Publications

Aviation weather refers to any type of weather that can affect the operation of an aircraft – anything from a brief delay in departure to a catastrophic accident during flight. Wind shear and events associated with convective weather were recognized as an aviation hazard long before Dr. Theodore Fujita began publishing his now-famous treatises. On July 28, 1943, American Airlines Flight 63 from Cleveland, Ohio, USA to Nashville, Tennessee crashed after the pilot lost control of the Douglas DC3. The pilots and numerous passengers were fatally injured. The aircraft was destroyed by impact and post crash fire. The weather report at the time included warnings for storms, heavy rain, lightning and severe turbulence. The Civil Aeronautics Board found that the probable cause was a loss of control of the aircraft due to unusually severe turbulence and violent downdraft caused by a thunderstorm. In the ten-year period from 1987 through 1996, 24% of all U.S. accidents were judged to be "weather related". For the twenty-year period 1976 to 1996 fully 43% of U.S. accidents were judged to have involved wind or wind shear, and 2.3 % thunderstorm, although the two data elements are not mutually exclusive. In the U.S., approximately 82% of accidents are general aviation; the rest are air carriers and commuters of various types. When general aviation accidents are negated, and only air carriers are considered, wind and wind shear issues account for 9.5% of accidents. The Weather Systems Processor (WSP) has been developed to reduce the impact of severe weather conditions on air traffic by providing information concerning weather conditions in the airport terminal environment. WSP provides warnings to air traffic controllers and supervisors of hazardous wind shear and microburst events in the terminal area, forecasts the arrival of gust fronts, and tracks thunderstorms, providing a complete picture of current and future terminal area hazardous weather conditions that may impact runway and airport usage. Common weather situation awareness allows Terminal Approach, Tower Controllers and other traffic management personnel to jointly plan with confidence and safely manage more arrivals and departures with less delay. Knowledge of the location, severity and movement of hazardous weather allows dynamic adjustments to be made in routing aircraft to runways, approach and departure corridors, terminal arrival and departure transition areas (i.e. gate-posts) and other air routes.

Automated air traffic decision support tools must compute the time it takes an aircraft to fly along a path. The estimation of Time-To-Fly (TTF) requires accurate knowledge of the wind. Two proposed sources of wind data for the Center-TRACON Automation System (CTAS) developed by NASA are the Rapid Update Cycle (RUC) and the Integrated Terminal Weather System (ITWS). The RUC is a mesoscale numerical weather prediction model run by the National Centers for Environmental Prediction. The ITWS was developed by MIT Lincoln Laboratory for the FAA. The ITWS winds product, Terminal Winds takes in RUC forecasts and refines them using recent local measurements of the wind from Doppler radars, aircraft, and ground stations. This report examines the question: does the use of RUC and ITWS wind fields lead to different Time-To-Fly estimates?

Adverse terminal weather is a key factor in the safety and efficiency of airline operations. Weather has been directly related to many of the air carrier accidents with fatalities in the 1990's, and the cost to airlines per year for weather delays is estimated to exceed one billion dollars, with at least half of this arising from convective weather. This paper discusses the airline operations center (AOC) use of information from the Federal Aviation Administration (FAA) terminal weather systems to improve safety and operational efficiency (e.g., reduce delays and diversions, improve predictability, and airline schedule integrity) during severe or rapidly changing conditions. Historically (e.g., prior to 1992), the FAA terminal weather information capability was fairly rudimentary, and airlines had no access to the information. However, with deployment of the ITWS, the ASR-9 Weather Systems Processor (WSP) production systems, and CDMnet (and perhaps Internet) product servers for ITWS and WSP airlines will have access to the products. Thus, it is important now to consider how these products could be used operationally and what refinements should be made to the ITWS/WSP products to better meet the needs of airline users.

Currently, the prototype Integrated Terminal Weather System (ITWS) displays six-level precipitation data generated from the Airport Surveillance Radar (ASR-9) and the Next Generation Weather Radar (NEXRAD). The ASR-9 data are updated every 30 seconds and provide a 0.5 nm spatial resolution to a distance of 60 nm (Weber, 1986). Since the ASR-9 is a fan beam radar, the data represent the average precipitation within the vertical column. As reported by Isaminger, et al., (1999), this sensor can significantly underestimate the precipitation intensity and areal coverage due to precipitation processing limitations and hardware failures. In particular, storms located near the sensor can be underestimated or missed entirely (Crow& et al., 1999). The NEXRAD data are updated every 5-6 minutes with a spatial resolution of 0.5 nm (2.2 nm) and a coverage region of 100 nm (200 nm). The maximum reflectivity value in the vertical column at each grid point is used to create the product. This sensor can overestimate the precipitation intensity near the surface due to bright band contamination and the composite technique (Crowe and Miller, 1999). The update rate can also become an issue if the storms are moving rapidly or developing quickly. In order to confront these issues, the specified ITWS product suite will include six-level precipitation derived from the Terminal Doppler Weather Radar (TDWR). The data from this sensor will be depicted in a high-resolution window (5-nm) around the airport. The TDWR one-minute update rate will provide timely information on rapidly moving or developing storm cells. In many regards, the data will be complimentary to that provided by the ASR-9 and NEXRAD. In others, the weather levels could vary significantly. This report will focus on a discussion of the 5-nm product capabilities and limitations based on an analysis of data collected in Memphis (MEM) and New York City (NYC). A discussion of key product enhancements will serve to illustrate the modifications required to improve this product suite. Finally, a list of recommendations will be presented to assist in product development.

Gust front detection is an important Initial Operational Capability (IOC) of the Integrated Terminal Weather System (ITWS). The Machine Intelligent Gust Front Algorithm (MIGFA) being deployed for ITWS uses multi-dimensional, knowledge-based signal processing techniques to detect and track gust fronts in Terminal Doppler Weather Radar (TDWR) data. Versions of MIGFA have also been developed for the ASR-9 Weather Systems Processor (WSP) and NEXRAD, and within the past year MIGFA was installed as the primary gust front detection algorithm for operational TDWRs throughout the United States. (Not complete.)

On December 6th, 1998, a fatal accident involving a twin engine Beech Baron occurred near the Max-Westheimer Airport at Norman Oklahoma (OUN). Although the National Transportation Safety Board (NTSB) conducted an extensive investigation into this accident, the probable cause for the accident has yet to be determined. Since the accident occurred outside of weather echoes that might be considered hazardous, it seems difficult to deduce a meteorological explanation for this accident. However, Doppler radar data suggested the presence of wave formations near the site of the accident. This report reflects examination of the data provided by the NTSB.

Air traffic delay due to convective weather reached historically high levels in 1999, as passengers blamed airlines and airlines blamed the FAA for the massive inconveniences. While coordination between the FAA's System Command Center and the regional centers and terminals can be expected to improve with the FAA's new initiatives, it is clear that air traffic management and planning during convective weather will ultimately require accurate convective weather forecasts. In addition to improving system capacity and reducing delay, convective forecasts can help provide safer flight routes as well. The crash of a commercial airliner at Little Rock, AR in June 1999 after a one-hour flight from Dallas/Ft. Worth illustrates the dangers and potential tactical advantage that could be gained with frequently updated one-hour forecasts of convective storms. The Terminal Convective Weather Forecast (TCWF) product has been developed by MIT Lincoln Laboratory as part of the FAA Aviation Weather' Research Convective Weather Product Development Team (PDT). Lincoln began by consulting with air traffic personnel and commercial airline dispatchers to determine the needs of aviation users (Forman, et. al., 1999). Users indicated that convective weather, particularly line storms, caused the most consistent problems for managing air traffic. The "Growth and Decay Storm Tracker" developed by Wolfson et al. (1999) allows the generation of up to 1-hour forecasts of large scale, organized precipitation features with operationally useful accuracy. This patented technology. represents a breakthrough in short-term forecasting capability, providing quantitative envelope tracking as opposed to the usual cell tracking. This tracking technology is now being utilized in NCAR's AutoNowcaster (Mueller, et al., 2000), the National Convective Weather Forecast running at the Aviation Weather Center (Megenhardt, et al., 2000) and by private sector meteorological data vendors. The TCWF has been tested in Dallas/Ft. Worth (DFW) since 1998, in Orlando (MCO) since 1999, and in New York (NYC) since fiscal year 2000 began. These have been informal demonstrations, with the FAA William J. Hughes Technical Center (WJHTC) assessing utility to the users, and with MIT LL modifying the system based on user feedback and performance analyses. TCWF has undergone major revisions, and the latest build has now been deployed at all sites. The TCWF is now in a formal assessment phase at the Memphis international Airport as a prerequisite to an FAA operational requirement. The FAA Technical Center will make a recommendation on whether TCWF is suitable for inclusion in the FAA's operational integrated Terminal Weather System (ITWS), which has an unmet requirement for 30+ minute forecasts of convective weather. Memphis was selected for the TCWF Assessment since it has not been exposed to the forecast product during prior demonstrations. Operations began on March 24, 2000 and operational feedback is being assessed by the FAA Technical Center (McGettigan, et al., 2000) and MCR Corporation is performing a quantitative benefits assessment (Sunderlin and Paull, 2000). This paper details the refined TCWF algorithm and display concept, gives examples of the operational impact of terminal forecasts, and analyzes the technical performance of the TCWF during the early stages of the Memphis Assessment.

Major terminals and the surrounding en route airspace are critical elements of the US National Air System (NAS). A large fraction of the US population lives near these terminals, and the bulk of the hub connecting operations are at these airports as well. Adverse weather in these terminal areas and surrounding en route airspace is a major safety concern for the NAS as well as causing a large fraction of all US aviation delays. The principal weather decision support tool for these terminals is the Integrated Terminal Weather System (ITWS) which commenced full-scale development by the FAA in 1995, with first articles to be deployed shortly. In this paper, we discuss how the initial ITWS operational capability needs to be extended to address performance problems identified in operational use and to meet the many new user needs that have developed in the past five years. The paper proceeds as follows. In Section 2, we provide some necessary background on the ITWS operational capability, followed by a discussion of new capabilities to meet urgent user needs. Section 3 discusses refinements to the initial capability to address problems/issues that have been identified based on five years of operational use of ITWS products from ITWS demonstration systems at eight major airports. Next, we consider extending planned ITWS coverage to other major terminals. The final section summarizes the paper's results and suggests additional studies.

The Integrated Terminal Weather System (ITWS) provides runway-orientated wind shear and microburst alerts to enhance the safety of flight operations at major U.S. airports. The alerts are reported as either losses or gains of airspeed, representing performance decreasing or performance increasing wind shears. The performance of ITWS as a stand-alone system has been thoroughly documented in previous research. During the 1994 ITWS Demonstration and Validation testing, the probability of detection (POD) and probability of false alarm (PFA) at Memphis (MEM) and Orlando (MCO) for all loss events were > 90 and < 5 percent, respectively, based on single-Doppler truth. The Low-Level Windshear Alert System-Network Expansion (LLWAS-NE) also generates runway alerts in the same format as ITWS. LLWAS-NE is not subject to viewing angle problems such as those experienced by single-Doppler radar. However, false alarms caused by LLWAS-NE sensor failures at some Terminal Doppler Weather Radar (TDWR) sites have reduced user confidence in the system. At those ITWS sites with an LLWAS-NE, the ITWS alerts derived from TDWR data will be integrated with LLWAS-NE alerts, hopefully to improve the performance. The ITWS integration algorithm is identical to the TDWR version, with the exception of a few adaptable parameter changes. The ITWS/LLWAS-NE parameters were modified slightly to account for ITWS and TDWR algorithm performance differences. In this paper, the performance of a stand-alone ITWS and the ITWS/LLWAS-NE integration algorithm at the MCO and Dallas-Ft. Worth (DFW) demonstration sites will be discussed. This assessment is considered unique since the radar and anemometer data were combined to create the runway truth. The focus of this research is to identify the shortcomings of both systems in order to recommend modifications that will improve the integration algorithm performance.

In this paper we present results from a recently completed study of weather sensing and data fusion to improve safety and reduce delays at major west coast airports. With the exception of a summer stratus burn-off prediction project at San Francisco, these airports have received much less attention in terms of advanced FAA terminal weather decision support systems than major airports east of Los Angeles. This is because the principal concern for terminal weather decision support to date has been microburst-induced wind shear, which is very infrequent at the west coast airports. However, three factors warrant a reexamination of weather decision support provided to these major west coast airports: 1. The increased emphasis on significantly improving aviation safety while reducing delays at major airports in the face of expected increases in operations rates within the National Airspace System (NAS), 2. New air traffic management technology such as terminal automation, collaborative decision making (CDM), and weather adaptive wake vortex spacing systems, and 3. Advances in terminal weather decision support technology represented by the Integrated Terminal Weather System (ITWS) [including various P31 enhancements to ITWS (Evans and Wolfson, 2000)] The airports considered in this study were the Los Angeles (LAX), San Francisco (SFO), Portland (PDX) and Seattle (SEA) International Airports. It should be noted that because these airports did not receive a Terminal Doppler Weather Radar, there currently is no plan to provide them with an ITWS. LAX, SF0 and PDX are scheduled to receive an ASR-9 Weather System Processor (WSP). The paper proceeds as follows. Section 2 discusses the study's methodology and provides background information on delays and weather phenomena for these airports in the context of other major US airports as well as applicable air traffic management (ATM) and terminal weather system technology. Section 3 summarizes the principal findings for the four airports. We conclude with a summary of the potential benefits of improved weather sensing and data fusion that might be provided at these west coast airports by an augmented ITWS as well as recommendations for further studies.