Tatsuhiro Adachi

Observations of Tropospheric Temperature Fluctuations with the MU Radar-RASS

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 p...

Diurnal variations of the planetary boundary layer observed with anL-band clear-air doppler radar

Based on continuous observations of the planetary boundary layer (PBL) with anL-band (1357.5 MHz) boundary-layer radar (BLR) at a hilly location in Japan, we have discovered that on clear days, a thin enhanced echo layer corresponding to the top of the PBL (or mixed layer) appeared at about 500 m height in the morning and ascended to about 1500 m in the afternoon. Strong upward velocities were observed below the echo layer (or inside the PBL), reaching 1500 m in the afternoon.

Frequency spectra of wind velocity and temperature fluctuations observed with the MU Radar‐RASS

MU radar-RASS observations were conducted on July 28–31, 1994 at Shigaraki, Japan (35°N, 136°E), to determine the detailed time variations of all three wind components and virtual temperature in the lower troposphere. We discuss their frequency spectra in comparison with a theoretical model of gravity waves. Assuming hydrostatic equilibrium, we also estimated the vertical wind velocity from the time derivative of temperature variations at each altitude. We found that the frequency spectra of the derived vertical wind were more consistent with a gravity wave model than those spectra calculated from direct measurements of the vertical wind.

Effects of the acoustic and radar pulse length ratio on the accuracy of radio acoustic sounding system (RASS) temperature measurements with monochromatic acoustic pulses

This paper is concerned with the accuracy of temperature measurements with radio acoustic sounding system (RASS) consisting of a pulsed Doppler radar and an acoustic source, where the latter excites short monochromatic pulses. Through the use of a numerical model and middle and upper atmosphere (MU) radar experiments, we found that the accuracy is significantly affected by the acoustic and radar pulse length ratio. When the Bragg condition is not strictly satisfied, a numerical model predicted that the mean frequency shift fm of a RASS echo spectrum is detected between the Doppler-shifted frequency corresponding to the sound speed fs and the transmitted acoustic frequency fa. When the ratio is close to or larger than unity, fm becomes almost identical with fa, while fm approaches fs as the ratio decreases. RASS experiments involving the MU radar operating at 46.5 MHz (6.45-m wavelength) and an acoustic transmitter with a frequency of about 100 Hz showed that the observed characteristics of RASS echoes for various acoustic pulse lengths agreed quite well with model predictions. Although the numerical model suggested that a small value of the ratio is preferable for accurate measurement of temperature with RASS, the minimum value of the ratio was determined to be about 0.2 by taking into account the system sensitivity of the MU radar, since the RASS echo intensity decreases as the acoustic pulse length becomes shorter. When the lengths of the acoustic and radar pulses were set equal to about 60 m (18 acoustic wave cycles) and 300 m (l μs in pulse duration), respectively, which gives a ratio of the acoustic pulse length to the radar pulse length of approximately 0.2, we were able to obtain temperature profiles at 5 to 9 km every 3 min with an accuracy of about 0.5°C.

MU radar observations of tropopause variations by using clear air echo characteristics

We present in this paper the characteristics of clear air echoes revealed by the MU radar experiments in Shigaraki, Japan (34° 51′ N, 136° 06′ E). In particular, we study a relation between atmospheric stability, represented by Brunt Väisälä frequency squared, N2, and both the vertical echo power and the aspect sensitivity. On August 24–25, 1991, echo power was collected by steering the zenith angle of the antenna beam of the MU radar from the zenith to 28° with a step of 2°. Aspect sensitive echoes were detected in the lower stratosphere and some regions in the troposphere. We found that a ratio of vertical echo power to oblique echo power at 10° can represent the magnitude of aspect sensitivity. We compared profiles of both the vertical echo power and the aspect sensitivity with N2, inferred from a radiosonde sounding at the radar site. Cross correlation analysis indicates that a rapid increase of both vertical echo power and the aspect sensitivity near the tropopause usually coincides with a step-wise enhancement of N2.We also analyzed four other MU radar observations, continued 4–5 days each, and obtained a statistical results that the mean CCF values between profiles of N2 and vertical echo power and the aspect sensitivity are 0.89 and 0.86, respectively. Their mean lag distance was about 71 m and 134 m, with a standard deviation of 77 m and 176 m, respectively. That is, the increase in the vertical echo power and the aspect sensitivity generally occurred slightly higher altitude relative to a sharp increase in N2. However, we also found some exceptional cases when the lag distance was as large as several hundred meters or even negative.

The Spatial Structure of RASS Echoes

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.

Analysis of the radio acoustic sounding system using a chirped acoustic wave

We investigated radio acoustic sounding system (RASS) echoes backscattered from refractive index fluctuations produced by an acoustic pulse linearly modulated in frequency (a chirped acoustic pulse). We have numerically simulated errors of the Doppler frequency shift which are caused by the fact that the widths of the acoustic and radar pulse are finite. We analytically showed that a RASS with a chirped acoustic pulse can correctly measure a wide range of Doppler shifts. The error of the Doppler shift measurement is found to be a function of the radar range resolution, the rate at which acoustic frequency is swept, and the width of the acoustic pulse. The error is also shown to be almost independent of the radar frequency and the lapse rates of the atmospheric temperatures. We have also numerically showed that when the chirped acoustic wave frequency is 50 Hz/s, errors are within 0.018°C when the radar frequency is 1357.5 MHz and the range resolution is 75 m. At the chirped wave frequency, errors are within 0.055°C for a 46.5-MHz radar with a 150-m range resolution. We also propose a method for designing a chirped acoustic pulse to measure atmospheric temperature.

Estimating wind speed in the lower atmosphere wind profiler based on a genetic algorithm

A lower atmosphere wind profiler, which is a pulse Doppler radar, is a useful operational tool that can provide unattended and continuous observation of wind profiles. Since the observed signals are usually degraded by ground clutters which are overlapped with the wind signal, it is necessary to develop a method to estimate the wind signal from the observed signals. In this paper, we propose to use a genetic algorithm to resolve the overlapping signals.

A real multi-parent tri-hybrid evolutionary optimization method and its application in wind velocity estimation from wind profiler data

Abstract A real-coded multi-parent tri-hybrid evolutionary algorithm (EA) for problem optimization is presented. The hybrid EA algorithm combines the features of Simplex, stochastic relaxation (SR) and multi-parent EA reproduction in a model that encourages competition among the best individual solutions from various operations. Its strength has been evaluated using standard test functions and shown to do better than other methods. The algorithm’s ability to handle noise is evident when applied to experiments involving resolution of overlapping wind profiler (WP) data. Results obtained using raw data closely matched those obtained with data preprocessed by a low-pass FFT filter. Resolution of low-speed wind and clutter signals in various degrees of overlap is made possible, thereby allowing the determination of wind velocity and variance to be executed with ease.