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Hiroki Ando
,
Takeshi Imamura
, and
Toshitaka Tsuda

Abstract

Vertical wavenumber spectra of Martian gravity waves were obtained for the altitude range 3–32 km from temperature profiles acquired by the Mars Global Surveyor (MGS) radio occultation experiments. The spectra, which cover vertical wavelengths from 2.5 to 15 km, generally show a decline of the spectral density with wavenumber similar to those obtained in the terrestrial stratosphere and mesosphere. The power-law spectral index is typically around −3 except near the low-wavenumber end, and the spectra frequently lie along the theoretical spectrum for saturated gravity waves developed for Earth’s atmosphere. The results suggest that gravity wave saturation occurs in the atmosphere of Mars as well as that of Earth.

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Simon P. Alexander
and
Toshitaka Tsuda

Abstract

The first campaign-based measurements of virtual temperature in the upper-troposphere and lower-stratosphere (UTLS) region were made with the middle- and upper-atmosphere (MU) radar radio acoustic sounding system (RASS) during 4 days in August 1995. This dataset was examined in order to study high-frequency changes in the stability below 20 km, but especially in the UTLS region. Calculations of the WMO tropopause and cold-point tropopause heights showed the latter to be (1.0 ± 0.6) km higher, where 0.6 km is the standard deviation. A diurnal cycle of temperature and wind dominated the spectra, which was identified as the diurnal solar tide. Its phase maximum occurred in the afternoon between 5 and 15 km and showed upward energy propagation above this height. Changes in the UTLS kinetic energy dissipation rate ε showed significant high-frequency fluctuations embedded within layers that persisted for at least 1 day. Relative to the WMO tropopause height, the median ε increased from (0.5 ± 0.1) × 10−3 m2 s−3 in the upper troposphere to (0.7 ± 0.1) × 10−3 m2 s−3 in the lower stratosphere.

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Simon P. Alexander
,
Toshitaka Tsuda
, and
Junichi Furumoto

Abstract

The very high frequency (VHF) middle and upper atmosphere radar radio acoustic sounding system (MU-RASS) in Shigaraki, Japan, is able to provide tropospheric virtual temperature data with high temporal resolution on the order of a few minutes. The objective of this paper is to test the usefulness of MU-RASS as a tool for examining high-frequency changes in atmospheric stability and its effects on wave and energy propagation. For this study, temperature and wind data below 8-km altitude during a 2-day campaign period in October 2001 were used. A long-lasting inversion layer at 3.5-km altitude dominated the observation period. Large vertical wind perturbations with periods of less than 30 min were observed inside this inversion layer. Wavelet analysis was used to identify the dominant wave period for calculating the wind and temperature variances. The temperature variance characteristics exhibited a combination of the horizontal and vertical wind variance characteristics. In conclusion, the high temporal resolution of the MU-RASS enabled the study of short time-scale wind and temperature perturbations. These perturbations were related to the atmospheric stability, wave propagation, and energy in the troposphere, demonstrating the usefulness of the MU-RASS for this kind of study.

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Toshitaka Tsuda
,
Masaki Miyamoto
, and
Jun-ichi Furumoto

Abstract

This paper is concerned with the development of a new stratosphere–troposphere radar technique for determining a height profile of specific humidity (q) by using characteristics of clear-air echoes in the troposphere. Turbulence echoes have been observed with the middle and upper atmosphere (MU) radar in Shigaraki, Japan (34.8°N, 136.1°E), on 2–6 August 1995 and 29–30 July 1999. Applying the radio acoustic sounding system (RASS) technique to the MU radar, the detailed time–height structure of the virtual temperature was simultaneously measured as well.

The volume reflectivity (η) for turbulence echoes mainly depends on the refractive index gradient (M), the Brunt–Väisälä frequency squared (N 2), and the turbulence energy dissipation rate (ε). Because N 2 and ε can be estimated from the RASS temperature profiles and the spectral width of a Doppler spectrum of turbulence echoes, respectively, a linear relation between η and M 2 can be derived where a proportion constant should be calibrated by referring to a simultaneous radiosonde measurement.

On the other hand, radiosonde results indicate that M below about 10 km is mainly contributed first by dq/dz and second by the atmospheric temperature (T) and pressure. Then, a first-order differential equation between q and M can be derived. The sign of |M|, which is delineated from the MU radar observations of η, is inferred considering a correlation between dq/dz and N 2, where N 2 is measured with RASS. A q profile is analyzed from the MU radar–RASS observations in August 1995 and July 1999, assuming an upper boundary condition at 10 km. The inferred q profile agrees reasonably well with a simultaneous radiosonde result at 1.5–10 km.

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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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Jun-ichi Furumoto
,
Tomonori Shinoda
,
Atsushi Matsugatani
, and
Toshitaka Tsuda

Abstract

The present study applies the range imaging (RIM) technique to radio acoustic sounding system (RASS) measurements in an attempt to improve the vertical resolution of temperature profiles obtained by RASS measurements.

When an FM-chirped acoustic wave is used for RASS observations, the transmitted radio wave is backscattered from the section of the FM-chirped acoustic pulse where the Bragg condition between the radar and acoustic wavelengths is satisfied. The Bragg resonance region propagates upward with the upward movement of the acoustic pulse. To improve the height resolution of the RIM results, complex time series are extracted for an adequate period corresponding to the effective scattering length of the RASS echo.

In the RIM analysis for RASS echo, a model temperature profile is required to compensate the Doppler shift bias due to the shape of range-gate weighting. To remove this bias, an iteration algorithm was developed so that the temperature profiles of the previous RIM results could be used for the bias correction of the next RIM step.

The RIM technique was applied to RASS imaging measurements performed using the middle and upper atmosphere (MU) radar on 29–31 October 2006 in an attempt to improve the height resolution to approximately 60 m from the nominal range gate width of 150 m. The temperature profiles inside the radar range volume with a temporal resolution of 26 min were successfully retrieved. The detailed temperature structure, which cannot be revealed by conventional RASS observations, was clarified in the RIM results. In particular, the small-scale inversion layers inside the radar range gate were clearly revealed by the MU radar–RASS imaging data. These detailed temperature variations within the radar range gate agreed well with the simultaneous rawinsonde results.

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M. Joan Alexander
,
Toshitaka Tsuda
, and
Robert A. Vincent

Abstract

Knowledge of the latitudinal variations in the occurrence of gravity waves is important for their parameterization in global models. Observations of gravity waves with short vertical scales have shown a pronounced peak in wave activity at tropical latitudes. In this paper, it is shown that such a peak may be a natural consequence of the latitudinal variation in the Coriolis parameter, which controls the lower limit for gravity-wave intrinsic frequencies ω̂ . Two distinct but related effects of this parameter on observations of gravity-wave activity are explained and explored with a simple model. The results are also compared to observed latitudinal variations in gravity-wave activity. The authors formally distinguish between observed gravity-wave spectra and what is called gravity-wave “source spectra,” the latter being appropriate for input to gravity-wave parameterizations. The results suggest that the ω̂ −5/3 dependence of the gravity-wave energy spectrum commonly assumed as input to parameterizations is likely too steeply sloped. Much more shallowly sloped spectra for gravity-wave parameterization input ∝ ω̂ −0.6 ω̂ −0.7 show better agreement with observations. The results also underscore the potential importance of intermittency in gravity-wave sources to the interpretation of gravity-wave observations.

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David C. Fritts
,
Toshitaka Tsuda
,
Susumu Kato
,
Toru Sato
, and
Shoichiro Fukao

Abstract

Radial velocity and temperature data obtained at the MU Radar Observatory during October and November 1986 are used to examine the character of the motion spectrum in the troposphere and lower stratosphere. It is found that the spectrum is dominated by low-frequency gravity waves with an upward sense of propagation in the lower stratosphere and both upward and downward propagation in the troposphere. Vertical wavenumber spectra of velocity and temperature are used to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith et al. Results indicate excellent agreement of the observed and predicted velocity and temperature spectra in both amplitude and slope. Vertical wavenumber spectra in area-preserving form reveal a dominant vertical wavelength of ∼2.5 km, systematic variations in energy density and the dominant vertical scale with time, and consistency between the temporal variations of velocity and temperature variance. Taken together, our results provide strong support both for the view that velocity and temperature fluctuations are due primarily to internal gravity waves and for the saturated spectrum theory and its imposed constraints on wave amplitudes and spectral shape.

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Peter T. May
,
Tatsuhiro Adachi
,
Toshitaka Tsuda
, and
Richard J. Lataitis

Abstract

An experiment to observe the spatial distribution of radio acoustic sounding system (RASS) echo intensity and Doppler shift using the MU radar is described. Various transmitting configurations are used to confirm that the RASS signal is focused onto a diffraction limited spot approximately the size of the transmitting antenna, except when a very small transmitting array is used where turbulence acting on the acoustic wave smears the spot. The signal fades away from the central spot with values about 6-10 dB lower in intensity next to the main spot. Significant gradients of Doppler shift across the radar antenna are seen in the lower height ranges. This may result in errors as large as a degree in the RASS virtual temperature estimates when large radar antennas and a single acoustic source are used.

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Toshitaka Tsuda
,
Tatsuhiro Adachi
,
Yoshihisa Masuda
,
Shoichiro Fukao
, and
Susumu Kato

Abstract

Applying the RASS (radio acoustic sounding system) technique to the MU (middle and upper atmosphere) radar, profiles of both temperature and wind velocity were observed every 90 s in the height range of about 1.5–7.0 km, with a height resolution of 300 m, for about 40 h on 6–8 August 1990. The temperature profiles obtained with RASS agreed well with the virtual temperature derived from radiosonde sounding, where the mean difference between the temperature values was approximately 0.3°C. The observed frequency spectra above about 2.5-km altitude, having an asymptotic slope of −5/3 and approximately 0 for temperature and vertical wind velocity fluctuations, respectively, were reasonably consistent with a model spectrum of gravity waves. But, below 2.5 km, low-frequency components were conspicuously enhanced, especially for vertical wind velocity, presumably affected by convection. Wavelike temperature fluctuations with a dominant period of 6–8 h clearly showed downward phase progression and a π/2 phase lag between temperature and vertical wind velocity. In addition, short-period components were also recognizable for both temperature and vertical wind velocity fluctuations. However, for low-frequency components, which were sometimes enhanced at the lowest altitudes of the observation range, the time variations of temperature and vertical wind velocity were in phase. The covariance between temperature and vertical wind velocity was also determined, and heat flux profiles were further estimated. Although a major part of the fluctuations above 2.5 km could be explained by gravity waves, those below 2.5-km altitude seemed to be due to effects of convective motions in the planetary boundary layer.

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