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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.

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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.

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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.

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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.

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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.

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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.

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Taishi Hashimoto, Akinori Saito, Koji Nishimura, Masaki Tsutsumi, Kaoru Sato, and Toru Sato

Abstract

The Program of the Antarctic Syowa Mesosphere–Stratosphere–Troposphere/Incoherent Scatter (PANSY) radar is a large atmospheric radar located at the Antarctic Syowa Station (69.01°S, 39.59°E). The PANSY radar performed the first incoherent scatter (IS) measurements in the Antarctic region in 2015. Several specific observations were undertaken in 2017 including a 24-h observation of the ionosphere using a peripheral antenna array to suppress interference from the field-aligned irregularities (FAIs). This paper presents the preliminary results derived from the IS measurements using the PANSY radar and the adaptive signal processing techniques to suppress FAIs. The norm-constrained and directionally constrained minimization of power (NC-DCMP) algorithm was applied to the 24-h ionosphere observations by the PANSY radar with a weighting applied to the directional constraint based on the gain differences of the subarrays. When compared with the conventional nonadaptive approach, the number of usable power profiles was increased by about 24% by the gain-weighted NC-DCMP algorithm, suggesting its effectiveness for FAI clutter suppression in ionosphere observations. Furthermore, detection of FAIs using the dedicated antenna array was found valuable in assessing the reliability of estimations of electron density based on VHF-band IS radar data.

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Yoshihiro Tomikawa, Masahiro Nomoto, Hiroaki Miura, Masaki Tsutsumi, Koji Nishimura, Takuji Nakamura, Hisao Yamagishi, Takashi Yamanouchi, Toru Sato, and Kaoru Sato

Abstract

Characteristically strong vertical wind disturbances (VWDs) with magnitudes larger than 1 m s−1 were observed in the Antarctic troposphere using a new mesosphere–stratosphere–troposphere (MST) radar called the Program of the Antarctic Syowa MST/incoherent scatter (IS) Radar (PANSY) during 15–19 June 2012 at Syowa Station (69.0°S, 39.6°E). In the same period, two synoptic-scale cyclones approached Syowa Station and caused a strong wind event (SWE) at the surface. The VWDs observed during the SWE at Syowa Station had a nearly standing (i.e., no phase tilt with height) phase structure up to the tropopause and a power spectrum proportional to the − power of frequency. On the other hand, the observed VWDs were not associated with systematic horizontal momentum fluxes. Meteorological fields around Syowa Station during the SWE were successfully simulated using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). A strong VWD was also simulated at the model grid of 70.0°S, 40.0°E in NICAM, which had a standing phase structure similar to the observed ones. An analysis based on the Froude number showed that the simulated VWD was likely due to a hydraulic jump leeward of the coastal mountain ridge. The Scorer parameter analysis indicated that the observed VWDs at Syowa Station during 16–17 June 2012 were likely due to the hydraulic jump similar to that in NICAM. On the other hand, a possibility of lee waves was also suggested for the VWD observed on 18 June 2012.

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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.

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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.

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