Publications
Aspect angle dependence of outflow strength in Denver microbursts: spatial and temporal variations
October 22, 1990
Conference Paper
Published in:
16th Conf. on Severe Local Storms/Conf. on Atmospheric Electricity, 22-26 October 1990, pp. 397-402.
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Summary
MIT Lincoln Laboratory is being sponsored by the Federal Aviation Administration (FAA) to develop and test the Terminal Doppler Weather Radar (TDWR) wind shear surveillance system. As part of this program Lincoln has developed algorithms for automatically detecting microbursts, or thunderstorm outflows using the radial velocity data gathered from a single TDWR. Output from the detection algorithms will be used to warn aircraft of microburst hazards. While the success in automatically detecting microbursts using the Lincoln Laboratory microburst detection algorithm has been encouraging, one issue which continues to cause concern is microburst asymmetry. Asymmetry, or aspect angle dependence, in microbursts refers to outflows that have a divergent surface outflow strength or extent what varies depending on the aspect (or viewing) angle of the radar. The TDWR detection algorithms utilize input from a single Doppler radar; therefore, an asymmetric microburst may be underestimated or go undetected if the radar is viewing the event from an aspect angle where the strength of the outflow is weak. Additionally, the size and location of the event may be distorted when the outflow extent is significantly asymmetric. Most of the present outflow modeling and detection methods are based on the assumption of axial symmetry both in the strength and extent of outflows. Asymmetry in microbursts, therefore, is a major concern for TDWR microburst detection performance. Past work by Wilson et al. and Eilts has indicated that some microbursts are highly asymmetric, for at least a portion of their lifetime. However, this previous work has been limited in scope to single "snap-shots" of the microbursts, generally at their peak outflow strength. Strength asymmetries from these previous studies indicated asymmetry ratios (maximum over minimum strength) ranging from 1.3:1 to as high as 6:1. None of the studies dealt with shape (or extent) asymmetries. This paper describes the results from a detailed study of 96 individual observations from 27 microburst events. Measurements were taken to determine both the strength and extent of each microburst at multiple aspect angles. The data clearly show that microbursts, on average, have maximum strengths and extents which are 1.9:1 and 1.5:1 asymmetric, respectively.
Summary
MIT Lincoln Laboratory is being sponsored by the Federal Aviation Administration (FAA) to develop and test the Terminal Doppler Weather Radar (TDWR) wind shear surveillance system. As part of this program Lincoln has developed algorithms for automatically detecting microbursts, or thunderstorm outflows using the radial velocity data gathered from a...
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Observed differences between Denver and Kansas City gust fronts and their impact upon the performance of the gust front detection algorithm
October 22, 1990
Conference Paper
Published in:
16th Conf. on Severe Local Storms/Conf. on Atmospheric Electricity, 22-26 October 1990, pp. 236-239.
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Summary
The Terminal Doppler Weather Radar (TDWR) testbed radar (known as FL-2) collected data near Denver's Stapleton Airport during 1988 and near the Kansas City International Airport (MCI) during 1989. One objective of the TDWR Program is to detect gust fronts and their associated wind shifts. This information can be used by an Air Traffic Control (ATC) supervisor to plan runway changes and for warnings of potentially-hazardous gust front-related wind shears to arriving and departing pilots. This function is performed by the gust front detection algorithm. An ongoing assessment of the performance of the current TDWR gust front algorithm is necessary to ensure that the algorithm performs consistently in different environments. Such assessments were performed after the 1988 TDWR Operational Test and Evaluation in Denver and after the 1989 operational season in Kansas City. This paper presents a comparison of gust front characteristics such as length, duration, strength, and propagation speed and direction that occurred in Denver and Kansas City and a comparison of algorithm performance at each location. In the following, the term gust front refers to the leading edge of the thunderstorm outflow throughout its life cycle. A gust front event is a single observation of a gust front (on a radar volume scan) by the National Severe Storms Laboratory (NSSL) ground-truth analyst.
Summary
The Terminal Doppler Weather Radar (TDWR) testbed radar (known as FL-2) collected data near Denver's Stapleton Airport during 1988 and near the Kansas City International Airport (MCI) during 1989. One objective of the TDWR Program is to detect gust fronts and their associated wind shifts. This information can be used...
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Predicting summer microburst hazard from thunderstorm day statistics
October 22, 1990
Conference Paper
Published in:
16th Conf. on Severe Local Storms/Conf. on Atmospheric Electricity, 22-26 October 1990, pp. 383-387.
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Summary
Low-altitude wind shear, specifically, the aviation-hazardous form of wind shear known as the microburst, has been cited as the cause of several aviation disasters over the past two decades. Microbursts are strong, small-scale convective storm downdrafts that impact the ground and cause a violent divergent outflow of wind. The Federal Aviation Administration (FAA) recently awarded a contract for the production of 47 Terminal Doppler Weather Radars (TDWRs) to detect microbursts. Since the TDWR systems are expensive, only a limited number will be available for use at major U.S. airports. In deciding which airports will receive the TDWRs or any other advanced detection equipment, such as the ASR-9 with wind shear detection capability or the Enhanced Low Level Wind Shear Alert System, a detailed cost-benefit study will be performed. One factor that would aid in determining the benefit of advanced wind shear detection equipment is a knowledge of the average relative microburst threat at each major airport. Using "thunderstorm day" statistics and the results of measurements by the FAA TDWR testbed systems, we propose a method for predicting this threat.
Summary
Low-altitude wind shear, specifically, the aviation-hazardous form of wind shear known as the microburst, has been cited as the cause of several aviation disasters over the past two decades. Microbursts are strong, small-scale convective storm downdrafts that impact the ground and cause a violent divergent outflow of wind. The Federal...
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Vertical reflectivity profiles: averaged storm structures and applications to fan-beam radar weather detection in the U.S.
October 22, 1990
Conference Paper
Published in:
16th Conf. on Severe Local Storms/Conf. on Atmospheric Electricity, 22-26 October 1990, pp. 213-218.
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Summary
The FAA is deploying over 100 next generation airport surveillance radars (ASR-9) at selected major airports across the country. Like previous ASRs, the ASR-9 utilizes dual broad elevation fan beams Figure 1) along with a rapid scan rate (12.5 RPM to exercise its primary function of detecting aircraft over a 60 nmi radius. In addition, the ASR-9 has a separate dedicated weather reflectivity channel which allows air traffic controllers to display quantitative precipitation intensity reports corresponding to the NWS six-level intensity scale on their PPI display. The 30 second update rate of the weather channel coupled with the large sample volume swept by the ASR-9 fan-beam combine to provide timely and useful indications of precipitation intensity within the terminal airspace. The PPI display of precipitation intensity which is presented to the air traffic controller is essentially a 2-D representation of the 3-D reflectivity field sampled by the fan-shaped beam of the ASR-9. Since the antenna gain varies with elevation angle (Figure 1), the parameter reopned by the ASR-9 weather channel represents a beam-weighted, vertically averaged estimate of storm intensity. Previous research has shown that the vertically integrated reflectivity automatically reported by fan-beam radars such as the ASR-9 correlates well with estimates of vertically integrated liquid water content (VIL), a useful meteorological parameter which is a measure of overall storm intensity. Dobson found a linear relationship between W and fan-beam reflectivity from 30 to 60 dBZ assuming the beam is filled with precipitation (see discussion in Section 4). If the beam is non-uniformly or only partially filled with precipitation, then the inherent vertical integration introduced by the fan-beam may cause an underestimation of the storm intensity. This beam filling loss is most acute at long range, where the vertical extent of the beam intercepts more than 10 km of altitude. The magnitude of this error depends on the complex interaction between the vertical reflectivity structure of the storm and its interception by the fan-shaped beam. If the shape and altitude extent of the vertical reflectivity profile (such as could be provided by a pencil-beam radar) are known, then a suitable adjustment can be calculated and applied to the fan-beam reflectivity estimate in order to produce the desired reflectivity report. The six-level weather thresholds are stored in processor memory for each range sate as functions of receive beam (high or low). The thresholds can be adjusted to compensate for beam filling losses. The adjustments initially implemented in the ASR-9 were derived using a reflectivity profile model which assumes the maximum reflectivity of the storm is distributed constantly from the surface up to 4 km, and then falls off at 3 dBZ per km above 4 km. The success of the reflectivity correction depends on how well the model profile matches actual storm profiles. If regional variations in general storm morphology are significant, then different beam filling loss correction models may need to be developed for specific regions. Understanding the significance of these regional variations in storm vertical reflectivity structure and their impact on ASR-9 weather report accuracy provided the motivation for this study.
Summary
The FAA is deploying over 100 next generation airport surveillance radars (ASR-9) at selected major airports across the country. Like previous ASRs, the ASR-9 utilizes dual broad elevation fan beams Figure 1) along with a rapid scan rate (12.5 RPM to exercise its primary function of detecting aircraft over a...
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Understanding and predicting microbursts
October 22, 1990
Conference Paper
Published in:
16th Conf. on Severe Local Storms/Conf. on Atmospheric Electricity, 22-26 October 1990, pp. 340-351.
Summary
Wind shear is a major cause of aircarrier accidents in the United States, and most of these accidents have been caused by one particular form of wind shear called a microburst (Zorpette, 1986). Microbursts have been defined as small scale, low-altitude, intense downdrafts which impact the surface and cause strong divergent outflows of wind. We know they are associated with thunderstorms and are usually but not always accompanied by heavy rainfall at the ground. However, a number of meteorologically distinct phenomena associated with thunderstorms can give rise to strong downdrafts and high surface winds. Most microburst research has focused on the main precipitation driven downdraft of thunderstorms, both with and without significant surface rainfall. But other downdraft types such as the dynamically driven downdrafts at low altitude associated with "vortices" at the leading edge of expanding thunderstorm outflows and with "roll clouds" have also been associated with the microburst problem. In this paper, I discuss these two primary forms of low altitude downdraft phenomena in thunderstorms. This differentiation is essential to discovering exactly what atmospheric conditions lead to the development of the most hazardous microbursts. A physically based predictive model for thunderstorm downdraft strength is presented which shows that the radar reflectivity of a storm alone cannot be used as a hazard index; information about the static stability of the atmosphere is also essential. I then show that the downdrafts associated with the gust front around a cold outflow from a small isolated thunderstorm, a microburst, are inherently stronger at low altitudes than those found in more straight-line gust fronts. Finally, I reexamine the most recent fatal U.S. microburst accident, the crash of Delta 191 at Dallas/Ft. Worth in 1985, and show that both types of low altitude downdrafts were encountered as part of the "microburst", although the downdrafts came from different storms.
Summary
Wind shear is a major cause of aircarrier accidents in the United States, and most of these accidents have been caused by one particular form of wind shear called a microburst (Zorpette, 1986). Microbursts have been defined as small scale, low-altitude, intense downdrafts which impact the surface and cause strong...
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Beam filling loss adjustments for ASR-9 weather channel reflectivity estimates
October 22, 1990
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-177
Summary
The FAA is deploying over 100 new ariport surveillance radars (ASR-9) across the country. In contrast to earlier ASRs, the ASR-9 utilized a separate digital weather processing channel to provide air traffic controllers with timely, calibrated displays of precipitation intensity. The ASR-9 utilizes dual selectable fan-shaped elevation beams designed to track aircraft over a large volume. As a consequence, weather echoes received from these fan-shaped beams represent vertically-averaged quantities. If the precipitation only partially or nonuniformly fills the beam, then the vertically integrated reflectivity may underestimate the actual intensity of the storm. The ASR-9 weather channel corrects for this by adjusting the range-dependent six-level relfectivity thresholds. The appropriateness of the currently implemented correction has not been carefully examined and may require modification to take into account regional and morphological variability in storm structure. This report discusses the method used to derive new beam filling loss adjustments. An extensive database of volumetric pencil-beam radar data were used in conjunction with our ASR-9 simulation facility to derive adjustments aimed at calibrating the precipitation intensity reports to the maximum perceived hazard. The new corrections yield substantially improved results over the current corrections in producing these reflectivity reports.
Summary
The FAA is deploying over 100 new ariport surveillance radars (ASR-9) across the country. In contrast to earlier ASRs, the ASR-9 utilized a separate digital weather processing channel to provide air traffic controllers with timely, calibrated displays of precipitation intensity. The ASR-9 utilizes dual selectable fan-shaped elevation beams designed to...
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Divergence detection in wind fields estimated by an airport surveillance radar
October 15, 1990
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-172
Summary
This report assesses a technique for automatic detection of hazardous divergence in velocity fields estimated by an Airport Surveillance Radar (SAR). We evaluate a least-squares approach to radial divergence estimation through a performance analysis based on simulated data. That approach is compared to an existing decision-based radial shear finding method used for the Terminal Doppler Weather Radar (TDWR). Empirical results derived by the application of two techniques to data collected at ASR testbeds in Huntsville, Alabama and in Kansas City, Missouri are presented. Results indicate that a simple, least-squares divergence estimator combined with time association logic to increase temporal continuity of algorithm output is an equally effective means of detecting divergent wind shear in velocity fields estimated from ASR signals.
Summary
This report assesses a technique for automatic detection of hazardous divergence in velocity fields estimated by an Airport Surveillance Radar (SAR). We evaluate a least-squares approach to radial divergence estimation through a performance analysis based on simulated data. That approach is compared to an existing decision-based radial shear finding method...
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Planning horizon requirements for automated terminal scheduling
October 1, 1990
Conference Paper
Published in:
Proc. 35th Annual Air Traffic Control Association Mtg., 16-20 September 1990, pp. 438-451.
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Summary
This paper presents the results of an engineering analysis of the ability of an automated terminal scheduling process to achieve efficient use of runways. The motivation for the analysis is the need to understand possible architectures for an implementation of the proposed Terminal Air Traffic Automation (TATCA) system. The performance of TATCA is dependent upon metering precision and the controllability that TATCA can apply to aircraft entering the scheduling process. Controllability refers to the amount of time by which the flight time of an aircraft can be lengthened or shortened between the scheduling horizon and the chosen runway. The analysis concludes that when current en route metering mechanisms are used to deliver traffic to the terminal, the terminal scheduler meets a controllability window of 300 seconds or so in order to achieve full runway utilization. Because this amount of controllability is often achievable within the terminal area itself, a TATCA system can provide significant benefits prior to the implementation of further improvements in the en route metering process.
Summary
This paper presents the results of an engineering analysis of the ability of an automated terminal scheduling process to achieve efficient use of runways. The motivation for the analysis is the need to understand possible architectures for an implementation of the proposed Terminal Air Traffic Automation (TATCA) system. The performance...
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Integrated use of GPS and GLONASS in civil aviation navigation I: coverage and data models
September 19, 1990
Conference Paper
Published in:
Institute of Navigation, 3rd Int. Technical Mtg. of the Satellite Division, 19-21 September 1990, pp. 425-435.
Summary
Pursuant to a bilateral agreement signed in 1988, both US and USSR are currently in the process of examining integrated use of GPS and GLONASS for sole-means civil aviation navigation. This paper presents results from the initial phase of a program underway at MIT Lincoln Laboratory to support this effort. Specifically, we present results on satellite coverage and quality of the range measurements from GPS and GLONASS. The coverage results highlight the extent to which each system alone falls short of providing a self-contained system integrity check. In integrated use, however, there are enough redundant measurements to make receiver autonomous integrity monitoring (RAIM) practical. The data quality results are based on statistical analysis of the range measurements from GPS, at various levels of selective availability (SA), collected over extended periods. We present empirical cumulative distribution function of the range error, and RMS value of its component, defined as the 'effective' range error, relevant to position estimation. These results are used to project the position estimation. These results are used to project the position estimation accuracy achievable globally with GPS, when operational. Comparable results for GLONASS are being developed. The coverage and data quality results together provide a basis for development of the navigation and RAIM algorithms for the integrated use. This will be addressed in the next phase of the program. The important considerations in the design of these algorithms, including the differences in the reference systems for space and time employed by the two systems, are briefly reviewed.
Summary
Pursuant to a bilateral agreement signed in 1988, both US and USSR are currently in the process of examining integrated use of GPS and GLONASS for sole-means civil aviation navigation. This paper presents results from the initial phase of a program underway at MIT Lincoln Laboratory to support this effort...
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Real-time simulation for air traffic control research and development
September 1, 1990
Conference Paper
Published in:
Proc. 1990 Air Traffic Control Association Conf., September 1990, pp. 378-384.
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Summary
An approach is suggested for the incremental use of real-time ATC simulations for concept development and human factors evaluation of automation systems. Emphasis is placed on the characteristics that distinguish research simulators from those used primarily for training. Four general levels of simulator fidelity are identified for two ATC environments of interest: the radar room and the control tower. Fidelity requirements are generated by the specific needs of the particular human factors study to be conducted, ranging from part-task single-controller simulation used for concept demonstration to full-mission simulation of multiple ATC facilities to examine issues of interaction among automation systems. This approach is applicable to smaller simulations performed at an R&D contractor site as well as large-scale system integration studies conducted at a high-fidelity, centralized, simulation facility. It has been applied to the design of simulations of ATCT and TRACON environments that are being used for the evaluation of displays, controls, and procedures for the Airport Surface Traffic Automation (ASTA) and Terminal ATC Automation (TATCA) projects.
Summary
An approach is suggested for the incremental use of real-time ATC simulations for concept development and human factors evaluation of automation systems. Emphasis is placed on the characteristics that distinguish research simulators from those used primarily for training. Four general levels of simulator fidelity are identified for two ATC environments...
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