Showing papers on "Weather radar published in 1989"

Apparatus and method for detecting and indicating weather conditions for aircraft

Abstract: An apparatus and a method for detecting and indicating severe weather conditions such as wind shear and clear air turbulence includes a sensor for detecting the weather parameter of air temperature differential and a computer for comparing the parameter value with a stored constant value. The sensor can be an infrared scanner in an aircraft or on the ground. When the parameter value exceeds the constant value, a severe weather condition warning indication is generated by the computer as a visual and/or audio signal. The method and apparatus can also include sensor modules for detecting wind speed and direction, barometric pressure and air temperature to be compared by the computer. A steam generator can generate steam bubbles for detecting winds aloft with the infrared scanner or weather radar.

Development of an automated windshear detection system using Doppler weather radar

Abstract: The US Federal Aviation Administration (FAA) is developing the Terminal Doppler Weather Radar (TDWR) system to determine the location and severity of LAWS (low-altitude windshear) phenomena and other weather hazards (e.g. tornadoes and turbulence) and to provide the pertinent information to real-time air traffic control users. The FAA program for developing and evaluating the TDWR is described, with emphasis on the resolution of key technical issues such as separation of the radar return due to the low-altitude weather phenomena from that caused by various clutter sources and the automatic detection of the phenomena by means of pattern recognition applied to images depicting the weather reflectivity and Doppler shift. These technical issues have been addressed using experimental data obtained using a testbed radar in representative meteorological regimes. The system performance has been assessed using numerous experimental windshear data sets with corresponding 'truth' developed by experienced radar meteorologists from a number of organizations. It is shown that the system provides very reliable detection of strong microbursts in a variety of environments with a gust-front detection capability that supports effective planning of airport runway use. >

Meteorological Considerations Used in Planning the NEXRAD Network

Abstract: A systematic and objective approach was used to optimize the siting of the individual radars forming the Next Generation Weather Radar (NEXRAD) network. Prime consideration was given to meteorological factors, in conjunction with the user agencies' needs and the population distribution. The latter was assessed by a novel technique using weather satellite photographs showing urban illumination at night. Priority coverage areas were identified for population centers based on the expected paths of storms and their travel speeds. Radar viewing of the priority coverage areas down to low altitudes is needed so that approaching storms can be detected and warnings issued as early as possible. Other siting criteria taken into account included consideration of terrain features and local obstructions, locations of airways and civilian and military airports, electromagnetic interference, and integration of NEXRAD data into the national weather system. The methodology for selecting the network is described. Environmen...

Thunderstorm-Generated Solitary Waves: A Wind Shear Hazard

Abstract: Observations of a boundary-layer solitary wave sensed with the National Severe Storms Laboratory's Doppler weather radar and a 444-m-tall instrumented tower suggest that solitary and other nonlinear waves are a source of significant wind shear hazard to safe flight and thus should be studied both experimentally and theoretically. Wave transport of the horizontal momentum of the vertically sheared ambient air contributed much to the observed wind perturbations and horizontal wind shear. Observations are compared with, and shown to agree fairly closely with, the waveform predicted by steady state, weakly nonlinear, internal wave theory.

Areal reduction factors for design storm construction: Joint use of raingauge and radar data

Abstract: The paper describes an analysis of the spatial variability of rainfall in an upland region of the UK using both raingauge and weather radar data. Two differing methodologies are outlined for the re-evaluation of areal reduction factors in the study area for a range of durations. Areal reduction factors for durations of 1, 2, 4 and 8 days are presented which are slightly lower than those currently accepted as applicable throughout the UK. Areal reduction factors are found to decrease with return period, but to vary little with location within the study area. Preliminary results of the analysis of radar data are presented. BACKGROUND TO THE STUDY Analysis of the spatial and temporal variability of storm rainfall allows design storms to be constructed. These provide the fundamental design criteria for engineering projects concerned with river management and drainage works. This paper presents a study of the spatial variations of extreme rainfall in an upland region of England, with reference to the applications of this work to flood hazard assessment and reservoir safety. Details of an analysis of raingauge data for rainfall durations of 1 to 8 days are given. In addition, the use of radar data in providing areal rainfall measurements of high temporal and spatial resolution is illustrated, and preliminary results of an analysis of short duration rainfalls are presented.

The FLOWS Automatic Weather Station Network

Abstract: This report describes in detail the FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) automatic weather station network which is being used in the Terminal Doppler Weather Radar program to assess the radar detectability of wind shear and to help gain an understanding of microburst forcing mechanisms. The weather stations are descended from the PROBE stations originally operated by the Bureau of Reclamation. The current instrumentation has been modified slightly but is largely the same as that originally used as is the hardware structure, but the data collection platforms are entirely new. Each station in the 30-station network transmits 1 min averages of temperature, relative humidity, barometric pressure, wind speed, wind direction and precipitation amounts, as well as peak wind speed, on a single GOES satellite channel. Performance results from the first 3 yr (1984–86) of mesonet operations are presented. During June and July 1986 the FLOWS network was collocated with the NCAR PAM-II n...

Advances in weather technology for the aviation system

Abstract: It is affirmed that there is a critical need for the development of aviation weather warning and forecast system products that give attention to storm-scale weather. This specifically addresses weather events that occur on the 0-2 h time and 0.5 mile space scales, which are significantly smaller than the scales used in current weather systems. Through a series of weather sensor modernization systems, including Doppler weather radar vertical wind profiling radars, automated surface weather-sensing stations, and advanced weather satellites, the capability to obtain high-resolution advanced weather products will be enhanced. Examples both of deficiencies in the current aviation weather system and of expected advanced products in a modernized weather system are given. For example, a precise 30-60 min thunderstorm point forecast should become available during the next decade. In addition, a series of fully automatic weather hazard warning products, including those for low-altitude wind shear, tornadoes, and turbulence, will become available for air traffic controllers and flight crews. It is noted that, once validated advanced weather products are routinely made available, it is incumbent upon the operational users of the aviation system to develop more definitive means of making operational decisions that improve the safety and efficiency of the system. >

Doppler compensated airborne weather radar system

Abstract: An airborne weather radar system having a capability of compensating for variable Doppler shift caused by the forward motion of the aircraft and the relative motion of the antenna sweep which includes a local oscillator having a single low phase noise crystal which is operated in its fundamental mode at a predetermined frequency for increasing the pullability away from the tuned frequency of the local oscillator.

Studies of Precipitation Processes in the Troposphere Using an FM-CW Radar

Abstract: In the field of FM-CW tropospheric radar, new breakthroughs have become possible due to progress in FM-CW radar hardware, in FM-CW radar signal processing and in the availability of the Doppler polarimetric FM-CW radar DARR. DARR has a high resolution in range, Doppler and polarization. Doppler resolution of a few centimeters per second combined with a maximum Doppler range of ±9 meters per second can be obtained. Two mechanical polarizers, located directly behind the center feeds of the transmit and receive antenna, are employed, operating over polarization angles or ±90 degrees. Measured Doppler and polarimetric reflectivity profiles are shown. The melting process during stratiform rain and the raindrop size distribution have been modeled and verified, based on these profiles.

Adaptive clutter rejection filters for airborne Doppler weather radar applied to the detection of low altitude windshear

Abstract: An optimum adaptive clutter rejection filter for use with airborne Doppler weather radar is presented. The radar system is being designed to operate at low-altitudes for the detection of windshear in an airport terminal area where ground clutter returns may mask the weather return. The coefficients of the adaptive clutter rejection filter are obtained using a complex form of a square root normalized recursive least squares lattice estimation algorithm which models the clutter return data as an autoregressive process. The normalized lattice structure implementation of the adaptive modeling process for determining the filter coefficients assures that the resulting coefficients will yield a stable filter and offers possible fixed point implementation. A 10th order FIR clutter rejection filter indexed by geographical location is designed through autoregressive modeling of simulated clutter data. Filtered data, containing simulated dry microburst and clutter return, are analyzed using pulse-pair estimation techniques. To measure the ability of the clutter rejection filters to remove the clutter, results are compared to pulse-pair estimates of windspeed within a simulated dry microburst without clutter. In the filter evaluation process, post-filtered pulse-pair width estimates and power levels are also used to measure the effectiveness of the filters. The results support the use of an adaptive clutter rejection filter for reducing the clutter induced bias in pulse-pair estimates of windspeed.

Experimental tests of range-profiling algorithms

Abstract: The natural tendency in the design of a spaceborne weather radar is to select frequencies at 10 GHz or higher. A major disadvantage of this approach is that increased attenuation limits the maximum rain rate that can be accurately estimated. Here, a way to lessen this problem is suggested which uses estimates of path attenuation to reconstruct a profile of the rain rate.

The effects of clutter-rejection filtering on estimating weather spectrum parameters

Abstract: The effects of clutter-rejection filtering on estimating the weather parameters from pulse Doppler radar measurement data are investigated. The pulse pair method of estimating the spectrum mean and spectrum width of the weather is emphasized. The loss of sensitivity, a measure of the signal power lost due to filtering, is also considered. A flexible software tool developed to investigate these effects is described. It allows for simulated weather radar data, in which the user specifies an underlying truncated Gaussian spectrum, as well as for externally generated data which may be real or simulated. The filter may be implemented in either the time or the frequency domain. The software tool is validated by comparing unfiltered spectrum mean and width estimates to their true values, and by reproducing previously published results. The effects on the weather parameter estimates using simulated weather-only data are evaluated for five filters: an ideal filter, two infinite impulse response filters, and two finite impulse response filters. Results considering external data, consisting of weather and clutter data, are evaluated on a range cell by range cell basis. Finally, it is shown theoretically and by computer simulation that a linear phase response is not required for a clutter rejection filter preceeding pulse-pair parameter estimation.

Intercomparison of radar measurements of rain by single- and dual-wavelength techniques

Abstract: We are experimenting with three different techniques of rain rate estimation to assess their appropriateness by analyzing the data on simultaneous rain measurements with a C band weather radar and a dual-wavelength (3- and 0.86-cm) radar scatterometer. These techniques are (1) a conventional single-wavelength technique based on an empirical relation between a reflectivity factor and a rain rate; (2) a dual-wavelength technique based on radar wave attenuation at one wavelength, where it is assumed that the raindrops sampled are Rayleigh scatterers at both wavelengths; and (3) an alternative dual-wavelength technique using the method of least squares to minimize the difference between measurement-derived quantities and their corresponding quantities derived from a radar equation. The rain rate estimates from the above three techniques are compared with each other, and the causes of their differences are discussed. The rain rate estimates obtained with each technique and gauge rain rates measured near the radar site are also compared to evaluate their absolute accuracy. The advantages and disadvantages of each rain rate estimation technique are discussed for specific situations and applications. As a result of investigation the following conclusions are obtained: The single-wavelength technique gives fairly good rain rate estimates if an external radar calibration is made accurately. The dual-wavelength technique based on radar wave attenuation needs verification of the estimates by auxiliary means to avoid unrealistic estimates, and the dual-wavelength technique by means of a least squares method is useful for an airborne or a spaceborne application where an external calibration is difficult to perform.

Radar data processing for hydrology in the Cevennes region

Abstract: This paper describes a specific experiment called "Cevennes 86-88", designed to evaluate weather radar capabilities for a flash flood early warning system in a mountainous area. A general description of the experiment is provided, with an overview of the hydrological context and details concerning the radar characteristics and the collected data set. The different methods used to process the radar data on an hourly time step are presented together with results illustrating the improvements obtained.

Middle Atmosphere Program. Handbook for MAP. Volume 30: International School on Atmospheric Radar

Abstract: Broad, tutorial coverage is given to the technical and scientific aspects of mesosphere stratosphere troposphere (MST) meteorological radar systems. Control issues, signal processing, atmospheric waves, the historical aspects of radar atmospheric dynamics, incoherent scatter radars, radar echoes, radar targets, and gravity waves are among the topics covered.

Variation of Microwave Radar Cross - Section Of Wheat During The Initial Hours Of A Rainfall

Abstract: paired observations near the beginning of the rain and near the end of the rain-halted program.

Analysis of records from four airliners in the Denver microburst, July 11, 1988

Abstract: Flight and radar position records are analyzed to determine the winds encountered by four airliners that penetrated a multicell microburst on approach to Denver's Stapleton International Airport The results show significant expansion in the size of the microburst, and indicate that there were fluctuations in the internal wind velocity At its peak strength, as experienced by the second aircraft, the microburst produced a head-wind-to-tail-wind shear of 115 ft/sec The wind patterns derived from the flight-data analysis are in general agreement with results derived from Doppler weather radar and from a numerical microburst simulation

ASR-9 weather channel test report

Abstract: : The ASR-9, the next generation airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the U.S.. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. Issues addressed in this report are: 1) whether the ASR-9 weather channel performs according to FAA specifications; and 2) whether the ASR-9 weather channel adequately represents weather reflectivity for ATC purposes. Comparisons between data from an ASR-9 in Huntsville, AL, recorded during design qualification and testing, and data from two other 'reference' radars, were used as the basis for the assessment. Several storm cases were analyzed, comprised of stratiform rain, isolated convective storms, squall lines, and cold fronts containing multiple simultaneous convective storms. Results suggest that, with the exception of an apparent 3 dB discrepancy between the weather products of the ASR-9 and the 'reference' radars, the ASR-9 weather channel seems to perform according to FAA specifications. Although the ASR-9 products give a reasonable representation of the extent and severity of potentially hazardous weather in Huntsville, the results suggest that the static storm model used to determine beamfill corrections for the ASR-9 should be optimized for the particular climatic region in which an ASR-9 will be operated. Keywords: Fan-beam radar; Ground clutter suppression; Beamfilling compensations; Pencil-beam radar. (edc)

Discrimination between hail and rain precipitation types from dual polarization radar, raingage and hailpad data

Abstract: Dual polarization radar data (ZH andZDR) are analysed in 4 convective cells. Quite different relationships betweenZH andZDR are induced by the different hydrometer characteristics in these cells. The radar data, compiled with raingage and hailpad data obtained at the ground, show that in the mixed-phase (hail and rain) type of precipitations the differential reflectivityZDR is about 1.5 dB smaller than in the rain type of precipitations, without reaching zero values as in the hail type. In zones of strong precipitations the differential reflectivityZDR increase as the horizontal reflectivityZH, reaching values > 4 dB which indicate the existence of very big drops.

Airborne Radar And Radiometer Experiment For Quantitative Remote Measurements Of Rain

Abstract: An aircraft experiment has been conducted with a dual-frequency (10 GHz and 35 GHz) radar/radiometer system and an 18-GHz radiometer to test various rain-rate retrieval algorithms from space. In the experiment, which took place in the fall of 1988 at the NASA Wallops Flight Facility, VA, both stratiform and convective storms were observed. A ground-based radar and rain gauges were also used to obtain truth data. An external radar calibration is made with rain gauge data, thereby enabling quantitative reflectivity measurements. Comparisons between path attenuations derived from the surface return and from the radar reflectivity profile are made to test the feasibility of a technique to estimate the raindrop size distribution from simultaneous radar and path-attenuation measurements.

Computer simulation of radome effects for the Doppler weather radar antenna system

Abstract: The authors present the results of an analysis of the performance of a Doppler weather antenna inside a protective radome. Computer calculations are based on existing Raytheon programs that simulate radome panel insertion effects and seam scattering of an antenna located within a radome structure. These calculations are compared to measure weather antenna system radiation patterns and shown to be consistent. These analyses demonstrate that seam scattering computer simulations are a viable tool for radome evaluation. >

The Development of Numerically-Based and Expert System Approaches for Airfield Nowcasting/Very Short Range Forecasting

Abstract: : Among the new capabilities that most USAF weather stations will acquire are NEXRAD Doppler weather radar, DMSP and GOES satellite imagery and soundings, conventional observations and model-generated analysis and forecast gridded fields via a system called AWDS (Automated Weather Distribution System), automated airfield observations and, in time, wind and thermodynamic vertical profiles on an hourly-updated basis. Each of these systems will comprise individual computer-based processing and display systems, none of which will be able, in their initial configuration, to effectively communicate or share data with other systems in the weather station. An R&D program at AFGL was recently undertaken to seek effective ways to process these data from diverse sources into an integrated database. The program called AMPS (for Advanced Meteorological Processing System), has the objective to develop, test and evaluate both numerically-based and expert system procedures for the assessment, analysis and prediction of weather events critical to safe and efficient aviation activities on and around air bases. Current emphasis is on (1) The development of procedures to nowcast NEXRAD reflectivity and doppler wind fields via statistically based extrapolation methods and to (at a later date) link these methods with a limited area numerical forecast model designed to account for convective initiation and development and (2) Expert system approaches to terminal area fog prediction. Research progress in these areas and plans regarding the establishment of an AMPS Test Facility involving prototype versions of the system capabilities cited above are discussed. Reprints.

Considerations in Estimating Horizontal Wind Gradients from an Individual Doppler Radar or a Network of Wind Profilers

Abstract: Applications of Doppler weather radar data to the analysis of wind fields are reviewed. Radial velocity measurements from a single radar are used to estimate horizontal wind vectors within small azimuthal sectors using two different models. One assumes a uniform wind, the other a linear wind within the sector. Errors in wind estimates owing to gradients of wind are derived using harmonic analysis. The radar data analysis techniques are tested on complex wind patterns which were reconstructed from dual-Doppler radar measurements.

Quality Control and Archiving of Digital Data Gathered by the Bureau of Reclamation's Weather Radar

Abstract: This paid describes the logic and process used to process high volumes of digital radar data recorded on magnetic tape to a compressed quality-checked archival format. The basic Philosophy in the processing is to retain all echo data that exceed the background noise threshold, and to convert fixed-length raw data records to a space-saving variab1e-length format, having about 70 data quality conditions checked and flagged in binary masks. Edit flags provide information to analysts regarding errors or potential errors within the header fields (e.g. azimuth, elevation, date, time, range delay, range interval, and samples per averaged return) and data fields (e.g. suspicious reflectivity gradients, anomalously high reflectivities and reflectivities significantly below the noise threshold).