Browsing by Author "Maekawa, Yasuyuki"
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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.