Publications
Low-altitude wind shear characteristics in the Memphis, TN area based on mesonet and LLWAS data
November 1, 1985
Conference Paper
Published in:
Proc. 14th Conf. on Severe Local Storms, 29 October -1 November 1985, pp. 322-327.
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Summary
As part of the 1984-85 FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) Project, mesonet and Doppler radar data are being collected on rain and thunderstorms in the Memphis, TN area. One of the key goals of the FLOWS Project is to characterize and evaluate the various form of potentially aviation-hazardous low-altitude wind shear in parts of the country where this type of high spatial and temporal resolution meteorological data have not previously been collected. The 1982 JAWS (Joint Airport Weather Studies) Project revealed that the "microburst", a small scale, intense downdraft which hits the surface and causes a strong divergent outflow of wind, has been the source of much of the hazardous wind shear in the Denver area. The 1978 NIMROD (Northern Illinois Meteorological Research on Downbursts) Project revealed that microbursts occur there on convectively unstable days along with gust fronts and "macrobursts" (scale 4-40 km). Other experiments have largely failed to detect microbursts because their observational networks have not been dense enough to resolve this small scale. A compilation of pioneering studies of microburst-related aircraft accidents around the world by Fujita (1985) illustrates clearly the inherent danger of the microburst wind pattern to jet aircraft, wherever it occurs. In developing ways to best meet the goal of providing warning and protection from low-altitude wind shear in the airport terminal areas, the FAA will need to characterize the problem in different parts of the country. It may be misleading, for example, to use the results on wind shear in the Denver area, or any other single geographical locale, to typify the requirements for microburst warnings at all airports in the country. An important region in terms of its frequency of commercial air traffic control and of thunderstorms, in which high resolution measurements capable of revealing microburts have never before been collected, is the southeastern part of the United States (excluding Florida). During 1984 Lincoln Laboratory continuously collected surface meteorological data from 25-30 mesonet stations and FAA Low Level Wind Shear Alert System (LLWAS) data from the 6 anemometers at the Memphis International Airport from May through November (212 days total). Presented here are preliminary results on the characteristics of wind shear events in the Memphis area. Microburst statistics for Memphis are contrasted with those computed by Fujita and Wakimoto (1983) for the Denver area during JAWS and the Chicago area during NUMROD. A detailed analysis of a microburst that occurred on August 11, 1984 is also presented.
Summary
As part of the 1984-85 FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) Project, mesonet and Doppler radar data are being collected on rain and thunderstorms in the Memphis, TN area. One of the key goals of the FLOWS Project is to characterize and evaluate the various form of potentially aviation-hazardous low-altitude...
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Evaluation of the ASR-9 weather reflectivity product
June 21, 1985
Conference Paper
Published in:
Proc. Second Int. Conf. on the Aviation Weather System, 19-21 June 1985, pp. 196-202.
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Summary
The ASR-9 is a modern airport surveillance radar (ASR) under procurement by the United States Federal Aviation Agency. The radar operates at S-band, providing range-azimuth position information on aircraft targets within a 111-km radius. A fully-coherent klystron amplifier, large dynamic range and digital signal processing enable high integrity target processing and display under condition of ground clutter, weather, angel clutter, RF interference and ground vehicular traffic. To aid controllers in the identification of hazardous weather conditions, the processor will also generate two- or six-level weather reflectivity contours for display at the terminal radar control center and (potentially) remote sites. In this paper, we present an overview of the ASR-9 and its weather processor, emphasizing those features that raise issues with respect to the utility of the weather reflectivity product in an air-traffic control environment. We then describe a simulation procedure that utilizes pencil-beam Doppler weather radar data and ground clutter measurements to preview the ASR-9 product and assess the effects of the radar's configuration on the weather intensity reports. Examples of the simulated weather reports are used to illustrate" (a) partial beamfilling die to the fan-shaped surveillance antenna pattern; (b) attenuation of low velocity weather by the clutter filters' (c) the effects of the spatial filters used in weather processing.
Summary
The ASR-9 is a modern airport surveillance radar (ASR) under procurement by the United States Federal Aviation Agency. The radar operates at S-band, providing range-azimuth position information on aircraft targets within a 111-km radius. A fully-coherent klystron amplifier, large dynamic range and digital signal processing enable high integrity target processing...
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The FAA/MIT Lincoln Laboratory Doppler Weather Radar Program
June 21, 1985
Conference Paper
Published in:
Proc. Second Int. Conf. on the Aviation Weather Systems, 19-21 June 1985, pp. 76-79.
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Summary
Adverse weather is the leading cause of aircraft accidents in the United States. In order to improve hazardous weather detection and warning capability for aviation, the Federal Aviation Administration (FAA) is pursuing a two part Doppler weather radar program. The first part consists of a joint program with the National Weather Service (NWS) and United States Air Force Weather Service (AWS) is to develop and install the Next Generation Weather Radar (NEXRAD). The NEXRAD Systems will meet the FAA enroutw hazardous weather detection requirements and will replace the existing obsolete NWS and AWS weather radars. The second part of the FAA program is the development of a Terminal Doppler Weather Radar (TDWR), which could be procured and installed at major airports to detect weather hazards to terminal aviation operations. The TDWR couls be either a derivative of NEXRAD or a separate radar system. In order to support both of these efforts, the FAA contracted with M.I.T. Lincoln Laboratory to develop and fabricate a NEXRAD-like transportable weather radar support facility. This facility along with a second Doppler radar and a network of meteorological measurement stations are installed near Memphis, Tennessee. These facilities will be used to validate and refine scanning strategies, data processing techniques, and weather detection algorithms. The utility of weather radar products for air traffic control (especially for pilots and controllers) will be evaluated.
Summary
Adverse weather is the leading cause of aircraft accidents in the United States. In order to improve hazardous weather detection and warning capability for aviation, the Federal Aviation Administration (FAA) is pursuing a two part Doppler weather radar program. The first part consists of a joint program with the National...
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A gust front case studies handbook
January 10, 1985
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-129
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Summary
Gust fronts produce low altitude wind shear that can be hazardous to aircraft operations, especially during takeoff and landing. Radar meteorologists have long been able to identify gust front signatures in Doppler radar data, but in order to use the radars efficiently, automatic detection of such hazards is essential. Eight gust front case studies are presented. The data include photographs of the Doppler weather radar displays, thermodynamic and wind measurements from a 440 m high tower, environmental soundings and tables of gust front characteristics. The tabulated characteristics are those thought to be most important in developing rules for automatic gust front detection such as length and height, maximum and minimum values of reflectivity, velocity and spectrum width, and estimates of radial shear. For the cases studied, outflows could be detected most reliably in the velocity field, but useful information also could be gleaned from the spectrum width and reflectivity fields. The signal-to-noise ratio threshold was found to be a major factor in the ability of an observer to discern the gust front signature in the Doppler radar displays. Detection within the spectrum width field required a higher SNR than did the radial velocity field.
Summary
Gust fronts produce low altitude wind shear that can be hazardous to aircraft operations, especially during takeoff and landing. Radar meteorologists have long been able to identify gust front signatures in Doppler radar data, but in order to use the radars efficiently, automatic detection of such hazards is essential. Eight...
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An automatic weather station network for low-altitude wind shear investigations
September 18, 1984
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-128
Summary
During the summer of 1983 an experimental network of automatic weather stations (a mesonet) was operated in the vicinity of Hanscom Field, northwest of Boston, as part of a larger effort to collect Doppler radar and meteorological data on thunderstorms and other potentially hazardous weather events in this area. This report describes the mesonet system used and presents in detail the data collected on 21-22 July 1983. Conclusions about the limitations and the future use of the mesonet system are also included.
Summary
During the summer of 1983 an experimental network of automatic weather stations (a mesonet) was operated in the vicinity of Hanscom Field, northwest of Boston, as part of a larger effort to collect Doppler radar and meteorological data on thunderstorms and other potentially hazardous weather events in this area. This...
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A comparison of storm tracking and extrapolation algorithms
July 31, 1984
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-124
Summary
The FAA requires short-term forecasts of the development and motion of high reflectivity regions to plan for weather avoidance in the en route and terminal areas. Specific needs include choice of air routes and anticipating when to open or close approach/departure gates, descent corridors, and runways. This report compares storm-tracking algorithms for making short-term (0-30 minute) forecasts of high reflectivity areas, to serve these air traffic control needs. The area forecasts are made by moving the key features of the current reflectivity map according to the velocities derived from the storm trackers. The NEXRAD centroid, correlation, and Crane peak-cell trackers are compared against themselves, persistence, and a best-fit extrapolation. Two performance measures are used: (a) overlap of predicted versus actual areas (b) accuracy in flight-path choice. The second method is a new way of scoring the predictor performance and is particularly suited to aviation needs. Five storms are considered, three in Massachusetts and two in Oklahoma. The correlation and peak-cell trackers generally performed well in the Massachusetts storms, close to a best correlation fit extrapolator. The centroid tracker behaves erratically, due to contour merging and splitting. The centroid tracker performed well on compact, Oklahoma storms where the correlation and peak-cell trackers were misled by storm propagation, an effect to be expected when there is high vertical shear of the horizontal wind. It is recommended that either the correlation or centroid tracker be used, depending on the type of storm expected. The centroid tracker would be used on compact storms; the correlation tracker would be used on storms without substantial propagation. The forecasts appear to be skillful in predicting high-reflectivity areas; however, they are less skillful in anticipating flight-paths which do not intersect these areas. Inclusion of forecasts of storm growth and decay will probably be required to improve the performance; anticipating growth and decay will also be important for forecasts of greater than 30 minutes.
Summary
The FAA requires short-term forecasts of the development and motion of high reflectivity regions to plan for weather avoidance in the en route and terminal areas. Specific needs include choice of air routes and anticipating when to open or close approach/departure gates, descent corridors, and runways. This report compares storm-tracking...
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A high stability TWTA for ground surveillance applications
March 13, 1984
Conference Paper
Published in:
Proc. 1984 IEEE Natl. Radar Conf., 13-14 March 1984, pp. 110-114.
Summary
A high stability, C-band transmitter capable of supporting -68 dBr subclutter visibility over a wide range of pulse widths and duty factors was developed for use in a multimode, battlefield surveillance radar. The transmitter was mode-switched between groups of 1/4, 20 and 65 microsecond radar pulses and long duration (100ms) FSK burst. A key feature was the use of a high speed, interpulse regulator to maintain proper TWT voltage and to limit ripple independent of waveform without the need for excessively large energy storage. Actual measured performance met the 100 mV cathode ripple specification without the use of PKF synchronization, independent of PKI, duty cycle and pulse width and was confirmed via direct evaluation of electrode voltages, serrodyne phase jitter and the radar pulsed-Doppler spectrum.
Summary
A high stability, C-band transmitter capable of supporting -68 dBr subclutter visibility over a wide range of pulse widths and duty factors was developed for use in a multimode, battlefield surveillance radar. The transmitter was mode-switched between groups of 1/4, 20 and 65 microsecond radar pulses and long duration (100ms)...
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Ground clutter cancellation for the NEXRAD system
October 19, 1983
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-122
Summary
Returns from the ground and associated obstacles surroudning a NEXRAD weather radar (i.e., ground clutter) will contaminate the estimates of weather echo spectral features (e.g., reflectivity, mean velocity, and spectral width). The ground clutter returns are particularly large at low elevation angles and close range (e.g., within 40 km). Additionally, the pulse repetition frequency (PRF) values necessary to obtain the desired weather Doppler features result in ground clutter contamination at ranges that are multiples of the unambiguous range interval (e.g., 115-175 km for a typical NEXRAD). Fortunately, the groung clutter power spectrum is localized around zero velocity so that one can reduce its effect by appropriate Doppler signal processing. Automatic reduction of clutter contamination is essential if NEXRAD is to achieve the desired automatic weather product generation capability. The results of an analytical/experimental study oreinted toward development of a clutter cancellation specification and assiciated quality assurance tests for the NEXRAD system are described.
Summary
Returns from the ground and associated obstacles surroudning a NEXRAD weather radar (i.e., ground clutter) will contaminate the estimates of weather echo spectral features (e.g., reflectivity, mean velocity, and spectral width). The ground clutter returns are particularly large at low elevation angles and close range (e.g., within 40 km). Additionally...
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Doppler radar observations of an Oklahoma downburst
September 23, 1983
Conference Paper
Published in:
21st Conf., on Radar Meteorology, 19-23 September 1983, pp. 590-595.
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Summary
"Downbursts", first discovered by Dr. T. T. Fujita, are small intense downdrafts at very low altitudes which impact the surface and cause a divergent outflow of wind. They can occur under a variety of meteorological conditions as was demonstrated during the JAWS 1982 field experiment which took place in the vicinity of Stapleton airport in Denver, CO. Many downbursts were detected but most of them were of the type now being called "dry" or "cumulus" or "virga" downbursts. A distinction must be made between these and the "wet" or "thunderstorm" downbursts which are the subject of this study. The two phenomena are very different. They are easy to distinguish: the former come from benign looking cumulus clouds and fall through a very deep and dry subcloud layer and the latter are associated with thunderstorms. Thunderstorm downbursts have been detected throughout the Great Plains and the Midwest, on the east coast, and in Florida, while the virga downbursts have been detected mainly over the high plains east of the Rockies. The word "downburst" was first introduced by Fujita (1976) after the investigation of a plane crash at JFK airport, to describe the situation in which a thunderstorm downdraft becomes hazardous to the operation of jet aircraft on take-off or landing. At first, Fujita (1979) thought that the downburst and the well known thunderstorm downdraft were essentially the same but that, in the same way a funnel cloud aloft is not called a tornado, a mid-level downdraft in a thunderstorm would not be called a downburst. The concept was later refined when it was decided that the downburst must induce "an outburst of damaging winds on or near the ground" (Fujita and Wakimoto, 1981) where "damaging winds" refers to winds that can be estimated on the F-scale (for which there minimum threshold is 18 m/s). These damaging winds can be either straight or curved but they must be highly divergent (Fujita, 1981). Thus, even in its most recent and more meteorological definition, the term downburst is meant to signify a potential human hazard. Whether of not it also signifies a dynamically distinct phenomenon in thunderstorms is a matter of some debate and one which will be investigated in the current work.
Summary
"Downbursts", first discovered by Dr. T. T. Fujita, are small intense downdrafts at very low altitudes which impact the surface and cause a divergent outflow of wind. They can occur under a variety of meteorological conditions as was demonstrated during the JAWS 1982 field experiment which took place in the...
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Short-term prediction of high reflectivity contours for aviation safety
June 9, 1983
Conference Paper
Published in:
Proc. Ninth Conf. Aerospace and Aeronautical Meteorology, 6-9 June 1983, pp. 118-122.
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Summary
Airspace utilization and safety could benefit significantly from accurate, real-time, short-term predictions of hazardous weather regions (e.g., 5-30 minutes). For some hazards, such as heavy turbulence, the detection process itself is in an immature stage. No universally accepted algorithm exists for indicating the regions of current turbulence - let alone predicting it. For other hazards, such as hail and more particularly for heavy rain, the detection process is in a more mature state. In fact heavy rain may be unambiguously associated with high dBZ (reflectivity), if no ice phases are present. Hail is also associated with high reflectivities. We have therefore chosen to place our initial emphasis on the prediction of reflectivity contours in the context of ATC (air traffic control) operations. For all or our prediction techniques, we begin by collecting fixed dBZ-level contours on a fixed-elevation scan by fixed-elevation scan basis, and then combining these elevation cell slices into volume cells as is done in the algorithm of Bjerkaas and Forsyth (1980). To these volume cells we attach translations vectors to make the desired prediction: at this time no provision is made for the growth or decay of reflectivity cells. We generate our translation vectors using each of several algorithms which have already been described elsewhere. Firstly, we use the centroid-tracking approach of Bjerkaas and Forsyth (1980). This is the current tracker of choice in the NEXRAD (Next Generation Weather Radar) program. Secondly, we use tracking vectors of clusters of volume cells, as described ny Crane (1979): much of this work was performed under the sponsorship of the Federal Aviation Administration (FAA). Thirdly, we generate translation vectors by cross-correlating low-altitude (0-4 cm) CAPPIs (constant-altitude plan position indicators): this correlation is done either for the entire storm, or for 30 km by 30 km segments of the storm. This approach has been motivated by the work of Rinehart and Garvey (1978), although we generally use a CAPPI of liquid water content. Fourthly, we use as a prediction the current, composite reflectivity map - our so-called status-quo prediction.
Summary
Airspace utilization and safety could benefit significantly from accurate, real-time, short-term predictions of hazardous weather regions (e.g., 5-30 minutes). For some hazards, such as heavy turbulence, the detection process itself is in an immature stage. No universally accepted algorithm exists for indicating the regions of current turbulence - let alone...
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