Browsing by Author "Kato, Susumu"
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Item Open Access A cooperative synchronous observation of winds and tides in the tropical lower stratosphere and mesosphere using VHF radars at Jicamarca and Arecibo(Society of Geomagnetism and Earth, Planetary and Space Sciences, 1986-04) Maekawa, Yasuyuki; Aso, Takehiko; Róttger, Jürgen; Czechowsky, Peter; Rüster, Rüdiger; Schmidt, Gerhard; Hirota, Isamu; Woodman Pollitt, Ronald Francisco; Kato, SusumuA cooperative synchronous observation of winds in the tropical lower stratosphere and mesosphere was carried out using VHF radars at Jicamarca (11.95°S, 76.87°W) and at Arecibo (18.35°N, 66.75°W) on 19-21 or 23 November 1981. The comparison between observations about symmetrical north and south of the equator is focussed on the mean and tidal wind characteristics. Vertical structures of the mean zonal winds at both observational stations are in fairly good agreement with the CIRA (1972) zonal circulation model at the corresponding latitudes. The nature of lower stratospheric diurnal tides is similar between Jicamarca and Arecibo. Their vertical profiles generally correspond to the theoretical predictions based on the fundamental tidal modes. The vertical scale of the tidal components including higher-order modes is shown to be longer than 10km. In the mesosphere, vertically propagating diurnal tide which is expected to be predominant at lower latitudes is partially detected in daytime at Arecibo, while it is not so well-defined at Jicamarca, suggesting a variable nature of the relevant tidal modes.Item Restricted First observation of the upper stratospheric vertical wind velocities using the Jicamarca VHF radar(American Geophysical Union, 1993-10) Maekawa, Yasuyuki; Fukao, Shoichiro; Yamamoto, Mamoru; Yamanaka, Manabu D.; Tsuda, Toshitaka; Kato, Susumu; Woodman Pollitt, Ronald FranciscoThe Jicamarca VHF radar (50 MHz) has detected atmospheric echoes in the so-called "gap region" from 30 to 60 km heights. These echoes are, for the first time, discriminated from clutter echoes, using both co-polarized ( co-pol) and cross-polarized (xpol) arrays to monitor the clutter component which may enter the antenna sidelobes. The atmospheric scatterings in this region are shown to be composed of refractivity layers with thickness as of 1-2 km as in other middle atmospheric height ranges, suggesting that these scatterings are caused by thin turbulent layers. The height profile of infcrred vertical wind velocities indicates a wavy structure. The wave amplitude, as a whole, increases with height, suggesting the activity of upward-propagating gravity waves.Item Open Access Gravity waves observed by the Jicamarca VHF radar in the equatorial upper stratosphere(ISEA, 1995) Maekawa, Yasuyuki; Soichiro, Fukao; Yamamoto, Mamoru; Yamanaka, Manabu D.; Tsuda, Toshitaka; Kato, Susumu; Woodman Pollitt, Ronald FranciscoInternal gravity waves are known to play an important role in dynamics of the middle atmosphere. They can transport momentum flux from lower atmosphere to upper atmosphere and sometimes break mean flows or large-scale long-period motions due to dissipation process associatcd with wave flow interaction. However, an entire profile of short period gravity waves has not yet been obtained, since it has been very difficult to detect extremely weak atmospheric echoes from so-called "gap region" of 30-60 km heights. Recently, the Jicamarca VHF radar system in Peru ( Woodman and A.Guillen, 1974), which is one of the largest high-power VHF /UHF radars in the world, has been greatly improved in height resolution, and reliable atmospheric echoes have been, for the first time, detected in the gap regían by the Jicamarca radar with the high-altitude resolution (500 m). This paper presents new observational results on the upper stratospheric short-period vertical wind oscillations, which have never been resolved by the VHF /UHP. radars other than the present revised Jicamarca radar.Item Open Access Internal inertia-gravity waves in the tropical lower stratosphere observed by the Arecibo Radar(American Meteorological Society, 1984-08-01) Maekawa, Yasuyuki; Fukao, Schoichiro; Sato, Toru; Kato, Susumu; Woodman Pollitt, Ronald FranciscoHigh-resolution upper tropospheric and lower stratospheric (5-30 km) wind data were obtained during three periods from 1979 to 1981 with the aid ofthe high-power UHF radar at Arecibo, Puerto Rico (18.4°N, 66.8°W). A quasi-periodic wind oscillation with ai;t apparent period of 20-50 h was observed between 16 and 20 km in every experiment.The amplitude of both zonal and meridional wind components was ~2 m s-1, and the vertical wavelength ~2 km. The direction of the wind associated with this oscillation rotated clockwise with time, as seen for inertia-gravity waves in the Northern Hemisphere. The wave disappeared near 20 km where the mean zonal flow had easterly shear with height. This phenomenon is discussed in terms of wave absorption at a critical leve!. lt is suggested that the wave had a westward horizontal phase speed of 10-20 m s-1. The intrinsic period and the horizontal wavelength at the wave-generated height are, inferred to be 20-30 h and ~2000 km, respectively, from the relationship based on f plan e theory that the Doppler-shifted wave frequency approaches the Coriolis frequency at the critical level. The vertical group velocity estimated from the dispersion equation on the fplane closely agrees with the ascending rate of the observed wave packets at each height. In addition, each observation showed the presence of another type of oscillation with somewhat longer vertical wavelength in the lower stratosphere. If we assume the same intrinsic period and horizontal scale for this oscillation as for the abovementioned smaller vertical-scalé wave at the tropopause level, the observed period and vertical structure are well described in terms o( an internal inertia-gravity wave propagating to the opposite side in the horizontal plane.Item Restricted Mid‐latitude E region field‐aligned irregularities observed with the MU radar(American Geophysical Union, 1991-09) Yamamoto, Mamoru; Fukao, Shoichiro; Woodman Pollitt, Ronald Francisco; Ogawa, Tadahiko; Tsuda, Toshitaka; Kato, SusumuFine structures E region field-aligned irregularities were observed on June 24–25, 1989, with the MU radar at Shigaraki, Japan (34.9N, 136.1E; geomagnetic latitude 25.0N). The 3.2-m scale irregularities were observed with the MU radar in five main beam directions, each of which was nearly perpendicular to the geomagnetic field at 100 km altitude. Doppler spectra were obtained every 20 s with a range resolution of 600 m. Field-perpendicular echoes appeared from 2130 to 2330 LT and from 0400 to 1100 LT, times that correspond to postsunset and postsunrise period in the E region. A preliminary examination of the Doppler spectra indicates spectral widths of 50–120 m s−1 and the mean Doppler velocities are well below the ion acoustic speed. These spectral characteristics are consistent with those obtained in the equatorial and auroral electrojets, and have been attributed to the gradient drift instability. The echoes observed during the postsunset and postsunrise periods showed quite different morphologies in the time-height distribution. For this reason, they are classified into two types, ‘continuous’ and ‘quasi-periodic.’ The appearance of the ‘continuous’ echoes was mainly continuous in time and situated between 90 and 100 km altitude during the postsunrise period. The appearance of the ‘quasi-periodic’ echoes was intermittent with periods of 5–10 min and situated above 100 km altitude during the postsunset period. The quasi-periodic echoes showed phase propagation toward the radar, while the averaged mean Doppler velocity was away from the radar. By measuring the time delays in echo regions from five directions, an apparent westward motion (approximately 120 m s−1) of the irregularity regions was estimated.Item Restricted Three-antenna poststatistic steering using the MU radar(American Geophysical Union, 1990-11) Palmer, Robert D.; Woodman Pollitt, Ronald Francisco; Fukao, Shoichiro; Tsuda, Toshitaka; Kato, SusumuThis paper derives useful expressions for the synthesized beam statistics using poststatistic steenng (PSS), when three antennas are used in a spatial interferometer configuration for twodimensional steering. These antennas may have arbitrary position and can have nonorthogonal baselines. The use of three (noncollinear) antennas allows altitude dependent two-dimensional steering. This technique is verified using spatial interferometer data obtained in October 1989 using the MU radar, including PSS in directions parallel and perpendicular to the wind direction which is a very convenient advantage of the technique.Item Restricted VHF radar interferometry measurements of vertical velocity and the effect of tilted refractivity surfaces on standard doppler measurements(American Geophysical Union, 1991-03) Palmer, Robert D.; Larsen, Miguel F.; Woodman Pollitt, Ronald Francisco; Fukao, Shoichiro; Yamamoto, Mamoru; Tsuda, Toshitaka; Kato, SusumuAt VHF wavelengths, aspect sensitivity may result in an apparent beam direction that is off vertical even for a nominally vertically pointing beam direction if the refractivity surfaces responsible for the scatter are tilted with respect to the horizontal plane. Middle and upper atmosphere radar measurements obtained by using the system in a standard multireceiver configuration typical for radar interferometry (RI) and spaced antenna measurements have been analyzed for evidence of such effects. The analysis is based on the linear variation of the cross-spectral phase as a function of the radial velocity in the frequency doma.Jofor the RI cross spectra. I rue-verucal velocity escimates are obtained by using the fact that the phase difference between two antennas should be equal to zero when the echoes are being received from the vertical direction. The tilt angles of the refractivity surfaces were obtained from the phase of the cross-correlation function at zero lag, and the radial velocity in that direction was determined from the cross spectra. The results indicate that the vertical velocity derived from standard Doppler analyses is actually the velocity perpendicular to the refractivity surfaces and thus can be biased by the projection of the horizontal wind along the effective pointing direction.