[1] Gravity waves in the mesopause region (80-105 km) may induce perturbations in OH Meinal Band emissions at $87 km. These perturbations can be observed by ground-based OH airglow imagers. In this paper, we present observations of concentric gravity waves (CGW) by the all-sky OH imager at Yucca Ridge Field Station (40.7°N, 104.9°W) near Fort Collins, Colorado. We find that expanding rings of concentric gravity waves were observed on 9 out of 723 clear nights from 2003 to 2008. In particular, on 11 May 2004, concentric rings were observed for $1.5 h, with nearly perfect circular rings entirely in the field of view during the first 30 min. The centers of the concentric rings occurred at the geographic locations of two strong convective plumes which were active in the troposphere $1 h earlier. We measured the horizontal wavelengths and periods of these gravity waves as functions of both radius and time. These results agreed reasonably well with the internal Boussinesq gravity wave dispersion relation with an assumed zero background wind. Similarly, for the other 8 cases, strong convective plumes occurred prior to the CGW observations near the apparent center of each of the arcs or rings. For the 7 out of the 9 cases, radiosonde data were available up to z = 30-35 km. These data showed that the wind speeds from the tropopause to $30-35 km were smaller than $20-30 m/s. Because 8 of the 9 cases occurred when the total horizontal mean winds were weak and because the horizontal winds below $87 km were less than $20 m/s on 11 May 2004 (according to radiosonde and TIME-GCM model data), we postulate that weak background horizontal winds are likely a necessary condition for gravity waves excited from convective overshooting to be observed as concentric arcs or rings in the OH layer.

The vertical eddy diffusivity K due to atmospheric turbulence with spatial scales of 10ø-102 m has been computed from the echo power spectral width observed by the middle and upper atmosphere radar for almost every month from January 1986 to December 1988. The method of analysis follows Lilly et al. [1974], $ato and Woodman [ ], and Hocking [1983a[ , 1988, and the contamination due to beam broadening, vertical shear, and transience has been removed. Although observations for horizontal wind speeds larger than approximately 40 m/s, such as occur near the tropopause jet stream in winter, have been omitted because of excessive beam broadening, sufficient numbers of observations have been accumulated to produce a reasonable climatology for the upper troposphere and lower stratosphere (6-20 km altitude) and for the mesosphere (60-82 km altitude). The monthly median of K shows a local maximum near the tropopause jet stream altitude. It becomes larger in the mesosphere, increasing gradually with height. Maxima of K are observed in winter near the tropopause and in summer in the mesosphere, and the seasonal variability of K reaches approximately an order of magnitude. A semiannual variability is apparent in the mesosphere with minima in the equinoctial seasons. effects of seasonal and meridional variations [Johnson and Wilkins, 1965; Justus, 1973; McElroy et al., 1974; Shimazaki and Ogawa, 1974; Crutzen, 1974; Johnston et al., 1976; Ogawa and $himazaki, 1975; Blum and $chuchardt, 1978; Massie and Hunten, 1981; Allen et al., 1981; $trobel et al., 1987]. Such an ad hoc description has also been used in dynamical models of gravity-wave dissipation [Matsuno, Paper number 94JD00911. 0148-0227/94/94JD-00911 $05.00 1982]. Chemically deduced values of K are often affected by the lifetime of each constituent and include not only the true diffusion effect due to microscale turbulence but also an advection effect due to the meridional circulation [Strobel, 1989; Mcintyre, 1989] and/or planetary waves [cf. Matsuno, 1980; Holton, 1981]. Since Lindzen [1981] proposed his well-known parameterization scheme for K due to purely monochromatic internal gravity waves, modelers have incorporated it into their models [e.g., Holton, 1982; Garcia and Solomon, 1985].Recently, mesosphere-stratosphere-troposphere (MST) and mesosphere/lower-thermosphere (MLT) radars have provided a powerful measurement technique for determination of K over a quite broad altitude range, with far better temporal resolution than previously afforded with the other techniques. There are two main procedures which may be used to infer K. Firsfly, we may estimate characteristics of the target scattering the radio wave from the radar echo power intensity [Gage et al., 1980;Balsley and Garello, 1985;Sato et al., 1985]. We then use relationships between the refractive index gradient and the turbulence parameters [e.g., Tatarski, 1971] to infer K, but the accuracy of this method decreases when temperature and humidity are not known with high resolution. Alternatively, we can e...

A wideview CCD imager for OH airglow observations was operated at the MU radar site in Shigaraki, Japan (35 • N, 136• E). From the 18 months' observation, dominant gravity wave components in the OH images are extracted, and seasonal variation of the characteristics of the waves is investigated. These waves typically have short horizontal wavelengths (5 km-60 km) and short periods (5 min-30 min), with horizontal phase speeds of 0-100 m/s. All the wave events are separated into two groups by a boundary of a horizontal wavelength of 17.5 km, which is close to the boundary between ripples and bands. For the waves with larger horizontal wavelengths, the horizontal propagation direction showed clear seasonal variation with summer eastward and winter westward preferences, with a change of direction in mid-March and at the end of September. This suggests that these waves are propagated from the lower atmosphere and filtered in the middle atmosphere by the mean winds. However, the small scale waves propagate in almost all azimuths with a slight seasonal variation. Therefore, in-situ generation would be the major source of such waves although the wavelength as a physical boundary between the two groups could be smaller than 17.5 km. The seasonal variation of the wave parameters especially between summer/winter and equinoctial months is also discussed. The waves with small horizontal wavelengths (<15 km), longer periods (>10 min), and slow horizontal phase speeds (<30 m/s) are mainly seen in summer/winter.

The Optical Mesosphere Thermosphere Imagers (OMTI) have been developed to investigate the dynamics of the upper atmosphere through nocturnal airglow emissions. The OMTI consist of an imaging Fabry-Perot interferometer, three all-sky cooled-CCD cameras, three tilting photometers, and a Spectral Airglow Temperature Imager (SATI) with two container houses to install them in. These instruments measure wind, temperature and 2-dimensional airglow patterns in the upper atmosphere at multi-wavelengths of OI (557.7 nm and 630.0 nm), OH (6-2) bands, O 2 (0, 1) bands, and Na (589.3 nm), simultaneously. Examples of the data are shown for the cameras, the photometers, and the SATI based on the airglow observation at a mid-latitude station in Japan. Good correlation of the photometer and SATI observations is obtained. A comparison is shown for small-and large-scale wave structures in airglow images at four wavelengths around the mesopause region using four cooled-CCD cameras. We found an event during which large-scale bands, small-scale row-like structures, and large-scale front passage occur simultaneously.

[1] Using airglow images of the near-infrared OH band (720-910 nm) and OI (557.7 nm) line, we investigated seasonal, latitudinal, and local time variations of short-period gravity waves. The images were obtained at two locations in Japan that are $1200 km apart, Rikubetsu (43.5°N, 143.8°E) and Shigaraki (34.9°N, 136.1°E), between October 1998 and October 1999. Our analysis has focused on small-scale gravity waves with wavelengths less than 40 km and dominant phase speeds of $20-50 m/s. Wave occurrences for both OH and OI at Rikubetsu and Shigaraki are significantly higher than 60%, with a slightly larger value in summer. The occurrences increase from evening to midnight. There are no obvious local time dependencies in horizontal wavelength, propagation direction, and phase speed. The propagation directions in summer are either northward or northeastward at both locations. However, in winter the propagation directions at Rikubetsu are generally westward (NW, W, and SW), whereas those at Shigaraki are only southwestward. From simultaneous wind observation by the MF radars at Wakkanai (45.4°N, 141.7°E) and Yamagawa (31.2°N, 130.6°E), we discuss possible influences of Doppler and thermal ducting, wind filtering, and source distribution of gravity waves propagating from the lower atmosphere to the airglow heights in the mesopause region.

[1] A westward propagating zonal wave number 1 wave with a period near 6.5 days was a prominent feature in the mesosphere and lower thermosphere (MLT) during the 1994 equinoxes. The meridional structure of the wave in the upper stratosphere and the MLT is consistent with the 5-day wave structure predicted by normal mode theory. However, the amplitude increases sharply above 80 km, where the wave exhibits a highly organized baroclinic circulation. The eddy fluxes and the background state suggest that the wave is amplified by instability of the mesospheric winds.

[1] A VHF Doppler radar with an active phased-array antenna system, called the Equatorial Atmosphere Radar (EAR), was established recently at the equator near Bukittinggi, West Sumatra, Indonesia (0.20°S, 100.32°E, 865 m above sea level). The EAR is a large monostatic radar which operates at 47.0 MHz with peak output power of 100 kW. The EAR uses a circular antenna array, approximately 110 m in diameter, which consists of 560 three-element Yagi antennas. Each antenna is driven by a solid-state transmitter-receiver module. This system configuration allows the antenna beam to be steered electronically up to 5,000 times per second. The scientific objective of the EAR is to advance knowledge of dynamical and electrodynamical coupling processes in the equatorial atmosphere from the near-surface region to the upper atmosphere. The equatorial atmosphere over Indonesia is considered to play an important role in global change of the Earth's atmosphere. This paper presents the system description of the EAR, including observational results of the equatorial atmosphere made for the first time with altitude resolution of 75-150 m.

The volume depolarization ratio of the molecular backscatter signal detected with polarization lidar varies by a factor of nearly 4 depending on whether the rotational Raman bands are included in the detected signals of the individual system or not. If the rotational Raman spectrum is included partially in the signals, this calibration factor depends on the temperature of the atmosphere. This dependency is studied for different spectral widths of the receiving channels. In addition, the sensitivity to differences between the laser wavelength and the center wavelength of the receiver are discussed.