Search Results

You are looking at 1 - 10 of 18 items for

  • Author or Editor: Toru Sato x
  • Refine by Access: All Content x
Clear All Modify Search
Koji Nishimura
,
Takuji Nakamura
,
Toru Sato
, and
Kaoru Sato

Abstract

Aspect-sensitive backscattering of the atmosphere causes a small error in an effective line-of-sight direction in vertical beam observations leading to a serious degradation of vertical wind estimates due to contamination by horizontal wind components. An adaptive beamforming technique for a multichannel mesosphere–stratosphere–troposphere (MST) radar is presented, which makes it possible to measure the vertical wind velocity with higher accuracy by adaptively generating a countersteered reception beam against an off-vertically shifted echo pattern. The technique employs the norm-constrained direction-constrained minimization of power (NC-DCMP) algorithm, which provides not only robustness but also higher accuracy than the basic direction-constrained minimization of power algorithm in realistic conditions. Although the technique decreases the signal-to-noise ratio, the ratio is controlled and bound at a specified level by the norm constraint. In the case that a decrease of −3 dB is acceptable in a vertical beam observation, for which usually a much higher signal-to-noise ratio is obtained than for oblique beams, the maximum contamination is suppressed to even for the most imbalanced aspect sensitivity.

Full access
Toru Sato
and
Ronald F. Woodman

Abstract

Preliminary results of wind velocity measurements made using the Arecibo 430 MHz radar are presented. These measurements were made in the altitude range between 10 and 30 km, with a time resolution of 1–2 min, and an improved altitude resolution of 150 m. A few interesting phenomena such as a quasi-stationary wavy structure and short period sinusoidal oscillations are discussed.

Full access
Toru Sato
and
Ronald F. Woodman

Abstract

Stratospheric turbulence is now observed by the Arecibo 430 MHz radar with an improved altitude resolution of 150 m. Turbulence is observed to occur in thin layers with a thickness usually less than the altitude resolution, and estimated to be about 50 m. A clear correlation is found between the power, wind shear and spectral width of the echoes. A simple method of estimating the energy dissipation rate and the eddy diffusivity is examined. Values of the order of 0.2 m2 s−1 are found for the eddy diffusivity coefficient in the lower stratosphere, showing the importance of turbulence on vertical transport.

Full access
Taishi Hashimoto
,
Koji Nishimura
,
Masaki Tsutsumi
,
Kaoru Sato
, and
Toru Sato

Abstract

This paper presents a novel method for the automatic determination of the diagonal-loading level for robust adaptive beamforming on radar wind profilers. This method balances the degradation of the signal-to-interference ratio with that of the signal-to-noise ratio to maximize the detectability of the backscattered signals. Because radial wind velocities are usually estimated from the first moment of the spectrum of backscattered echoes, both the residual ground clutter and any increase in noise level degrade the detectability of atmospheric echoes. The proposed algorithm evaluates the power spectral density of the residual clutter and increased noise to determine the optimal diagonal-loading level by balancing these two factors. The results of numerical simulation show that, without the need to specify any user parameters, the proposed algorithm is stable and more effective at maximizing the signal-to-interference ratio than the conventional norm-constrained diagonal-loading approach. The stability and clutter suppression capability of the proposed algorithm are examined using data from the Program of the Antarctic Syowa Mesosphere–Stratosphere–Troposphere/Incoherent Scatter Radar.

Full access
Ryosuke Shibuya
,
Kaoru Sato
,
Yoshihiro Tomikawa
,
Masaki Tsutsumi
, and
Toru Sato

Abstract

Multiple tropopauses (MTs) defined by the World Meteorological Organization are frequently detected from autumn to spring at Syowa Station (69.0°S, 39.6°E). The dynamical mechanism of MT events was examined by observations of the first mesosphere–stratosphere–troposphere (MST) radar in the Antarctic, the Program of the Antarctic Syowa MST/Incoherent Scatter (IS) Radar (PANSY), and of radiosondes on 8–11 April 2013.

The MT structure above the first tropopause is composed of strong temperature fluctuations. By a detailed analysis of observed three-dimensional wind and temperature fluctuation components, it is shown that the phase and amplitude relations between these components are consistent with the theoretical characteristics of linear inertia–gravity waves (IGWs).

Numerical simulations were performed by using a nonhydrostatic model. The simulated MT structures and IGW parameters agree well with the observation. In the analysis using the numerical simulation data, it is seen that IGWs were generated around 65°S, 15°E and around 70°S, 15°E, propagated eastward, and reached the region above Syowa Station when the MT event was observed. These IGWs were likely radiated spontaneously from the upper-tropospheric flow around 65°S, 15°E and were forced by strong southerly surface winds over steep topography (70°S, 15°E). The MT occurrence is attributable to strong IGWs and the low mean static stability in the polar winter lower stratosphere.

It is also shown that nonorographic gravity waves associated with the tropopause folding event contribute to 40% of the momentum fluxes, as shown by a gravity wave–resolving general circulation model in the lower stratosphere around 65°S. This result indicates that they are one of the key components for solving the cold-bias problem found in most climate models.

Full access
Taishi Hashimoto
,
Koji Nishimura
,
Masaki Tsutsumi
, and
Toru Sato

Abstract

Strong meteor trail echoes are interferences in the wind velocity estimates made from mesosphere radar observations. Contaminated spectra are detected by their discontinuity and are removed at the risk of greater fluctuations of spectra, leading to a severe reduction of the signal-to-noise ratio (SNR) and inaccurate wind estimates for weak atmospheric echoes. This paper presents an adaptive signal processing technique for the suppression of spectral contaminations by meteor trail echoes. The method is based on the norm-constrained and directionally constrained minimization of power (NC-DCMP), which balances the capability of canceling the clutter and the robustness of beam shaping, at the cost of a slight decrease in the SNR, which can be determined in advance. Simulation results show that with a 3-dB decrease of the SNR being allowed, the method improves the signal-to-interference ratio (SIR) by 15 dB, giving wind estimates that are about 8 m s−1 better in terms of root-mean-square error and providing 4 times as wide an observable range when compared with the results of the ordinary nonadaptive beamforming method. The results for an actual observation show that the improvement of both the SIR and the observable range are achieved as in the simulations, which implies that the method should provide the simulated accuracy for the estimation of wind velocity from actual observations.

Full access
Shoichiro Fukao
,
Manabu D. Yamanaka
,
Toru Sato
,
Toshitaka Tsuda
, and
Susumu Kato

Abstract

Upper-tmpospheric three dimensional air motions have been observed for the first time during the Baiu period in 1984 by using a 46.5 MHz Doppler radar in Japan. This radar, called the MU radar, operates with an antenna aperture of 8330 m2 and peak and average radiation powers of 1000 and 50 kW, respectively. It can steer the antenna beam up to 30° from the zenith in each interpulse period. With the aid of this fast beam steerability the MU radar can measure the three dimensional air motion. Resolutions in time and altitude of, the present observations are 100 s and 150 m, respectively. Referring to the routine rawinsande observations the following results are obtained on the air motion over the Baiu front: 1) the observed mean meridional motion is upward and northward as expected but deviates upward from the frontal surface and pseudo-isentropes, 2) the upper-tropospheric mesoscale wind variations are not strongly correlated with the lower-tropospheric frontal activity such as precipitation and 3) intense updrafts of 0.5–1 m−1 appear at an interval of approximately 22 h. This interval suggests that the updrafts are caused by neutral symmetric motion.

Full access
Shoichiro Fukao
,
Toru Sato
,
Norikazu Yamasaki
,
Robert M. Harper
, and
Susumu Kato

Abstract

Vertical profiles of horizontal winds in the lower stratosphere and upper troposphere were measured by the UHF Doppler radar at Arecibo, Puerto Rico (18.35°N, 66.75°W) on 26 days in August and September 1977. On comparing these with horizontal winds measured by routine rawinsonde balloons launched some 80 km east of Arecibo, fairly good agreement between every wind profile can be seen. Most of the difference between the two sets of measurements in the lower stratosphere is shown to be caused by the experimental error of the rawinsonde, while the spatial and/or temporal variations in the wind field seem to dominate the difference in the upper troposphere.

Full access
Yasuyuki Maekawa
,
Shoichiro Fukao
,
Toru Sato
,
Susumu Kato
, and
Ronald F. Woodman

Abstract

High-resolution upper tropospheric and lower stratospheric (5–30 km) wind data were obtained during three periods from 1979 to 1981 with the aid of the high-power UHF radar at Arecibo, Puerto Rico (18.4°N, 66.8°W). A quasi-periodic wind oscillation with an 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 level. It 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-plane 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 f-plane 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-scale wave at the tropopause level, the observed period and vertical structure are well described in terms of an internal inertia–gravity wave propagating to the opposite side in the horizontal plane.

Full access
Shoichiro Fukao
,
Toru Sato
,
Norikazu Yamasaki
,
Robert M. Harper
, and
Susumu Kato

Abstract

Wind oscillations of tidal periods that showed a marked downward phase progression were detected at the lower stratosphere using the Arecibo radar. The amplitudes of 1–5 m s−1 were inferred for both diurnal and semidiurnal components, much larger than the values predicted by the classical tidal theory. The vertical wavelengths inferred were also less than the theoretical values; ∼5 km for the diurnal component and 2–9 km for the semidiurnal component.

Full access