Jun-ichi Furumoto

Estimation of a Humidity Profile Using Turbulence Echo Characteristics

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–Vaisala frequency squared (N2), and the turbulence energy dissipation rate (ϵ). Because N2 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 M2 can be derived where a proportion constant should be calibrated by referri...

Characteristics of energy dissipation rate and effect of humidity on turbulence echo power revealed by MU radar-RASS Measurements

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.

Continuous Humidity Monitoring in a Tropical Region with the Equatorial Atmosphere Radar

Abstract A radar remote sensing technique that estimates humidity profiles using a wind profiler is applied to the equatorial atmosphere radar (EAR) to monitor detailed humidity variations in tropical regions. Turbulence echo power intensity is related to the vertical refractive index gradient squared (M2). Here, M is primarily determined by the vertical gradient of specific humidity in the lower troposphere. These relations are employed to estimate a humidity profile. EAR is a 47-MHz very high frequency (VHF) atmospheric radar installed at KotoTabang in West Sumatra, Indonesia. A humidity-profiling method, which was recently developed for the middle- and upper-atmosphere (MU) radar, was applied to EAR. The aim was to test this new method with a larger dataset observed in the tropical region, where detailed humidity variations have not been fully revealed. EAR observations were carried out in November 2002. Turbulence echoes from one vertical and four oblique beams were obtained with time and height resol...

Performance Analysis of Optimum Tilt Angle and Beam Configuration to Derive Horizontal Wind Velocities by Postset Beam Steering Technique

Software beam synthesis by postset beam steering (PBS) technique is used to derive the horizontal wind velocities from multireceiver phased-array radar data. In order to improve the efficiency of beam synthesis, the Capon beamforming method is used to synthesize time series signals in the desired line-of-sight angle within the radar transmit beam volume (3.6° beamwidth). This paper describes beam synthesizing in optimum directions to reduce the error in the measurement of horizontal wind velocities using the PBS technique. For this purpose, beams are synthesized at various tilt angles with multibeam configuration. From the synthesized beam, power spectral distribution is obtained by one of the subspace methods, i.e., eigenvector (EV). The mean Doppler frequency and radial wind velocity at the desired line-of-sight angle are derived by an adaptive-moment estimation method from an EV-produced power spectrum. Using the moments obtained in various radial directions, horizontal wind components are derived by the conventional method for wind profiler radars. Also, an analysis is done for the required number of beams to improve PBS-based wind estimation. The statistical comparisons of performances of various tilt angles and beam configurations are being studied using the data collected from a multireceiver phased-array radar system (middle and upper atmosphere radar, Shigaraki, Japan). The analyzed results show that beam synthesis at the tilt angle from 1.5 ° to 2.0° using a 16-beam configuration minimize the error in horizontal wind estimation by PBS technique.

The Variational Assimilation Method for the Retrieval of Humidity Profiles with the Wind-Profiling Radar

Abstract Recently, a humidity estimation technique was developed by using the turbulence echo characteristics detected with a wind-profiling radar. This study is concerned with improvement of the retrieval algorithm for delineating a humidity profile from the refractive index gradient (M) inferred from the echo power. To achieve a more precise estimate of humidity, a one-dimensional variational method is adopted. Because the radar data provide only the absolute value of M, its sign must be determined in the retrieval. A statistical probability for the sign of M [Pr(z)] is introduced to the cost function of the variational method to determine the optimum result with reduced calculation cost. GPS-derived integrated water vapor (IWV) was assimilated together with the radar-derived |M| for constraining the signs of |M| to agree with the radar-derived IWV and the GPS-derived IWV. Humidity profiles were retrieved from the Middle and Upper Atmosphere (MU) radar–Radio Acoustic Sounding System (RASS) data for July...

A Novel Approach to Mitigation of Radar Beam Weighting Effect on Coherent Radar Imaging Using VHF Atmospheric Radar

Multiple-receiver coherent radar imaging using very high frequency atmospheric radars is capable of imaging angular power distribution (termed brightness distribution) of the backscattered radar echoes with some inversion algorithms such as Capons method. The brightness distribution, however, is weighted by the radar beam weighting pattern. Modification of the brightness distribution with a simulated radar beam weighting pattern usually incurs spurious peaks around the edge of the distribution map. In view of this, an approach to mitigation of the radar beam weighting effect on brightness distribution is proposed, thereby giving more reliable estimates of echo center and brightness width. The proposed approach employs several pairs of symmetrically tilted radar beams to determine an effective weighting pattern of the radar beam that is adaptive to the signal-to-noise ratio (SNR) of the data, as well as the transmitting-receiving array configuration. Four radar experiments were carried out with the Middle and Upper atmosphere radar in Japan (34.85°N , 136.11°E) to demonstrate the proposed approach. One of the experiments was exhibited in more detail, and it showed the following: 1) Approximately 14% of the single-center cases turned into double-center situations; 2) the zenith angles of the corrected echo centers were larger than the original ones by ~0.75° on average; and 3) the brightness widths could be larger than the original ones by several degrees, depending on the SNR of the data. Based on these investigations, suitable corrections of the echo center and brightness width are expected to result in different estimates of some atmospheric parameters like scatterer anisotropy and tilt angle of the layer structure.

Effects of Atmospheric Stability on Wave and Energy Propagation in the Troposphere

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 te...

Denoising Atmospheric Radar Signals Using Spectral-Based Subspace Method Applicable for PBS Wind Estimation

This paper mainly focuses on the advantages of subspace-based eigenvector (EV) spectral estimator to improve the power spectrum and the quality of calculations in spectrum parameter estimation. In general, the spectrum produced by most of subspace methods is sharply peaked at the frequency of complex sinusoids. Although subspace methods exhibit the advantage of spectral resolution, the retrieval of the actual spectrum width is not well observed in many cases, compared with standard Fourier estimates. Several simulation works are carried out to determine the unknown order of the signal correlation matrix, which significantly helps in obtaining the equivalent Fourier spectrum using EV along with numerous advantages of the subspace method for better estimation of spectrum parameters. Such advantages are useful in precisely obtaining the atmospheric moments (Doppler frequency, spectrum width, etc.) from the synthesized beams required for wind estimation by the postset beam steering technique. In addition, the systematic improvements done in EV are much useful for complete wind profiling up to ~ 20 km with a temporal resolution of ~ 26 s, which is reported for the first time.

Measurement of Range-Weighting Function for Range Imaging of VHF Atmospheric Radars Using Range Oversampling

Multifrequency range imaging (RIM) used with the atmospheric radars at ultra- and very high-frequency (VHF) bands is capable of retrieving the power distribution of the backscattered radar echoes in the range direction, with some inversion algorithms such as the Capon method. The retrieved power distribution, however, is weighted by the range-weighting function (RWF). Modification of the retrieved power distribution with a theoretical RWF may cause overcorrection around the edge of the sampling gate. In view of this, an effective RWF that is in a Gaussian form and varies with the signal-to-noise ratio (SNR) of radar echoes has been proposed to mitigate the range-weighting effect and thereby enhance the continuity of the power distribution at gate boundaries. Based on the previously proposed concept, an improved approach utilizing the range-oversampled signals is addressed in this article to inspect the range-weighting effects at different range locations. The shape of the Gaussian RWF for describing the range-weighting effect was found to vary with the off-center range location in addition to the SNR of radar echoes—that is, the effective RWF for the RIM was SNR and range dependent. The use of SNR- and range-dependent RWF can be of help to improve the range imaging to some degree at the range location outside the range extent of a sampling gate defined by the pulse length. To verify the proposed approach, several radar experiments were carried out with the Chung-Li (24.98N, 121.18E) and middle and upper atmosphere (MU; 34.858N, 136.118E) VHF radars.

Measurement of Atmospheric Aspect Sensitivity Using Coherent Radar Imaging after Mitigation of Radar Beam Weighting Effect

AbstractThe aspect angle, a measurement of the aspect sensitivity of atmospheric refractivity irregularities, was estimated with multiple-receiver coherent radar imaging (CRI) of very high frequency (VHF) atmospheric radar. Two CRI parameters retrieved by Capon’s method were utilized to derive the aspect angle: brightness width from the vertical radar beam and the direction of arrival (DOA) of the echo center from the oblique radar beam. Differing from previous studies with CRI, the radar beam weighting effect on the CRI brightness distribution was considered, and moreover, the radar beamwidth used in study was adaptive to the signal-to-noise ratio (SNR) of data as well as the off-beam direction angle. The study is based on statistical results. It is shown that the brightness width, a representative of the aspect angle, obtained from the modified CRI brightness distribution of the vertical radar beam was generally larger than that without correction, and it was very close to the values derived from the DO...