Showing papers on "Weather radar published in 1999"

Cloud Microphysics Retrieval Using S-Band Dual-Polarization Radar Measurements

Abstract: Recent studies have shown the utility of polarimetric radar observables and derived fields for discrimination of hydrometeor particle types. Because the values of the radar observables that delineate different particle types overlap and are not sharply defined, the problem is well suited for a fuzzy logic approach. In this preliminary study the authors have developed and implemented a fuzzy logic algorithm for hydrometeor particle identification that is simple and efficient enough to run in real time for operational use. Although there are no in situ measurements available for this particle-type verification, the initial results are encouraging. Plans for further verification and optimization of the algorithm are described.

Polarimetry for Weather Surveillance Radars

Abstract: This paper is an overview of weather radar polarimetry emphasizing surveillance applications. The following potential benefits to operations are identified: improvement of quantitative precipitation measurements, discrimination of hail from rain with possible determination of sizes, identification of precipitation in winter storms, identification of electrically active storms, and distinction of biological scatterers (birds vs insects). Success in rainfall measurements is attributed to unique properties of differential phase. Referrals to fields of various polarimetric variables illustrate the signatures associated with different phenomena. It is argued that classifying hydrometeors is a necessary step prior to proper quantification of the water substance. The promise of polarimetry to accomplish classification is illustrated with an application to a hailstorm.

Aircraft weather information system

Abstract: A system and method that downlinks weather data, generated by existing weather and data sensors, to a groundstation. The groundstation utilizes data from multiple aircraft to form refined weather information, and uplinks the refined weather information to the aircraft. The refined weather information is stored at the aircraft and picture generating equipment, such as an existing on-board ground proximity terrain picture and symbol generator, generates pictorial information depicting the weather information. The pictorial information is displayed, for example by an existing EFIS or weather radar display, in the form of polygons.

An evaluation of NEXRAD precipitation estimates in complex terrain

Abstract: Next Generation Weather Radar (NEXRAD) precipitation estimates are used for hydrological, meteorological, and climatological studies at a wide range of spatial and temporal scales. The utility of radar-based precipitation estimates in such applications hinges on an understanding of the sources and magnitude of estimation error. This study examines precipitation estimation in the complex mountainous terrain of the northern Appalachian Mountains. Hourly digital precipitation (HDP) products for two WSR-88D radars in New York state are evaluated for a 2-year period. This analysis includes evaluation of range dependence and spatial distribution of estimates, radar intercomparisons for the overlap region, and radar-gage comparisons. The results indicate that there are unique challenges for radar-rainfall estimation in mountainous terrain. Beam blockage is a serious problem that is not corrected by existing NEXRAD algorithms. Underestimation and nondetection of precipitation are also significant concerns. Improved algorithms are needed for merging estimates from multiple radars with spatially variable biases.

Coordinate Transformations for Using NEXRAD Data in GIS-Based Hydrologic Modeling

Abstract: Gridded precipitation products created as part of the U.S. National Weather Service's Next Generation Weather Radar (NEXRAD) program are referenced to a national grid called the Hydrologic Rainfall Analysis Project (HRAP) grid, which is used to mosaic precipitation estimates from different radars into a single national precipitation map. The HRAP grid is defined in a polar stereographic map projection that is formed on a plane intersecting a spherical earth datum at 60° N. This paper describes a method for transforming HRAP grid cells into a coordinate system commonly used for mapping geographic information system data sets, quantifies mapping errors associated with using the HRAP coordinate system, and outlines an approach to reduce these mapping errors. The NEXRAD radar rainfall processing software assumes that the earth is spherical rather than using a more accurate ellipsoidal representation. This assumption causes east-west distances to be distorted relative to north-south distances. The magnitude of...

A procedure to calibrate multiparameter weather radar using properties of the rain medium

Abstract: The joint distribution characteristics of size and shape of raindrops directly translate into features of polarization diversity measurements in rainfall. Theoretical calculations as well as radar observations indicate that the three polarization diversity measurements, namely, reflectivity, differential reflectivity, and specific differential propagation phase, lie in a constrained space that can be approximated by a three-dimensional (3D) surface. This feature as well as the vertical-looking observation of raindrops are used to determine biases in calibration of the radar system. A simple procedure is developed to obtain the bias in the absolute calibration from polarization diversity observation in rainfall. Simulation study as well as data analysis indicate that calibration errors can be estimated to an accuracy of 1 dB.

Real-Time Radar Rainfall Estimation. Part I: Algorithm Formulation

Abstract: A multicomponent radar-based algorithm for real-time precipitation estimation is developed. The algorithm emphasizes the combined use of weather radar observations and in situ rain gauge rainfall measurements. The temporal and spatial scales of interest are hourly to storm-total accumulations for areas of 4 km2 to approximately 16 km2. The processing steps include beam–height-effect correction, vertical integration, convective–stratiform classification, conversion from radar observables to rainfall rate, range-effect correction, and transformation of the estimated rainfall rates from polar coordinates to a Cartesian grid. Additionally, the algorithm applies advection correction to the gridded rainfall rates to minimize the temporal sampling effect and, subsequently, aggregates the corrected rainfall rates to 1-hourly, 3-hourly, and storm-total accumulations. The system applies different parameter values for convective and stratiform regimes. The calibration of the system is formulated as a global...

Comparing Mean Areal Precipitation Estimates from NEXRAD and Rain Gauge Networks

Abstract: Mean areal precipitation values (MAPX) derived from next generation weather radar (NEXRAD) stage III data are compared with mean areal precipitation (MAP) values derived from a precipitation gauge network. The gauge-derived MAPs are computed using Thiessen polygon weighting, whereas the radar-based MAPXs utilize the gridded stage III radar precipitation products that have been conditioned with gauge measurements and have been merged with overlapping radar fields. We compare over 4,000 pairs of MAPX and MAP estimates over a 3-year time period for each of eight basins in the southern plains reigon of the United States. Over the long term, mean areal estimates derived from NEXRAD generally are 5–10% below gauge-derived estimates. In the smallest basin, the long-term MAPX mean was greater than the MAP. For storm events, a slight tendency for NEXRAD to measure fewer yet more intense intervals of precipitation is identified. Comparison of hydrologic simulations using the two forcings indicates that significant differences in runoff volume can result. This work is aimed at providing insight into the use of a data product that is becoming increasingly available for public use. It also is aimed at investigating the use of radar data in hydrologic models that have been calibrated using gauge-based precipitation estimates.

Automatic storm finding weather radar

Abstract: An automatic storm finding weather radar is disclosed that uses a storm finding algorithm to automatically control the weather radar to eliminate manual control. The storm finding algorithm uses the 0° C. isotherm altitude where precipitation is most likely to first occur to calculate an altitude search layer to find storms. The storm finding algorithm calculates the antenna upper and lower tilt angles and the number of scans to search the altitude search layer. A list of useable antenna tilt values is formed to drive the antenna controller. The antenna controller scans the antenna at the lower tilt angle and then moves to the next tilt angle until the search is complete. The search is then repeated using any new data.

A Variational Method for Real-Time Retrieval of Three-Dimensional Wind Field from Multiple-Doppler Bistatic Radar Network Data

Abstract: A variational method for the retrieval of the 3D wind field from bistatic multiple-Doppler radar network data is developed, and its performance is evaluated. This bistatic network consists of one S-band weather radar and two passive low-gain receivers at remote sites. To allow for measurement error, the method uses the Doppler velocities of the three receivers as weak constraints and uses the continuity equation as a strong constraint in a cost function in which the two horizontal wind components are the control variables. Improvements are brought to the classical upward integration of the continuity equation, using a weighted combination of upward and downward integrations and its adjoint. A unique characteristic of a bistatic network is that all Doppler velocity measurements from individual resolution volumes are collected simultaneously, which minimizes the errors on the vertical wind component arising from the local evolution of the airflow. However, the time required to sample a complete wea...

Systematic Phase Codes for Resolving Range Overlaid Signals in a Doppler Weather Radar

Abstract: This paper deals with the recovery of Doppler velocities in the presence of range overlaid echoes. Transmitted pulses are phase shifted to tag the echoes from scatterers, which are separated by the unambiguous range. A new systematic phase code and an algorithm for estimating the mean velocities of overlaid first- and second-trip signals are presented. The return samples are phase corrected to cohere the first- or the second-trip signal, leaving the other signal power spread in a deterministic manner across the Doppler spectrum. An algorithm has been developed to recover the velocity of the weaker signal even if the power ratio of overlaid signals is as large as 40 dB, for spectrum widths of 4 m s−1 or less, and an unambiguous velocity of 32 m s−1. Tests on simulated weather signals indicate that the method, employed in surveillance Doppler radars, can effectively double the unambiguous range without the sacrifice of the unambiguous velocity interval.

A space and time model for design storm rainfall

Abstract: Realistic rainfields that represent storms with a known return period are required as input to design calculations for hydrological projects that cover a wide range of hydrological scales. The current standard practice is to assume either that the storm is uniform in time and space or that it varies in some very simple manner. Multiaffine models of rainfall, based on the concept of a multiplicative cascade, provide the possibility of generating a stochastic series of space and time rainfall that reproduces the observed behavior. The spatial distribution of a field of instantaneous rain rates is modeled using the multiplicative cascade approach. The temporal development of the cascade weights at each level in the cascade is modeled with a simple autoregressive ARMA(1,1) model where the parameters vary in a systematic manner with scale. The model is verified using rain fields produced by a monsoonal depression that passed over a weather radar at Darwin, Australia. Radar data for the event were used to estimate the model parameters. The model was able to reproduce the observed temporal and spatial correlation functions over a range of scales, and the probability distributions over a range of scales, for both the instantaneous and the hourly accumulations.

A Comparison of Radar Reflectivity Estimates of Rainfall from Collocated Radars

Abstract: Radar reflectivity–based rainfall estimates from collocated radars are examined. The usual large storm-to-storm variations in radar bias and high correlation between radar estimates and rain gauge observations are found. For three storms in Colorado, the radar bias factor (the ratio between gauge observations and radar estimates) with the National Center for Atmospheric Research’s S-band, dual-polarization radar (S-Pol) varied from 0.78 (an overestimate with radar) to 1.88. The correlation coefficient between gauge and radar amounts varied from 0.78 to 0.90. For a collocated Weather Surveillance Radar-1988 Doppler (WSR-88D), the bias factor varied from 0.56 to 1.49, and the correlation between gauge and radar amounts ranged from 0.77 to 0.87. In Kansas, bias factors varied from 0.86 to 1.41 for S-Pol (10 storms) and 0.82 to 1.71 for a paired WSR-88D (9 storms). The spread in correlation coefficients was 0.82–0.95 for S-Pol and 0.87–0.95 for the WSR-88D. Correspondence between the radar-derived ra...

NOTES AND CORRESPONDENCE A Comparison of Radar Reflectivity Estimates of Rainfall from Collocated Radars

Abstract: Radar reflectivity‐based rainfall estimates from collocated radars are examined. The usual large storm-tostorm variations in radar bias and high correlation between radar estimates and rain gauge observations are found. For three storms in Colorado, the radar bias factor (the ratio between gauge observations and radar estimates) with the National Center for Atmospheric Research’s S-band, dual-polarization radar (S-Pol) varied from 0.78 (an overestimate with radar) to 1.88. The correlation coefficient between gauge and radar amounts varied from 0.78 to 0.90. For a collocated Weather Surveillance Radar-1988 Doppler (WSR-88D), the bias factor varied from 0.56 to 1.49, and the correlation between gauge and radar amounts ranged from 0.77 to 0.87. In Kansas, bias factors varied from 0.86 to 1.41 for S-Pol (10 storms) and 0.82 to 1.71 for a paired WSR-88D (9 storms). The spread in correlation coefficients was 0.82‐0.95 for S-Pol and 0.87‐0.95 for the WSR-88D. Correspondence between the radar-derived rainfall estimates for the paired radars was very high; correlation coefficients were 0.88 to 0.98. Moreover, the ratio between rainfall estimates (S-Pol/paired WSR-88D) varied only from 0.72 to 0.85 in Colorado and 0.82 to 1.05 in Kansas. The total variation in radar-to-radar rainfall estimates, roughly a factor of 1.2, is attributed primarily to nonmeteorological factors relating to radar hardware and processing. The radar-to-radar variation is small compared to the spread in storm-to-storm biases, which varied from a low of 1.64 with the S-Pol radar in Kansas to a high of 2.66 with the WSR-88D in Colorado. For this investigation, the storm-to-storm bias must have a large meteorological component—probably due to temporal and spatial changes in drop size distributions and consequently variations in the relationship between radar reflectivity and rainfall rate.

Attenuation in Rain for X- and C-Band Weather Radar Systems: Sensitivity with respect to the Drop Size Distribution

Abstract: This paper is devoted to a sensitivity study of the equation describing attenuation effects in rain for ground-based weather radar systems operating at X- or C-band wavelengths. First, the so-called attenuation equation, also termed the HB solution or HB algorithm in reference to the well-known paper by Hitschfeld and Bordan, is recalled. A procedure aimed at obtaining consistent relations between average values of the equivalent reflectivity factor Ze, the attenuation coefficient k, and the rain rate R as function of two parameters of the drop size distribution (DSD) is also presented. Then, a numerical simulation framework based on a simple description of rainfall characteristics and accounting for some of the radar measurement features is proposed to test the ability of the HB algorithm to perform attenuation correction of hypothetical rain-rate profiles. In a first step, the well-known instability of the solution is illustrated. For instance, it is shown that, even in the absence of radar cal...

Use of Weather Radar for Combined Sewer Overflows Monitoring and Control

Abstract: In the Mediterranean region, climatological factors make combined sewer overflows a major urban pollution problem that should be monitored and controlled. Some online management solutions require the use of real-time models to forecast flows at the most sensitive points of the combined sewer systems. These models need a very good time and spatial resolution of the rainfall field, which is not sufficiently reproduced by a conventional rain-gauge network. This paper presents a case study where the use of rainfall estimates from weather radar pictures is compared with the use of a rain-gauge network in terms of the ability to predict sewer flows in an urban basin in Barcelona, Spain. The results show that the use of radar data enables the combined sewer systems model to improve the reproduction of observed flows, and they provide support for the idea that the spatial description of rainfall is a key problem in modeling the events giving rise to combined sewer overflows.

Rain Measurement in Hilly Terrain with X-Band Weather Radar Systems: Accuracy of Path-Integrated Attenuation Estimates Derived from Mountain Returns

Abstract: The authors recently showed that when attenuating wavelengths are used mountain returns may allow estimation of path-integrated attenuations (PIAs) between a ground-based weather radar and a given mountain, an application of the well-known Surface Reference Technique originally proposed for spaceborne radar configurations. This information proved to be valuable for the quantitative interpretation of X-band weather radar data in terms of rainfall rate for an urban hydrological application in Marseilles, France. In this paper, a further verification of this concept is presented with the comparison of mountain-derived PIAs and direct measurements obtained by means of a receiving antenna installed in the Balcons de Belledonne mountain ridge near Grenoble, France. Maximum PIAs in the range of 8–16 dB are obtained over the considered 9-km propagation path for various rain events observed between May and July 1997. A physical model of the mountain return power is developed leading to the formulation of ...

Polarization Diversity Pulse-Pair Technique for Millimeter-WaveDoppler Radar Measurements of Severe Storm Features

Abstract: The Polarization Diversity Pulse-Pair (PDPP) technique can extend simultaneously the maximum unambiguous range and the maximum unambiguous velocity of a Doppler weather radar. This technique has been applied using a high-resolution 95-GHz radar to study the reflectivity and velocity structure in severe thunderstorms. This paper documents the technique, presents an analysis of the first two moments of the estimated mean velocity, and provides a comparison of the results with experimental data, including PDPP images of high-vorticity regions in supercell storms.

Detection of anomalous propagation echoes in weather radar data using neural networks

Abstract: The authors investigate a neural network-based methodology for detection of the anomalous propagation (AP) radar echo The methodology is devised to cope with the situations when only single scan data are available The output of the procedure is quantified in four classes corresponding to the upper limits of 25, 50, 75, and 100% of AP echo per scan The high dimension of the input data space is reduced by feature extraction based on physical considerations Fractal based, statistical, and wavelet analyses are performed, and their characteristics are used as features A feedforward neural network is used for classification in the four classes, with a fuzzy strategy used in the network training The authors test the methodology on real data and make a comprehensive assessment of the procedure's accuracy based on cross validation

A Comparison of NEXRAD WSR-88D Radar Estimates of Rain Accumulation with Gauge Measurements for High- and Low-Reflectivity Horizontal Gradient Precipitation Events

Abstract: Radar-estimated rainfall amounts from the NEXRAD Weather Surveillance Radar precipitation accumulation algorithm were compared with measurements from numerous rain gauges (1639 radar versus gauge comparisons). Storm total rain accumulations from 43 rain events from 10 radar sites were analyzed. These rain events were stratified into two precipitation types: 1) high-reflectivity horizontal gradient storms and 2) low-reflectivity horizontal gradient events. Overall, the radar slightly overestimated rainfall accumulations for high-reflectivity gradient cases and significantly underestimated accumulations for low-reflectivity gradient cases. Varying degrees of range effects were observed for these two types of precipitation. For high-reflectivity gradient cases, the radar underestimated rainfall at the nearest ranges, overestimated at the middle ranges, and had fairly close agreements at the farthest ranges. A much stronger range bias was evident for low-reflectivity gradient cases. The radar underes...

Real-time urban drainage system modelling using weather radar rainfall data

Abstract: This paper introduces an urban drainage system (UDS) modelling ;approach called RHINOS (Real-time urban Hydrological Infrastructure and Output modelling Strategy) developed specifically for use in predictive realtime control (RTC)' applications. RHINOS uses quantitative distributed rainfall forecasts to predict pipe flows at locations throughout the pipe network. The paper highlights, with the use of a case study, the contribution weather radar systems can make to the RTC of UDS. The RHINOS procedure differs from existing urban hydrological modelling approaches in several ways including: (i) the way in which the UDS is represented; (ii) the parameter identification/estimation algorithm, and (iii) the techniques developed for flow prediction. RHINOS has been developed to produce supplementary information to any sewer telemetry system and enable global predictive control strategies to be implemented.

Aviation Weather Surveillance Systems: Advanced Radar and Surface Sensors for Flight Safety and Air Traffic Management

Abstract: This book provides a comprehensive introduction to the science, sensors, and systems that form modern aviation weather surveillance systems. Focusing on radar-based surveillance, it presents logical, incremental detail on the fundamentals of the various disciplines involved and their complex interplay. This includes giving a background to aviation systems and control, atmospheric and meteorological aspects, weather issues in relation to aviation, and broad coverage of modern aviation weather surveillance and information systems, including detailed material on Doppler weather radar, plus new generation atmospheric sensors.Aviation Weather Surveillance Systems is an excellent resource for civil and military aviation professionals; electronic engineers, especially those working in radar; meteorologists; pilots; and flight crews.

Relationship between the Scavenging Coefficient for Pollutants in Precipitation and the Radar Reflectivity Factor. Part II: Applications

Abstract: The power-law dependences between the scavenging coefficient Λ for pollutants in precipitation and the radar reflectivity factor Z, theoretically derived in Part I, are discussed here from the point of view of applications. Possible problems in their use are related mainly to the uncertain characteristics of the pollutants involved and to common error sources in weather radar measurements of precipitation. The greatest usefulness of the Λ–Z relationships probably is obtained when the hydrometeor population that is producing the radar signal is the same as the population that is scavenging the pollutants. Because radar estimates Z in real time, the relationships can be utilized in short-term forecasts of the cleansing effect of precipitation and of wet deposition. This use is illustrated in the current paper with the aid of radioactivity and radar measurements in Finland following the Chernobyl accident. The Λ–Z relationships yielded estimates of radioactive fallout that were in good agreement wit...

Simulations of mirror image returns of air/space-borne radars in rain and their applications in estimating path attenuation

Abstract: The mirror image (MI) rain echo, received through the double reflection of the radar pulse from the surface, may provide useful information in estimating the rainfall rate from airborne and spaceborne weather radars. However, because of the complicated scattering mechanisms involving the surface and rain and the relatively small amount of measured data, studies of the MI effect have been few. In this paper, a more rigorous model of the MI return power has been constructed that yields the co-and cross-polarized components of the MI and bistatic returns as a functions of the radar parameters (antenna beamwidth and radar altitude) and the scattering properties of the rain and surface. As a test of the model, the mirror image return, as estimated from theory, is compared with the measured MI range profile, and reasonably good agreement is obtained. For meteorological applications, algorithms for estimation of the rain path attenuation are developed based on the difference of the direct and MI returns at the same distance from the surface. The accuracies of the algorithms are analyzed and compared with those of the surface reference technique (SRT) through the simulations for airborne and spaceborne [the case of the Tropical Rain Measuring Mission (TRRM)] geometries.

Algorithm application to improve weather radar snowfall estimates for winter hydrologic modelling

Abstract: Algorithms were applied to weather radar data to improve the precipitation estimation for winter hydrologic modelling. The radar data were adjusted to consider the occurrence of mixed precipitation at above freezing air temperatures, the shape of snow particles, and a site specific scaling phenomenon. Radar data, uncorrected and corrected gridded gauge precipitation data were used as input to the linked WATFLOOD/CLASS hydrologic model for simulation of streamflow. WATFLOOD performed the horizontal water routing and CLASS performed the vertical energy and water budgeting. Modelling of the Grand River watershed that is within the coverage of the Atmospheric Environment Service C-band radar in King City, Ontario, Canada for the five winters from 1993 to 1997 illustrated that on average the adjusted radar images produced ± 15% of the observed runoff volumes whereas the corrected gauge precipitation yielded 35% less runoff than observed. Substantial seasonal variation was observed. Radar provided more realistic winter precipitation quantities for streamflow modelling than the corrected gauge data. Application of the algorithms improved upon the raw radar estimates.

Polarimetric radar covariance matrix algorithms and applications to meteorological radar data

Abstract: Characterization of backscatter from complex or nonstationary targets, which is partially polarized, is considered first. The possibility of constructing the covariance matrix from coherency matrix measurements is investigated, and examples using practical polarization diversity radar data from meteorological targets are given. From examining the practical data, it is found that, for this type of target, reflection symmetry exists, thus giving indication about the physical properties of the medium. In particular, this shows that models, in which symmetrical distributions of both canting angle and shapes are assumed, are sufficient. Using the obtained matrix, propagation effects are examined under that assumption. Since the theory developed is amenable to implementation using a unique microwave circuit, results should have direct practical applications.

Meteorological structures shape description and tracking by means of BI-RME matching

Abstract: Images from meteorological satellites or weather radars must be often interpreted at a higher level than simple pixel classification, and shape analysis with reliable and fast methods may be necessary. For instance, it is useful to analyze the temporal evolution of a rain event by means of a reliable tracking of the rain patterns at different scales. However, this task of tracking objects continuously changing their shapes is challenging. In this paper, the author shows how the application of a recently introduced numerical technique, called boundary integral-resonant mode expansion (BI-RME), to weather radar and meteorological satellite data could be used to achieve more information about the evolution in time of rain patterns or other meteorological structures of interest. In order to demonstrate the efficiency and robustness of the approach, several examples of image processing are considered. Applications to both operational tracking of clouds to produce wind fields and hurricane tracking are presented.

The Kalman filter in the estimation of area precipitation

Abstract: The aim of this paper is to provide appropriate statistical methods to obtain accurate area rainfall estimates based on both weather radar and raingauge observations. As known from previous work, a good way to combine the two types of measurements is via a state space representation and the use of the Kalman filter. In this paper a three step procedure is proposed. Also, several state space representations are discussed and a new method of estimation of the parameters is proposed. Finally, a practical study is made, where the results of different models are compared. Copyright © 1999 John Wiley & Sons, Ltd.

Control method for weather radar

Abstract: PROBLEM TO BE SOLVED: To realize a control method, for a weather radar, in which a change in weather conditions can be observed at high density when the weather conditions are changed quickly. SOLUTION: When predetermined weather conditions are satisfied, the scanning range of an antenna beam is limited to the range of an azimuth angle at which the weather conditions are detected. When the antenna beam is sector scanned in the limited range of the azimuth angle, the weather conditions can be observed only in its region more quickly as compared with a case in which a set-altitude PPI scanning operation is performed in all directions. Consequently, even when the weather conditions are changed quickly, e.g. thunderclouds are changed quickly, it is possible to realize a control method, for a weather radar, in which their change can be observed at high density.