Publications
A comparison of anemometer and Doppler radar winds during wind shear events
October 22, 1990
Conference Paper
Published in:
16th Conf. on Severe Local Storms/Conf. on Atmospheric Electricity, 22-26 October 1990, pp. 356-361.
Summary
The Federal Aviation Administration (FAA) currently uses the anemometer-based Low Level Wind Shear Alert System (LLWAS) as the primary method of wind shear detection at major U.S. airports. With the upcoming deployment of the Terminal Doppler Weather Radar (TDWR) system, potential methods for integrating the two systems are being investigated. By integrating, advantages of both sensor systems can be utilized. Advantages of the LLWAS ground sensor network include true wind direction measurements, a high measurement frequency, a lack of sensitivity to clear air reflectivity, and few false alarms from radar point targets such as planes, birds, etc. Advantages of the radar include complete scan coverage of the region of concern, the ability to predict events, fewer terrain problems such as sheltering which can reduce the wind speed readings, and almost no false alarms due to non-hazardous wind shear such as thermals. The objectives of this study are to gain a clearer understanding of the basic relationship between the wind information provided by these two very different sensing systems, and to determine the impact this relationship may have on integration of the two operational systems. A proposed mathematical technique for "correcting" LLWAS winds where needed to better match radar winds is evaluated for cases of microburst (divergent) and gust front (convergent) wind shear.
Summary
The Federal Aviation Administration (FAA) currently uses the anemometer-based Low Level Wind Shear Alert System (LLWAS) as the primary method of wind shear detection at major U.S. airports. With the upcoming deployment of the Terminal Doppler Weather Radar (TDWR) system, potential methods for integrating the two systems are being investigated...
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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.
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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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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Contributions to the American Meterorological Society 16th Conference on Severe Local Storms
August 15, 1990
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-173
Summary
Eight papers contributed by the Lincoln Laboratory Weather Sensing Group to the American Meteorological Society's 16th Conference on Severe Local Storms, to be held October 22-26, 1990 in Kananaskis Provincial Park, Alberta, Canada, are compiled in this volume. The FAA sponsored the summer 1989 field test of the Terminal Doppler Weather Radar (TDWR) system in Kansas City, Missouri to detect wind shear aviation hazards at or near the airport. The papers are based on data collected through the summer 1989 field test and on subsequent analyses and product evaluation. The staff members of Group 43, Weather Sensing, have documented their studies of the following topics: a severe microburst; a prototype microburst prediction product; average summer microburst threat prediction at an airport; microburst asymmetry; the effect of radar viewing angle on the performance of the gust front detection algorithm; a comparison of Low-Level Wind Shear Alert System (LLWAS) anemometer-measured winds and Doppler-measured winds; and ASR-9 (Airport Surveillance Radar) adjustment of range-dependent storm reflectivity levels. The final paper is an invited paper for the Conference on microbursts. This paper discusses the precipitation-driven downdraft and the downdraft associated with the "vortex," or gust front, at the leading edge of an expanding thunderstorm outflow as two primary forms of low altitude downdraft phenomena in the microburst problem.
Summary
Eight papers contributed by the Lincoln Laboratory Weather Sensing Group to the American Meteorological Society's 16th Conference on Severe Local Storms, to be held October 22-26, 1990 in Kananaskis Provincial Park, Alberta, Canada, are compiled in this volume. The FAA sponsored the summer 1989 field test of the Terminal Doppler...
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Notes and correspondence - Correcting wind speed measurements for site obstructions
April 1, 1989
Journal Article
Published in:
J. Atmos. Oceanic Technol., Vol. 6. No. 2, April 1989, pp. 343-352.
Summary
The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem for scientific and operational wind shear measurement system This paper considers an improved version of the technique used by Fujita and Wakimoto for compensating the obstruction effects by the use of mathematical models relating the unobstructed wind speed to the measured wind speed and the observed obstructions at each site. Over eight million wind speed measurements gathered over 197 days (15 February–31 August) were used. The effects of obstructions at a particular site were evidenced by a strong negative correlation between the observed wind speed transmission factors and the obstruction angles as measured from panoramic photographs taken of the horizon around each station. The functional relationship between them was modeled as a decaying exponential plus a constant, and an iterative least squares regression technique was used on data from all of the stations at once in deriving the three parameters of the equation. It was found that the first 8° of obstruction have the greatest blockage effects, and that even a 2° or 3° high isolated clump of trees can have a pronounced effect on the measured wind speeds from that direction. The possibility that the transmission factors are scale dependent and time dependent is explored.
Summary
The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem...
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The FLOWS automatic weather station network
April 1, 1989
Journal Article
Published in:
J. Atmos. Oceanic Technol., Vol. 6, No. 2, April 1989, pp. 307-326.
Summary
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 network near Huntsville, Alabama to measure surface data on microbursts as part of the Cooperative Huntsville Meteorological Experiment (COHMEX). A preliminary assessment of the overall performance of the two networks suggests that they performed with comparable accuracy for those meteorological characteristics most important to the detection of microbursts. While differences and discrepancies were noted, none would preclude treating PAM-II and FLOWS data together as if they were generated by a single network.
Summary
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...
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Selected wind shear events observed during the 1987 evaluation of enhancements to the FAA low level wind shear alert system at Stapleton International Airport
February 15, 1989
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-158
Summary
The Federal Aviation Administration (FAA) Technical Center (FAATC) conducted a test of the enhancements to the FAA Low Level WInd Shear Alert System (LLWAS) at Denver Stapleton International Airport from 3 August through 4 September 1987. Upon completion of the test, the performance of the LLWAS during selected microburst and gust front test cases was investigated in detail. Additional sources of "true" wind shear information were sought to help evaluate the performance of the LLWAS. In support of these efforts, Lincoln Laboratory supplied complete data sets, including single Doppler radar data from the FAA-Lincoln Laboratory FL-2 radar, dual Doppler radar data from the FAA-Lincoln Laboratory FL-2 radar, dual Doppler analyses of the surface wind fields (when possible), mesonet data from the Lincoln network of 30 automatic weather stations in the vicinity of Stapleton, and LLWAS data to the FAATC. This report provides a summary of salient features for a number of FAATC selected wind shear events which occured during the evaluation of the enhanced LLWAS, and documents the data that Lincoln Laboratory has provided to the FAA as part of its project responsibilities.
Summary
The Federal Aviation Administration (FAA) Technical Center (FAATC) conducted a test of the enhancements to the FAA Low Level WInd Shear Alert System (LLWAS) at Denver Stapleton International Airport from 3 August through 4 September 1987. Upon completion of the test, the performance of the LLWAS during selected microburst and...
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Characteristics of microbursts observed in the Continental U.S.
February 26, 1988
Conference Paper
Published in:
15th Conf. on Severe Local Storms, 22-26 February 1988, pp. 372-379.
Summary
The topic of microbursts is explored in this paper through a historical perspective and review of the studies that have been performed since Fujita (1976) first introduced the concept. Taken as a whole, this body of work actually defines microbursts, and begins to take some of the initial steps toward their understanding. However, a number of dynamically distinct phenomena that give rise to strong surface outflows are being referred to as microbursts. The recent emphasis within the scientific and aviation communities on understanding microbursts makes it particularly important to categorize these various phenomena according to their meteorological nature and true aviation hazard potential. This paper takes some of the first steps toward this categorization, and emphasizes some of the differences in storms that can be expected in different climatological regimes.
Summary
The topic of microbursts is explored in this paper through a historical perspective and review of the studies that have been performed since Fujita (1976) first introduced the concept. Taken as a whole, this body of work actually defines microbursts, and begins to take some of the initial steps toward...
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Characteristics of microbursts in the continental United States
January 1, 1988
Journal Article
Published in:
Lincoln Laboratory Journal, Vol. 1, No. 1, Spring 1988, pp. 49-74.
Summary
Microbursts - powerful downdrafts generally associated with thunderstorms that occur in hot, humid weather - have caused a number of aircraft crashes. To prevent future accidents, air traffic controllers must be able to detect, and predict, microburst events. All microbursts are not alike, however; several distinct weather patterns can produce microbursts. Thus a categorization of the different types of microbursts is an essential part of understanding these hazardous phenomena Using this categorization, the relative hazard to aviation of the various types of microbursts can be assessed.
Summary
Microbursts - powerful downdrafts generally associated with thunderstorms that occur in hot, humid weather - have caused a number of aircraft crashes. To prevent future accidents, air traffic controllers must be able to detect, and predict, microburst events. All microbursts are not alike, however; several distinct weather patterns can produce...
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A comparison of PAM-II and FLOWS mesonet data during COHMEX
December 23, 1987
Project Report
Published in:
MIT Lincoln Laboratory Report ATC-154
Summary
Surface weather stations are being used in the Terminal Doppler Weather Radar program to assess the radar detectibility of wind shear and to help gain an understanding of microburst forcing mechanisms. During 1986, surface station networks operated by Lincoln Laboratory (FLOWS) and the National Center for Atmonspheric Research (PAM-II) were deployed in the Huntsville, AL area. A preliminary assessment of the overall performance of PAM-II and FLOWS networks suggests that they performed with comparable accuracy for those meterological characteristics most important to the detection of microbursts. While differences and discrepancies were noted, especially in the network total precipitation amounts, none would preclude treating PAM-II and FLOWS data together as if they were generated by a single network. We condlcue that the data can be directly combined for microburst detection analyses.
Summary
Surface weather stations are being used in the Terminal Doppler Weather Radar program to assess the radar detectibility of wind shear and to help gain an understanding of microburst forcing mechanisms. During 1986, surface station networks operated by Lincoln Laboratory (FLOWS) and the National Center for Atmonspheric Research (PAM-II) were...
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