT on the 15th day of each month, for 3 consecutive months during (a) the NH 2009 and (b) SH 2009/10 PMC seasons. The solid vertical line marks the zero wind line, and the horizontal dashed line indicates the
80 km altitude. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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observed in the MLT airglow emissions (at similar altitude as the PMC layer), which are most often manifested as preferential horizontal directions for the wave motions reaching the MLT region (e.g., Taylor et al., 1993; Nielsen et al., 2006; Suzuki et al., 2009). To investigate the effects of wind filtering on the observed wave spectrum in the summer polar regions, we have used the assimilated wind fields from the US Navy Operational Global Atmospheric Prediction System (NOGAPS) Advanced Level Physics High-Altitude (ALPHA) (NOGAPS-ALPHA) (Eckermann et al., 2009). These data are available from 2007 to early 2010 which cover 3 NH and 3 SH PMC seasons, coincident with our AIM/CIPS measurements. This assimilation system incorporates mesospheric temperature data from the Microwave Limb Sounder on the NASA Aura satellite and from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) experiment on the NASA TIMED satellite, creating an excellent resource for realistic examination of the wind and temperature fields in the MLT region (e.g. Hoppel et al., 2008). Fig. 11 plots the zonal mean zonal wind at 70°N and 70°S, for a time of 00 UT on the 15th day of each month, for 3 consecutive months during the NH 2009 and SH 2009/10 PMC seasons. During the summer, the zonal wind was consistently westward above the tropopause and exhibited maximum values in the mesosphere between 0.1 hPa and 0.01 hPa (
80 km). During 2009 NH summer months (Fig. 11a), the maximum wind speed decreased systematically during June through August from
45 to
35 m/s. A similar examination of the background winds for the SH 2009/10 summer season (Fig. 11b) shows that from December to February, the maximum wind speed also decreased systematically from
60 m/s down to
35 m/s during the PMC season. Both of these results would suggest an increase in wave activity with time, if critical level filtering was in effect. This is in contrast to the results as summarized in Fig. 10 (green curves) which exhibited consistent wave reduction throughout the summer seasons. Investigation of the other 4 seasons of NOGAPS-ALPHA wind data revealed the same results for the SH while the NH winds were variable and inclusive (results not shown here). Thus, the decreasing wave activity at the PMC level was apparently not associated with changes in the mean background wind field. On this note, winter-time measurements of gravity waves over Antarctica using observations of the mesospheric OH emission (nominal layer altitude
87 km, e.g. Baker and Stair, 1988) indicate that the observed phase speeds of medium gravity waves (λh 4100 km) often exhibited magnitudes 50–100 m/s (Taylor et al., 2009). If the summer-time gravity waves reported herein exhibited similar phase speeds, then we would not expect critical level filtering to be a major contributor to the observed intraseason reduction in wave activity. Two other possible causes of the observed reduction in wave activity during the summer seasons are (1) changes within the season in the wave sources, which are thought to be mainly tropospheric or lower stratospheric in origin (e.g. Lindzen, 1984; Yoshiki and Sato, 2000; Li et al., 2009; Sato et al., 2009); or (2) enhanced dissipation of the larger-scale waves as the summer season progresses, possibly by instability processes which can cause the waves to break before reaching the PMC layer. In general, the dominant seasonal sources of gravity waves observed at highlatitudes in the northern and southern hemispheres are not thought to be the same (e.g. Sato et al., 2009), due, for example, to different forcing by the polar jets and mountain terrain. Yoshiki and Sato (2000) have investigated gravity wave energy in the stratosphere using radiosonde observations from 33 stations located in both the Arctic and Antarctic regions over a 10 year period (1987–1996) prior to the AIM mission. They found that the gravity wave energy in the lower stratosphere (15–20 km) varied during the year, peaking in the NH winter and during the spring in the SH

and both exhibit a minimum toward the end of the summer. Of importance for our mesospheric wave studies is that the gravity wave induced temperature variances (their Fig. 3b and d) systematically decreased during the PMC months in both the Arctic and Antarctic stratosphere. The qualitative consistency between their stratospheric data and our mesospheric results suggests that the lower atmospheric wave sources and their variability may well play a direct role in the observed wave trend. Further examination of the stratosphere results of Fig. 4 in Liu et al. (2014) for the NH summer also supports the results of Yoshiki and Sato (2000), providing additional evidence for the influence of the lower atmospheric wave sources on the observed wave trends at PMC altitudes. Unfortunately, their SH summer time stratospheric PE were too small to investigate any commensurate changes. Identifying the effects of individual wave sources, propagation and dissipation would require detailed modeling of these data which is beyond the scope of this investigation. To examine hemispheric differences in the number of wave detections at PMC altitudes, Fig. 12 compares the results from Fig. 10 averaged over all six northern and southern hemispheres, respectively, for the range –20 to 60 DFS. On average, the wave activity in the SH was
10–15% higher than in the NH throughout the summer season. It is interesting to note that the stratospheric seasonal gravity wave temperature variance results of Yoshiki and Sato (2000) exhibit a more pronounced spring-time peak in the SH than the winter peak in the NH, which also results in a higher wave variance in the SH than in the NH at the beginning of the PMC season. This provide a possible explanation for the higher summertime SH wave activity observed in the PMC field, as the gravity waves propagate upwards into the mesosphere from their predominantly lower atmospheric sources. The six consecutive years of wave measurements have also enabled a limited, but informative, investigation of inter-annual variability in the wave activity in both summer hemispheres. Fig. 13 plots the mean number of waves (per data scan) for each season and their corresponding standard deviations, which is a measure of the geophysical variability in wave activity over each season. The 6 year mean of wave detections in the SH (average of the open circles) was 6.2 70.3, as compared with 5.6 70.2 waves

Fig. 12. Averaged results from Fig. 10 over all 6seasons in the NH (blue) and SH (red). The data are plotted over the range  20 to 60 DFS to capture the main PMC seasonal variability. The lines show the least-square fits identifying the
15% reduction in wave activity during the summer months. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i

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Fig. 13. Plots showing the mean number of waves per data scan and the corresponding standard deviation (indicated by the vertical bars), which is a measure of the geophysical variability in wave activity for each season. Note the often higher wave activity and variability in the SH, and the unusually low wave activity and variability during the NH 2011 season.

per data scan in the NH mean (solid circles). Together, these data support the general result that the gravity wave activity was systematically higher in the SH. Within the 12 seasons, the maximum number of wave detections occurred during the southern 2008/9 PMC season while the minimum number of waves occurred during the 2010 northern summer season. Over these 6 years, the wave activity appears to have decreased slightly in the SH, but no systematic trend is apparent in the NH. The NH 2010 and 2011 summers both exhibited significantly reduced wave activity, and 2011 also exhibited much less wave variability. Close inspection of Fig. 8 shows that these two seasons were not anomalous with respect to their duration and cloud activity. We do not yet know the origin of the reduced wave activity during these two seasons, but it is clearly not cloud-dependent. In summary, the SH generally exhibited more wave activity and more intra-seasonal variability than the NH. It also exhibited a small systematic decrease with wave activity from 2007 to 2013 that was not observed in the NH. Although the CIPS dataset is very extensive, this type of analysis is currently insufficient to investigate any longer-term trends in the wave activity in the summer polar MLT region.

6. Summary and conclusions In this study, a total of 12 polar summer seasons of CIPS measurements have been analyzed to derive the statistical characteristics and variations of waves seen in the PMC field in the upper mesosphere. The exceptionally large field-of-view of the CIPS imagery has enabled waves with horizontal scales as large as 2000 km to be measured, which is much larger than the scalesizes of the gravity waves usually measured using ground-based NLC and airglow imaging observations. Over
60,000 wave events were identified using the Lomb analysis technique as initially employed by Carbary et al. (2000) to measure transpolar PMC waves. While this measurement technique does not sample all of the wave data within the CIPS images, we have shown that the results are sufficient to investigate statistical properties of the waves in the summer polar mesosphere. Strong similarities were found in the distributions of the horizontal scales of wave structures for all 12 seasons and an abundance of waves was detected

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between 300 and 800 km range with reduced sensitivity at the shorter and longer horizontal wavelengths. The total number of waves detected each season in the NH was more than in the SH as the NH PMC seasons were longer than most of the SH seasons by
20 days, except for 2009/10 and 2012/13, which were unusually long due to the early start of the PMC season associated with the early breakdown of the polar winter vortex (Karlsson et al., 2011). For intra-seasonal, inter-annual and inter-hemispheric comparisons, the wave data were normalized to determine the average number of wave detections (per data scan) along the CIPS orbit. Wave activity in both the northern and southern polar regions was found to decrease systematically on average by
15% during the course of each summer season. Recent new results also from the AIM satellite provide independent support for this finding (Liu et al., 2014). Analysis of the NOGAPS-ALPHA wind data indicated that the observed decrease in wave activity was not associated with critical level filtering of the waves. Comparison of our mesospheric results with seasonal variability of gravity wave energy in the stratosphere (Yoshiki and Sato, 2000; Liu et al., 2014), suggest a direct influence of the lower atmospheric sources on the polar mesospheric dynamics. The wave activity in the SH mesopause region was also found to be 10–15% higher than in the NH. Finally, a limited inter-annual investigation determined that the highest wave activity occurred during the 2009/10 SH PMC season while the lowest wave activity was during the 2010 northern summer. Together, these results provide an important quantitative evaluation of the statistical properties of medium and large-scale gravity waves in the polar summer mesosphere. New groundbased winter time gravity wave measurements in the Antarctic (as part of the international Antarctic Gravity Wave Instrument Network, ANGWIN) and in the Arctic complementing AIM summer time measurements, are in progress, with a goal of providing year round information on polar mesospheric dynamics.

Acknowledgments The AIM satellite mission was developed as part of the NASA Small Explorer Program under contract NAS5-03132. The gravity wave research presented herein was supported as part of this mission through a subcontract from Hampton University to Utah State University # 05-17. We gratefully acknowledge the tremendous efforts of the entire AIM program, especially mission engineering, operations, data processing and science teams, for sustaining the excellent quality and continuity of the CIPS image data. We further acknowledge Dr. Dave Rusch for his considerable efforts and dedication as PI of the CIPS instrument during its development and initial on orbit measurements. We also thank the NOGAPS–ALPHA team for the use of their assimilated wind fields for investigating possible causes of the observed wave variability. We wish to acknowledge Dr. Damian Murphy and the reviewers for their very helpful comments and suggestions.

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Please cite this article as: Zhao, Y., et al., Investigating seasonal gravity wave activity in the summer polar mesosphere. Journal of Atmospheric and Solar-Terrestrial Physics (2015), http://dx.doi.org/10.1016/j.jastp.2015.03.008i