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S Fukao - One of the best experts on this subject based on the ideXlab platform.
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Comparisons of refractive index gradient and stability profiles measured by balloons and the MU radar at a high vertical resolution in the lower stratosphere
Annales Geophysicae, 2007Co-Authors: Hubert Luce, G. Hassenpflug, M. Yamamoto, S FukaoAbstract:Many experimental studies have demonstrated that VHF Stratosphere-Troposphere (ST) radar Echo Power is proportional to the generalized refractive index gradient squared M2 when using a vertically oriented beam. Because humidity is generally negligible above the tropopause, VHF ST radars can thus provide information on the static stability (quantified by the squared Brunt-Väisälä frequency N2) at stratospheric heights and this capability is useful for many scientific applications. Most studies have been performed until now at a vertical resolution of 150 m or more. In the present paper, results of comparisons between radar- and (balloon borne) radiosonde-derived M2 and N2 are shown at a better vertical resolution of 50 m with the MU radar (34.85° N, 136.15° E; Japan) by benefiting from the range resolution improvement provided by the multi-frequency range imaging technique, using the Capon processing method. Owing to favorable winds in the troposphere, the radiosondes did not drift horizontally more than about 30 km from the MU radar site by the time they reached an altitude of 20 km. The measurements were thus simultaneous and almost collocated. Very good agreements have been obtained between both high resolution profiles of M2, as well as profiles of N2. It is also shown that this agreement can still be improved by taking into account a frozen-in advection of the air parcels by a horizontally uniform wind. Therefore, it can be concluded that 1) the range imaging technique with the Capon method really provides substantial range resolution improvement, despite the relatively weak Signal-to-Noise Ratios (SNR) over the analyzed region of the lower stratosphere, 2) the proportionality of the radar Echo Power to M2 at a vertical scale down to 50 m in the lower stratosphere is experimentally demonstrated, 3) the MU radar can provide stability profiles with a vertical resolution of 50 m at heights where humidity is negligible, 4) stable stratospheric layers as thin as 50 m or less have at least a horizontal extent of a few km to several tens of kilometers and can be considered as frozenly advected over scales of a few tens of minutes.
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Horizontal maps of Echo Power in the lower stratosphere using the MU radar
Annales Geophysicae, 2004Co-Authors: M. Hirono, Hubert Luce, Mamoru Yamamoto, S FukaoAbstract:In recent works, zenithal and azimuthal angle variations of Echo Power measured by VHF Stratosphere-Troposphere (ST) radars have been analyzed in detail using different radar multi-beam configurations. It was found that the azimuthal angle corresponding to maximum Echo Power is closely related to the direction of the horizontal wind shear. These properties indicate that local wind shear affects the tilt of the scatterers. Moreover, horizontal maps of Echo Power collected using a large set of beams steered pulse-to-pulse up to 40 degrees off zenith revealed that the Power distribution pattern in the troposphere is often skewed. In this work, a three-dimensional description of Echo Power variations up to 24 degrees off zenith is shown for measurements in the lower stratosphere (i.e. up to approximately 20km) using a "sequential multi-beam" (SMB) configuration. Such a description was not possible above the tropopause with classical multi-beam configurations because of the loss of radar sensitivity due to the limited integration time by the use of a large number of beams. This work attempts to complete previous descriptions of the phenomenon by some observations in the lower stratosphere discussed in association with complementary balloon measurements. Key words. Meteorology and atmospheric dynamics (turbulence) – Radio Science (remote sensing)
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turbulence at the tropopause due to breaking kelvin waves observed by the equatorial atmosphere radar
Geophysical Research Letters, 2003Co-Authors: Masatomo Fujiwara, Hiroyuki Hashiguchi, Masayuki K Yamamoto, Takeshi Horinouchi, S FukaoAbstract:[1] The Equatorial Atmosphere Radar (EAR) installed in Sumatra Island, Indonesia, observed significant enhancement of turbulence in the tropopause region, 15–17 km, intermittently for ∼5 days in November 2001. The turbulence intensity was estimated with the spectral width of the radar Echo Power spectrum, and the turbulence during the period was a factor of up to ∼5 larger in kinetic energy than that in other periods. Further analyses confirm that the enhanced turbulence was convectively generated in the breaking phase of an equatorial Kelvin wave. Between July and December 2001, we observed at least three more prominent cases of the turbulence generation by breaking Kelvin waves in the tropopause region.
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rapid variations in Echo Power maps of vhf radar backscatter from the lower atmosphere
Journal of Atmospheric and Solar-Terrestrial Physics, 2000Co-Authors: R M Worthington, Robert D Palmer, S Fukao, Mamoru Yamamoto, Ivan AstinAbstract:Abstract For the first time, Echo Power maps of aspect-sensitive VHF backscatter are shown, with good time and spatial resolutions, for angles 0°–7° from zenith. Sequences of Power maps show large changes in appearance over timescales of a few minutes and height intervals of a few hundred metres. Often, individual Power maps are consistent with tilted and distorted specular-type scattering layers, rather than anisotropic turbulence, and the direction of maximum Echo Power is sometimes several degrees off-vertical. Nevertheless, after time-averaging the variable Echo-Power patterns, the average pattern can become almost circular and centred on zenith, as has been assumed in the past. Echo Power maps measured in strong windshear beneath the jet stream show a skewing of the Echo Power distribution. However, some Power maps in the lower stratosphere, despite stronger wind shear, appear more constrained and their maximum Echo Power remains closer to zenith.
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vertical eddy diffusivity in the lower and middle atmosphere a climatology based on the mu radar observations during 1986 1992
Journal of Atmospheric and Solar-Terrestrial Physics, 1996Co-Authors: S Kurosaki, Hiroyuki Hashiguchi, Toru Sato, Manabu D Yamanaka, S FukaoAbstract:Abstract Wind velocity variance due to turbulence in the inertial subrange (spatial scale: 100-102 m) has been estimated from a seven-year (1986–1992) database of the Echo Power spectral width observed by a VHF (46.5 MHz) Doppler radar (the MU radar) in Japan. We study a climatology of the vertical eddy diffusivity K in the tropo-stratosphere (altitude: 5–20 km) and mesosphere (60–90 km) which has been calculated from the observed wind velocity variance. We confirm completely the seasonal variabilities of K suggested by Fukao et al. (1994) based on a three-year database: an annual variation with winter maximum in the tropo-stratosphere, and a semi-annual variation with solstice maxima (plus a weak annual variation with summer maximum) in the mesosphere. In this paper we further point out some interannual variabilities of K with a dominant period of about two years, and discuss an overestimation of the median value of K near the tropopausal jet stream.
Peter Vary - One of the best experts on this subject based on the ideXlab platform.
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unbiased residual Echo Power estimation for hands free telephony
International Conference on Acoustics Speech and Signal Processing, 2002Co-Authors: Gerald Enzner, Rainer Martin, Peter VaryAbstract:Residual Echo arises in hands-free telephony equipment due to insufficient Echo canceler convergence, but can be suppressed using a postfilter. The most important control parameter for postfilter adaptation is therefore the residual Echo Power spectral density (PSD). In this contribution we present and compare residual Echo PSD estimation techniques. We introduce a new partitioned block-adaptive estimator delivering unbiased residual Echo PSD estimates in strongly reverberant and noisy acoustic environments.
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partitioned residual Echo Power estimation for frequency domain acoustic Echo cancellation and postfiltering
European Transactions on Telecommunications, 2002Co-Authors: Gerald Enzner, Rainer Martin, Peter Vary, Roland Auteur Du Texte MartinAbstract:Residual Echo arises in hands-free telephony equipment due to insufficient Echo canceler convergence, but can be suppressed using a postfilter. The residual Echo Power spectral density is the most crucial control parameter for both frequency-domain acoustic Echo cancellation and combined residual Echo and noise postfiltering. In this contribution we present and compare residual Echo Power spectral estimation techniques. We introduce a new partitioned block-adaptive estimation technique delivering considerably improved residual Echo estimates in strongly reverberant and noisy acoustic environments. We show that the adaptation loop of the frequency-domain adaptive filter (FDAF) can be used simultaneously for residual Echo Power estimation and tracking of the Echo path impulse response. In this way, the FDAF and the postfilter concept supplement each other in a true synergy with low complexity. The resulting Echo and noise control system proves to be robust in double talk situations as well.
Gerald Enzner - One of the best experts on this subject based on the ideXlab platform.
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unbiased residual Echo Power estimation for hands free telephony
International Conference on Acoustics Speech and Signal Processing, 2002Co-Authors: Gerald Enzner, Rainer Martin, Peter VaryAbstract:Residual Echo arises in hands-free telephony equipment due to insufficient Echo canceler convergence, but can be suppressed using a postfilter. The most important control parameter for postfilter adaptation is therefore the residual Echo Power spectral density (PSD). In this contribution we present and compare residual Echo PSD estimation techniques. We introduce a new partitioned block-adaptive estimator delivering unbiased residual Echo PSD estimates in strongly reverberant and noisy acoustic environments.
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partitioned residual Echo Power estimation for frequency domain acoustic Echo cancellation and postfiltering
European Transactions on Telecommunications, 2002Co-Authors: Gerald Enzner, Rainer Martin, Peter Vary, Roland Auteur Du Texte MartinAbstract:Residual Echo arises in hands-free telephony equipment due to insufficient Echo canceler convergence, but can be suppressed using a postfilter. The residual Echo Power spectral density is the most crucial control parameter for both frequency-domain acoustic Echo cancellation and combined residual Echo and noise postfiltering. In this contribution we present and compare residual Echo Power spectral estimation techniques. We introduce a new partitioned block-adaptive estimation technique delivering considerably improved residual Echo estimates in strongly reverberant and noisy acoustic environments. We show that the adaptation loop of the frequency-domain adaptive filter (FDAF) can be used simultaneously for residual Echo Power estimation and tracking of the Echo path impulse response. In this way, the FDAF and the postfilter concept supplement each other in a true synergy with low complexity. The resulting Echo and noise control system proves to be robust in double talk situations as well.
Jan M Rabaey - One of the best experts on this subject based on the ideXlab platform.
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“Echo” Reconfigurable Power Management Unit for Energy Reduction in Sleep-Active Transitions
IEEE Journal of Solid-State Circuits, 2013Co-Authors: Massimo Alioto, Elio Consoli, Jan M RabaeyAbstract:A novel reconfigurable switched-capacitor “Echo” Power Management Unit is introduced for ultra-low Power duty-cycled integrated systems (e.g., sensor nodes for critical event monitoring). “Echo” reduces the energy cost associated with sleep-to-active and active-to sleep transitions by 64% with an area overhead less than 1% and no impact on active mode operation. Analysis shows that approximately the same energy reduction is achieved over a very wide range of operating conditions and design constraints (e.g., ratio between flying and decoupling capacitances, granularity of the capacitor array). Measurements show 25-30% system Power saving for a 65-nm testchip implementation of the “Echo” PMU Powering a 16-kgate processing unit and a 2-kbit SRAM at 0.55-V voltage in active mode, assuming a 1-s wakeup cycle, 6.25-12.5% activity and a processing task of 250 cycles. The technique can be synergistically employed with traditional reconfiguration techniques that focus on efficiency in active mode (ignoring active-sleep transitions) to sum up the benefits.
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Echo Power management unit with reconfigurable switched capacitor converter in 65 nm cmos
Custom Integrated Circuits Conference, 2012Co-Authors: Massimo Alioto, Elio Consoli, Jan M RabaeyAbstract:In this paper, a novel reconfigurable Power management unit (PMU) is introduced. Its reconfigurable switched capacitor array permits for the first time to reduce the energy cost associated with sleep-to-active and active-to sleep transitions by 64%. This energy reduction comes at small area overhead (lower than 1%) and no penalty in active mode. Measurements on a 65-nm testchip comprising of PMU with integrated logic and memory show that energy savings enabled by our technique are in the order of 30% in practical cases.
Toshitaka Tsuda - One of the best experts on this subject based on the ideXlab platform.
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vertical structure of the lower troposphere derived from mu radar unmanned aerial vehicle and balloon measurements during shurex 2015
Progress in Earth and Planetary Science, 2018Co-Authors: Hubert Luce, Lakshmi Kantha, Masanori Yabuki, Tyler Mixa, Hiroyuki Hashiguchi, Dale Lawrence, Toshitaka TsudaAbstract:The ShUREX (Shigaraki UAV Radar Experiment) 2015 campaign carried out at the Shigaraki Middle and Upper atmosphere (MU) observatory (Japan) in June 2015 provided a unique opportunity to compare vertical profiles of atmospheric parameters estimated from unmanned aerial vehicle (UAV), balloon, and radar data in the lower troposphere. The present work is intended primarily as a demonstration of the potential offered by combination of these three instruments for studying the small-scale structure and dynamics in the lower troposphere. Here, we focus on data collected almost simultaneously by two instrumented UAVs and two meteorological balloons, near the MU radar operated continuously during the campaign. The UAVs flew along helical ascending and descending paths at a nearly constant horizontal distance from the radar (~ 1.0 km), while the balloons launched from the MU radar site drifted up to ~ 3–5 km in the altitude range of comparisons (~ 0.5 to 4.0 km) due to wind advection. Vertical profiles of squared Brunt-Vaisala frequency N2 and squared vertical gradient of generalized potential refractive index M2 were estimated at a vertical resolution of 20 m from pressure, temperature, and humidity data collected by UAVs and radiosondes. Profiles of M2 were also estimated from MU radar Echo Power at vertical incidence at a vertical sampling of 20 m and various time resolutions (1–4 min). The balloons and the MU radar provided vertical profiles of wind and wind shear S so that two independent estimates of the gradient Richardson number (Ri = N2/S2) could be obtained at a range resolution of 150 m. The two estimates of Ri profiles also showed remarkable agreement at all altitudes. We show that all three instruments detected the same prominent temperature and humidity gradients, down to decameter scales in stratified conditions. These gradients extended horizontally over a few kilometers at least and persisted for hours without significant changes, indicating that the turbulent diffusion was weak. Large discrepancies between N2and M2 profiles derived from the balloon, UAV, and radar data were found in a turbulent layer generated by a Kelvin-Helmholtz (KH) shear flow instability in the height range from 1.80 to 2.15 km. The cause of these discrepancies appears to depend on the stage of the KH billows.
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Characteristics of energy dissipation rate and effect of humidity on turbulence Echo Power revealed by MU radar-RASS Measurements
Journal of Atmospheric and Solar-Terrestrial Physics, 2001Co-Authors: Jun-ichi Furumoto, Toshitaka TsudaAbstract:Abstract We carried out an MU radar-RASS campaign during August 2–6, 1995, and collected both radar Echoes and virtual temperatures with good time resolution. We first investigated the accuracy of the Brunt–Vaisala frequency squared from the RASS temperature profiles, comparing them with radiosonde results launched from the radar site. We then evaluated the turbulence energy dissipation rate (e) by two radar methods: the Doppler spectral width and the radar Echo Power. Meteorological conditions were very calm throughout the campaign, which is ideal for deriving e by the spectral width method. Under calm conditions, the variation of e from spectral width is mainly influenced by the Doppler spectral width and is less dependent on the Brunt–Vaisala frequency. On the other hand, e from Echo Power is mainly determined by the Echo Power intensity. Though e values from both methods are roughly on the same order, their characteristics differ significantly. e with spectral width did not show large variations with time and height, while e with Echo Power showed significant fluctuations. We suspect that e from Echo Power is inaccurate because the refractive-index gradient in the calculation of e is mainly controlled by the humidity gradient in moist regions.
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zenith angle dependence of vhf specular reflection Echoes in the lower atmosphere
Journal of Atmospheric and Solar-Terrestrial Physics, 1997Co-Authors: Toshitaka Tsuda, T E Vanzandt, Hideya SaitoAbstract:Abstract We studied the characteristics of specular Echoes reflected from stratified layers in the troposphere and lower stratosphere. In particular, we observed Echoes at antenna-beam zenith angles, θ, from 0 ° to 28 ° in steps of 2 °. When the radar measurements were averaged over about 30 min, the zenith angle dependence of the Echo Power normalized by the vertical Power, S (θ) , was generally the same for sufficiently intense reflection Echoes. That is, the Echo Power was largest in the vertical direction, decreased to about − 10 dB at 6 °, and then gradually decreased to a constant level between − 15 and − 25 dB at θ ≥ 20 °. This constant level is interpreted as the isotropic turbulence scattering level S i . The width of S (θ) was significantly broader than expected for specular reflection from a perfectly horizontal layer. In order to explain this broadening, we developed two numerical models that describe statistically the slope of a reflection layer that has been distorted by vertical gravity-wave motions. With realistic gravity-wave spectra, the shape of S (θ) for θ = 0–6 ° was successfully explained. However, from 8 ° to 18 ° the observed S (θ) was enhanced by as much as 7 dB over the model. From the observations we showed that all of the reflection Echoes, including the enhanced Echoes at θ = 8–18 °, are probably due to the same process. Then we showed that the discrepancy with the model may be the result of our neglect of the horizontal component of gravity-wave motions, which was done in order to constrain the number of calculations.
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seasonal variability of vertical eddy diffusivity in the middle atmosphere 1 three year observations by the middle and upper atmosphere radar
Journal of Geophysical Research, 1994Co-Authors: S Fukao, Toshitaka Tsuda, Mamoru Yamamoto, Toru Sato, Manabu D Yamanaka, W K Hocking, Takuji Nakamura, Susumu KatoAbstract:The vertical eddy diffusivity K due to atmospheric turbulence with spatial scales of 100–102 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], Sato and Woodman [1982], and Hocking [1983a, 1985, 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.
