Showing papers by "Guifu Zhang published in 2019"

Current Status and Future Challenges of Weather Radar Polarimetry: Bridging the Gap between Radar Meteorology/Hydrology/Engineering and Numerical Weather Prediction

Abstract: After decades of research and development, the WSR-88D (NEXRAD) network in the United States was upgraded with dual-polarization capability, providing polarimetric radar data (PRD) that have the potential to improve weather observations, quantification, forecasting, and warnings. The weather radar networks in China and other countries are also being upgraded with dual-polarization capability. Now, with radar polarimetry technology having matured, and PRD available both nationally and globally, it is important to understand the current status and future challenges and opportunities. The potential impact of PRD has been limited by their oftentimes subjective and empirical use. More importantly, the community has not begun to regularly derive from PRD the state parameters, such as water mixing ratios and number concentrations, used in numerical weather prediction (NWP) models. In this review, we summarize the current status of weather radar polarimetry, discuss the issues and limitations of PRD usage, and explore potential approaches to more efficiently use PRD for quantitative precipitation estimation and forecasting based on statistical retrieval with physical constraints where prior information is used and observation error is included. This approach aligns the observation-based retrievals favored by the radar meteorology community with the model-based analysis of the NWP community. We also examine the challenges and opportunities of polarimetric phased array radar research and development for future weather observation.

Seasonal Variations of Observed Raindrop Size Distribution in East China

Abstract: Seasonal variations of rainfall microphysics in East China are investigated using data from the observations of a two-dimensional video disdrometer and a vertically pointing micro rain radar. The precipitation and rain drop size distribution (DSD) characteristics are revealed for different rain types and seasons. Summer rainfall is dominated by convective rain, while during the other seasons the contribution of stratiform rain to rainfall amount is equal to or even larger than that of convective rain. The mean mass-weighted diameter versus the generalized intercept parameter pairs of convective rain are plotted roughly around the “maritime” cluster, indicating a maritime nature of convective precipitation throughout the year in East China. The localized rainfall estimators, i.e., rainfall kinetic energy–rain rate, shape–slope, and radar reflectivity–rain rate relations are further derived. DSD variability is believed to be a major source of diversity of the aforementioned derived estimators. These newly derived relations would certainly improve the accuracy of rainfall kinetic energy estimation, DSD retrieval, and quantitative precipitation estimation in this specific region.

Raindrop Size Distributions and Rain Characteristics Observed by a PARSIVEL Disdrometer in Beijing, Northern China

Abstract: Fourteen-month precipitation measurements from a second-generation PARSIVEL disdrometer deployed in Beijing, northern China, were analyzed to investigate the microphysical structure of raindrop size distribution and its implications on polarimetric radar applications. Rainfall types are classified and analyzed in the domain of median volume diameter D 0 and the normalized intercept parameter N w . The separation line between convective and stratiform rain is almost equivalent to rain rate at 8.6 mm h−1 and radar reflectivity at 36.8 dBZ. Convective rain in Beijing shows distinct seasonal variations in log 10 N w – D 0 domain. X-band dual-polarization variables are simulated using the T-matrix method to derive radar-based quantitative precipitation estimation (QPE) estimators, and rainfall products at hourly scale are evaluated for four radar QPE estimators using collocated but independent rain gauge observations. This study also combines the advantages of individual estimators based on the thresholds on polarimetric variables. Results show that the blended QPE estimator has better performance than others. The rainfall microphysical analysis presented in this study is expected to facilitate the development of a high-resolution X-band radar network for urban QPE applications.

Recent Progress in Dual-Polarization Radar Research and Applications in China

Abstract: Dual-polarization (dual-pol) radar can measure additional parameters that provide more microphysical information of precipitation systems than those provided by conventional Doppler radar. The dual-pol parameters have been successfully utilized to investigate precipitation microphysics and improve radar quantitative precipitation estimation (QPE). The recent progress in dual-pol radar research and applications in China is summarized in four aspects. Firstly, the characteristics of several representative dual-pol radars are reviewed. Various approaches have been developed for radar data quality control, including calibration, attenuation correction, calculation of specific differential phase shift, and identification and removal of non-meteorological echoes. Using dual-pol radar measurements, the microphysical characteristics derived from raindrop size distribution retrieval, hydrometeor classification, and QPE is better understood in China. The limited number of studies in China that have sought to use dual-pol radar data to validate the microphysical parameterization and initialization of numerical models and assimilate dual-pol data into numerical models are summarized. The challenges of applying dual-pol data in numerical models and emerging technologies that may make significant impacts on the field of radar meteorology are discussed.

Dual-Polarization Antennas With High Isolation and Polarization Purity: A Review and Comparison of Cross-Coupling Mechanisms

Abstract: Existing dual-polarization antennas are categorized into three separate groups according to their topologies. Various cross-coupling mechanisms between orthogonal polarizations are specifically discussed in three categories and related to port isolation and cross-polarization radiation. Some representative antennas of each group are presented, and their performance is compared in terms of port isolation, cross polarization, frequency bandwidth (BW), dimension, and complexity. The techniques to enhance the polarization purity is discussed in the context of example antennas. Finally, further research opportunities in each category are introduced.

Ensemble Kalman Filter Assimilation of Polarimetric Radar Observations for the 20 May 2013 Oklahoma Tornadic Supercell Case

Abstract: Real polarimetric radar observations are directly assimilated for the first time using the ensemble Kalman filter (EnKF) for a supercell case from 20 May 2013 in Oklahoma. A double-moment m...

Microphysical Characteristics of Three Convective Events with Intense Rainfall Observed by Polarimetric Radar and Disdrometer in Eastern China

Abstract: Polarimetric radar and disdrometer observations obtained during the 2014 Observation, Prediction, and Analysis of Severe Convection of China (OPACC) field campaign are used in this study to investigate the microphysical characteristics of three primary types of organized intense rainfall events (meiyu rainband, typhoon outer rainband, and squall line) in eastern China. Drop size distributions (DSDs) of these three events on the ground are derived from measurements of a surface disdrometer, while the corresponding three-dimensional microphysical structures are obtained from the Nanjing University C-band polarimetric radar (NJU-CPOL). Although the environmental moisture and instability conditions are different, all three events possess relatively high freezing level favorable for warm-rain processes where the high medium to small raindrop concentration at low levels is consistent with the high surface rainfall rates. Convection is tallest in the squall line where abundant ice-phase processes generate large amounts of rimed particles (graupel and hail) above the freezing level and the largest surface raindrops are present among these three events. The storm tops of both the typhoon and meiyu rainbands are lower than that in the squall line, composed of less active ice processes above the freezing level. The typhoon rainrate is more intense than that of meiyu, enhanced by higher coalescence efficiency. A revised generalized intercept parameter versus mass-weighted mean diameter (Nw-Dm) space diagram is constructed to describe the DSD distributions over the three events and illustrate the relative DSD positions for heavy precipitation. DSDs of these intense rainfall convections observed in this midlatitude region of eastern Asia somewhat represent the typical DSD characteristics in low latitudes, suggesting that the parameterization of microphysical characteristics in eastern China in numerical models needs to be further investigated to improve rain fall forecasts in these heavy rainfall events.

Low Cross-Polarization, High-Isolation Microstrip Patch Antenna Array for Multi-Mission Applications

Abstract: The design and development of a high-performance, dual-polarization, hybrid aperture-coupled microstrip patch antenna are presented. In the presented design, the horizontal and vertical polarizations are excited using the aperture coupling method. The isolation level of better than 51 dB and cross-polarization level of less than −30 dB are achieved from the measurement results of the designed single element. To achieve better cross-polarization levels, the $2 \times 2$ element subarrays of the proposed unit cell, while the horizontal polarization ports are mirrored, are designed and fabricated, and a lower than −39-dB cross-polarization level is achieved in the measurement results. Finally, to characterize the scan performance of the unit cell and subarray, a $4 \times 10$ element array of the proposed single element is fabricated and a better than −45-dB cross-polarization level is observed while scanning up to 45°.

Phased-Array Radar System Simulator (PASIM): Development and Simulation Result Assessment

Abstract: In this paper, a system-specific phased-array radar system simulator was developed, based on a time-domain modeling and simulation method, mainly for system performance evaluation of the future Spectrum-Efficient National Surveillance Radar (SENSR). The goal of the simulation study was to establish a complete data quality prediction method based on specific radar hardware and electronics designs. The distributed weather targets were modeled using a covariance matrix-based method. The data quality analysis was conducted using Next-Generation Radar (NEXRAD) Level-II data as a basis, in which the impact of various pulse compression waveforms and channel electronic instability on weather radar data quality was evaluated. Two typical weather scenarios were employed to assess the simulator’s performance, including a tornado case and a convective precipitation case. Also, modeling of some demonstration systems was evaluated, including a generic weather radar, a planar polarimetric phased-array radar, and a cylindrical polarimetric phased-array radar. Corresponding error statistics were provided to help multifunction phased-array radar (MPAR) designers perform trade-off studies.

A Bayesian Hydrometeor Classification Algorithm for C-Band Polarimetric RADAR

Abstract: A hydrometeor classification algorithm is developed by applying Bayes’ theorem to C-band polarimetric weather radar measurements. The Bayesian hydrometeor classification algorithm (BHCA) includes eight hydrometeor types: hail, rain, graupel, dry snow, wet snow, crystal, biological scatterers (BS) and ground clutter (GC). The conditional likelihood probability distribution functions (PDFs) for each hydrometeor type are constructed with training data from radar observations. The prior PDFs include not only temperature information but also background information about occurrence frequency of hydrometeor types at each altitude, which is incorporated by a hydrometeor classification algorithm for the first time. The BHCA is evaluated by comparing with the Marzano-Bayesian hydrometeor classification algorithm (MBHC) and NCAR fuzzy logic classifier (NFLC). Results show that wet snow is largely missed in MBHC, while crystals are not adequately identified by NFLC. This may be due to the inappropriate conditional likelihood PDFs or membership functions. The prior PDFs in the MBHC may cause unexpected hail due to unreasonable variation above 0 °C. In addition, the prior PDFs of graupel and dry snow in the MBHC appear below −52 °C, which is not realistic. The BHCA proposed in this study overcomes these shortcomings in the prior PDFs and produces an overall reasonable classification product over the Yangtze-Huaihe River Basin (YHRB), Eastern China.

Uncertainty in Retrieving Raindrop Size Distribution from Polarimetric Radar Measurements

Abstract: Drop size distribution (DSD) is a fundamental parameter in rain microphysics. Retrieving DSDs from polarimetric radar measurements extends the capabilities of rain microphysics research and...

Ground Clutter Detection for Weather Radar Using Phase Fluctuation Index

Abstract: Our aim, in this paper, is to develop a clutter detection algorithm to provide more representative weather radar observations. The new discriminant function based on the phase fluctuation index (PFI) is introduced to achieve a better performance for clutter detection algorithms. Statistical properties of the PFI for pure weather and ground clutter are presented. A Bayesian classifier is used to make an optimal decision to detect clutter mixed with weather echoes. The performance improvements are demonstrated by applying the PFI detection algorithm to radar data collected by a WSR-88D polarimetric weather radar. Our proposed clutter detection algorithm is compared to several other detection algorithms and reveals the PFI algorithm yields the highest probability of detection.

Variational Retrieval of Rain Microphysics and Related Parameters from Polarimetric Radar Data with a Parameterized Operator

Abstract: A variational retrieval of rain microphysics from polarimetric radar data (PRD) has been developed through the use of S-band parameterized polarimetric observation operators. Polarimetric o...

Detection of Ground Clutter for Dual-Polarization Weather Radar Using a Novel 3D Discriminant Function

Abstract: A novel 3D discriminant function is introduced as part of a ground clutter detection algorithm for improving weather radar observations. The 3D discriminant function utilizes the phase fluc...

Evaluation of the Impacts of System Modules on Polarimetric Radar Data Quality Using a Phased Array Weather Radar System Simulator

Abstract: This paper combines a time-domain modeling and simulation method for evaluating the impacts of system modules on polarimetric data quality of phased array weather radars with both theoretical analysis and actual measurements. In the presented phased array radar system simulator (PASIM), the distributed weather targets are modeled by random multiple scattering centers, and Next-Generation Radar (NEXRAD) Level-II data or user-defined weather scenarios are utilized as weather truth fields. Besides, based on a generic patch element designed by the High Frequency Structural Simulator (HFSS), a specific dual-polarization phased array mobile demonstration system (Ten Panel Demonstrator, or TPD) is simulated. In addition, the biases of differential reflectivity, correlation coefficient and differential phase along beam direction away from broadside, in observation of uniform weather truth fields for both principal plane and non-principal plane are used as data quality metrics. It is shown that the simulation results are consistent with theoretical predictions, and measurements from TPD testbed in observation of a stratiform precipitation are presented, similarities and discrepancies between simulations and measurements are compared and explained.

Dual-polarised hybrid feed microstrip patch antenna for multifunction application

Abstract: In this Letter, a dual-polarised aperture-coupled microstrip patch antenna with low cross-polarisation and high isolation is presented. A microstrip patch antenna is designed and fabricated to meet the multifunction phased array radar requirements. The proposed single element is fabricated and the measurement results along with simulation results have been presented, showing the performance of the designed single element. An input isolation of better than 50 dB and a cross-polarisation level of <;-30 dB are achieved in the measurements.