Tian-You Yu

Coherent radar imaging using Capon's method

A linearly constrained mathematical formulation is provided for the problem of coherent radar imaging. In contrast to studies of field-aligned irregularities in the ionosphere, where the technique has previously been applied, lower atmospheric imaging is complicated by the fact that the scattering structures are not aligned along any single baseline. As a result, a two-dimensional generalization of the brightness distribution was required. It is shown that Fourier-based imaging is a special case of this general formulation. Furthermore, an imaging technique based on constrained optimization is introduced and shown to exhibit higher resolution and resistance to interfering signals. These techniques were applied to data from the middle and upper atmosphere radar in Shigaraki, Japan. The experiment was conducted during the Baiu season, which is characterized by significant precipitation events.

Range imaging using frequency diversity

The need exists for measurements with high vertical resolution when observing the variety of atmospheric processes with extremely small vertical extent, such as microscale turbulence and scattering layers associated with inertia gravity waves. For example, recent in situ observations have shown that both humidity and temperature “sheets,” with thicknesses of the order of meters, exist throughout the lower atmosphere. Hampered by bandwidth constraints, however, standard pulsed radar systems have shown only limited usefulness in the detection of such phenomena. Frequency domain interferometry can be used to estimate the position and thickness of a single scattering layer within the resolution volume. Using two closely spaced frequencies, the method is derived under the restrictive assumption of a single, Gaussian-shaped layer. We will now introduce range imaging (RIM), which fully exploits the general advantages of frequency diversity. Using a set of closely spaced transmitter frequencies, a generalized method based on constrained optimization will be used to reconstruct high-resolution images of the average power density as a function of range. The technique will be studied using simulated radar data and will be shown to be capable of resolving complex structures similar to Kelvin-Helmholtz billows, which can be much smaller in vertical extent than the resolution volume.

Implementation and Validation of Range Imaging on a UHF Radar Wind Profiler

The available range resolution of pulsed radar wind profilers is usually limited by bandwidth restrictions. Range imaging (RIM) has recently been developed as a means of mitigating these limitations by operating the wind profilers over a small set of distinct transmitter frequencies. A constrained optimization method can then be used to generate high-resolution maps of the reflectivity field as a function of range. This paper presents a description of how the RIM technique has been recently implemented on the Platteville 915-MHz tropospheric profiler, the first such implementation at UHF. Examples of data collected during a two-part experiment on 10 April 2001 using the Platteville 915-MHz tropospheric profiler are presented. In the first part, an intercomparison was made involving measurements from RIM and standard radar techniques. It is shown that available frequency bandwidth can be very effectively utilized through the RIM processing. In the second part of the experiment, RIM was applied to radar observations collected with a short (0.5 ms) transmit pulse. The resulting data include observations of a thin, persistent scattering layer attributed to a subsidence inversion and billows from a Kelvin‐ Helmholtz instability. Estimates of the width of the layer were found to be as small as 12 m.

Observations of the 10 May 2010 Tornado Outbreak Using OU-PRIME: Potential for New Science with High-Resolution Polarimetric Radar

A tornado outbreak occurred in central Oklahoma on 10 May 2010, including two tornadoes with enhanced Fujita scale ratings of 4 (EF-4). Tragically, three deaths were reported along with significant property damage. Several strong and violent tornadoes occurred near Norman, Oklahoma, which is a major hub for severe storms research and is arguably one of the best observed regions of the country with multiple Doppler radars operated by both the federal government and the University of Oklahoma (OU). One of the most recent additions to the radars in Norman is the high-resolution OU Polarimetric Radar for Innovations in Meteorology and Engineering (OU-PRIME). As the name implies, the radar is used as a platform for research and education in both science and engineering studies using polarimetric radar. To facilitate usage of the system by students and faculty, OU-PRIME was constructed adjacent to the National Weather Center building on the OU research campus. On 10 May 2010, several tornadoes formed near the c...

Beam Multiplexing Using the Phased-Array Weather Radar

Abstract The recently installed S-band phased-array radar (PAR) at the National Weather Radar Testbed (NWRT) offers fast and flexible beam steering through electronic beam forming. This capability allows the implementation of a novel scanning strategy termed beam multiplexing (BMX), with the goal of providing fast updates of weather information with high statistical accuracy. For conventional weather radar the data acquisition time for a sector scan or a volume coverage pattern (VCP) can be reduced by increasing the antenna’s rotation rate to the extent that the pedestal allows. However, statistical errors of the spectral moment estimates will increase due to the fewer samples that are available for the estimation. BMX is developed to exploit the idea of collecting independent samples and maximizing the usage of radar resources. An improvement factor is introduced to quantify the BMX performance, which is defined by the reduction in data acquisition time using BMX when the same data accuracy obtained by a...

A simulation study of coherent radar imaging

Coherent radar imaging (CRI) is used in an attempt to overcome the angular resolution limitation of conventional single-station radars and is used to image the horizontal structure inside the resolution volume. This recently developed technique has been successfully applied to radar observations of the ionosphere as well as the lower atmosphere. However, no statistical analysis of the robustness of the various techniques has been presented to date. In this work, three CRI techniques are reviewed: Fourier-based, Capons, and maximum entropy (MaxEnt) methods. The Fourier-based method is the simplest of the three algorithms but has inherent resolution limitations. Although quite different in nature and performance, both Capons and MaxEnt methods can be posed as constrained optimization problems. A statistical comparison of performance of the three CRI techniques, using various receiver configurations and two distinct cases of scattering structure, is made using simulated data. The results show that the MaxEnt method exhibits the best performance in the case of aspect-sensitive scattering with a broad characteristic. In the localized scattering case, however, Capons method shows superior performance for signals with high signal-to-noise ratio (SNR), but MaxEnt method outperforms all methods for low SNR. In general, both Capons and MaxEnt methods are able to reproduce the gross characteristics of the scattering media under observation.

Atmospheric radar imaging using multiple‐receiver and multiple‐frequency techniques

Atmospheric radar imaging techniques have shown promise in revealing the fine-scale structure of the atmosphere within the resolution volume of the radar. Enhanced resolution can be obtained in both angle and range by using spaced receivers and shifted frequencies, respectively. The distinct techniques have been termed coherent radar imaging (CRI) for angular resolution enhancement and range imaging (RIM) for radial resolution improvement. Because of the mathematical similarities between CRI and RIM it is possible to derive a generalization of both techniques. In this work, the three-dimensional (3-D) imaging technique, which uses multiple receivers and multiple frequencies simultaneously, is developed for the first time. Three-dimensional imaging has the advantage of mitigating the limitations of beam width as well as pulse width of a conventional radar to simultaneously improve both angular and range resolution. It is shown that CRI and RIM are special cases of 3-D imaging. The mathematical problem is formulated as an inverse problem with solutions provided by the Fourier, Capon, and maximum entropy (MaxEnt) methods. These three 3-D imaging methods are verified and statistically tested through numerical simulations.

Refractivity Retrieval Using the Phased-Array Radar: First Results and Potential for Multimission Operation

In this paper, an investigation of the potential of rapid refractivity retrieval is presented. The retrieval technique utilizes radar phase measurements of ground clutter to derive near-surface refractivity, which has been commonly used as a proxy for humidity, given its close relation to vapor pressure. Surface humidity is an important meteorological parameter and has been known to play an important role in convective initiation. In this paper, the refractivity retrieval technique is exploited by using smaller numbers of samples for phase calculation, which is a fundamental process in refractivity retrieval. The impetus for this paper is to explore the possibility of rapid refractivity retrieval by exploiting the rapid beam-steering capability of a phased-array radar. Using the National Weather Radar Testbed in Norman, OK, a 64-pulse per radial raw-data set was collected for conventional refractivity processing. Then, subsets of the 64 samples were extracted to emulate shorter dwell periods and the corresponding more rapid experiments. The test cases that were considered are 2, 4, 8, 16, and 32 samples. Refractivity fields retrieved using smaller numbers of samples are compared against the reference field, which was obtained using the entire 64-sample data set. It will be shown that, statistically, significant refractivity fields can be obtained from as short as a two-sample dwell.

SOMARE-99: Observations of tropospheric scattering layers using multiple-frequency range imaging

Results from an experimental implementation of multiple-frequency range imaging (RIM) are presented. The technique exploits the benefits of frequency diversity to improve range resolution of atmospheric radar systems. The theory has been described in the literature, and simulations have proven its usefulness. Nevertheless, experimental results have been extremely limited. Over a 5-day period in May 1999 we conducted experiments using RIM on the sounding system (SOUSY) radar in northern Germany to observe the layered structure in the troposphere. The experiment is referred to as the SOUSY Multifrequency Atmospheric Radar Experiment 1999 (SOMARE-99). Estimates of range brightness produced by the RIM analysis provide insight about the layered structure of the atmosphere. The RIM results show distinct similarities to previous in situ measurements, which have shown sharp refractive index discontinuities to exist throughout the troposphere and stratosphere. Examples from selected time periods show layers modulated by possible short-period gravity waves or advection of periodic structures as well as other layers with apparent downward motion possibly caused by the progression of a warm front or large-scale subsidence.

The Atmospheric Imaging Radar: Simultaneous Volumetric Observations Using a Phased Array Weather Radar

AbstractMobile weather radars often utilize rapid-scan strategies when collecting observations of severe weather. Various techniques have been used to improve volume update times, including the use of agile and multibeam radars. Imaging radars, similar in some respects to phased arrays, steer the radar beam in software, thus requiring no physical motion. In contrast to phased arrays, imaging radars gather data for an entire volume simultaneously within the field of view (FOV) of the radar, which is defined by a broad transmit beam. As a result, imaging radars provide update rates significantly exceeding those of existing mobile radars, including phased arrays. The Advanced Radar Research Center (ARRC) at the University of Oklahoma (OU) is engaged in the design, construction, and testing of a mobile imaging weather radar system called the atmospheric imaging radar (AIR). Initial tests performed with the AIR demonstrate the benefits and versatility of utilizing beamforming techniques to achieve high spatial...