Publications

A number of Federal Aviation Administration (FAA) aviation weather systems utilize Next Generation Weather Radar (NEXRAD) precipitation products including the Integrated Terminal Weather System (ITWS), Corridor Integrated Weather System (CIWS), Medium Intensity Airport Weather System (MIAWS), and the Weather and Radar Processor (WARP). The precipitation products from a NEXRAD [e.g., base reflectivity, composite reflectivity (CR), and vertical integrated liquid (VIL)] are generally only updated once with each NEXRAD volume scan, nominally at 5-6 minute intervals. Hence, the indicated position of storms may not correspond to the actual position due to movement of the storms since the last NEXRAD product update. This latency is particularly a concern in terminal applications such as MIAWS, which use the NEXRAD precipitation product to provide time critical information on moderate and heavy precipitation impacts on the final approach and departure corridors and runways. In order to provide a more accurate depiction, the MIAWS precipitation map is updated (advected) every 30 seconds based on the motion of the storms. The CIWS system performs a similar advection of NEXRAD data before mosaicing the precipitation products from individual NEXRADs. In both cases, motion vectors used for advection are generated by spatial cross-correlation of two consecutive precipitation maps (Chornoboy et al., 1994). This report addresses the accuracy of the advected precipitation map as compared to the current NEXRAD precipitation map using seven MIAWS cases from the Memphis, TN testbed and Jackson, MS prototype. We find that the advected precipitation product is significantly more accurate at providing a depiction of the current intensity of the storms as a fbnction of location. Without advection, the precipitation product from successive NEXRAD volume scans differs by at least one VIP level for over 47.5% of the one square kilometer pixels and has VIP level differences of two levels or more for 6.9% of the pixels in cases where both products had precipitation in a location. The advected precipitation product differs by one or more levels in only 17.2% of the pixels and a VIP level difference of two or more levels is observed in only 1.6% of the pixels. The percentage of cells in which there is precipitation in one map and no precipitation in the other is reduced from over 22% to less than 11% by use of advection. The analysis approach utilized did not quantitatively determine the relative importance of storm growth and decay over the period of the volume scan versus errors in storm motion estimation in causing the differences between the advected precipitation field and the current precipitation field.

The FAA is procuring aviation weather systems, which are designed to enhance safety/capacity and reduce delays at U.S. airports. The two most widely publicized systems currently being installed are the Integrated Terminal Weather System (ITWS) at airports equipped with a Terminal Doppler Weather Radar (TDWR) and the Weather System Processor (WSP) at those terminal areas covered by an Airport Surveillance Radar, Model 9 (ASR-9). At airports not slated to receive either an ITWS or WSP, an emerging system coined the Medium Intensity Airport Weather System (MIAWS) will be installed. Currently, either an ASR-7 or 8 provides terminal aircraft surveillance at these airports. Unfortunately, these platforms do not output calibrated precipitation intensity or storm motion information. Quantitative six-level weather reflectivity data will be available once the digitally enhanced ASR-11 radar system is operational at MIAWS supported sites. The Low Level Wind Shear Alert System - Relocation/Sustainment (LLWAS-RS) anemometer network will provide MIAWS with surface-based winds and wind shear alerts. The rationale for MIAWS evolved from the ITWS and WSP prototype testing. The premise is that the calibrated reflectivity and velocity data from state-of-the-art radar platforms can be utilized to produce a suite of current and forecasted storm positions to aid air traffic control decision making. The forecasted location is a critical issue if the storms are moving rapidly. This can lead to a scenario where the weather conditions deteriorate significantly within a matter of minutes. Once implemented, MIAWS will be an essential component of the National Airspace System by providing this evolving technology to airports whose traffic counts are not sufficient to warrant either an ITWS or WSP, but where commercial carriers could reap the benefits of a high-quality weather radar system. The FAA has contracted the Massachusetts Institute of Technology Lincoln Laboratory (MIT/LL) to undertake a proof-of-concept evaluation of MIAWS. To this end, MIT/LL installed two prototype systems at the Jackson, MS (JAN) and Memphis, TN (MEM) International Airports. The system at MEM is used solely for product evaluation and refinement, while the FAA is operationally evaluating the JAN MIAWS. The focus of this report is a preliminary assessment of the capabilities and limitations of MIAWS in its current implementation, i.e. precipitation based solely on NEXRAD data. Potential enhancements to the NEXRAD product data and MIAWS algorithms will also be discussed.

On June 1, 1999, American Airlines flight 1420 , arriving at Little Rock, AR from Dallas-Fort Worth, TX, was involved in a fatal accident upon landing, on runway 4R at Adams Field (LIT). There were eleven casualties, including the pilot, and numerous injuries among the 145 passengers and crew on board. At the time of the accident, 0451 UTC (11:51 PM CDT), severe thunderstorms existed in the vicinity of the airport. These storms were initiated by an approaching cold front and pre-frontal trough and were developmentally aided by veering low-level wind and warm air advection, which helped to further destabilize the atmosphere. This report will focus on the meteorological conditions preceding and immediately following the accident that could have played a contributing role in the crash. However, no theories on the actual cause will be put forth.

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.

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.

Low level wind shear has been identified as an aviation hazard which has caused or contributed to a significant number of aircraft accidents (Soffer, 1990). To protect aircraft from hazardous wind shear, the Federal Aviation Administration (FAA) developed a system called the Low Level Wind Shear Alert System (LLWAS), containing a collection of anemometers as well as data processing logic (Wilson and Gramzow, 1991). The LLWAS has undergone several advancements in both design and algorithmic computation. The latest deployment, known as the Network Expansion Low Level Wind Shear Alert System (LLWAS-NE), consists of additional sensors to the original LLWAS network, providing better coverage of the airfield. In addition, the LLWAS-NE is capable of providing runway-oriented wind shear and microburst alerts with loss and gain values. The alerts from LLWAS-NE will be integrated with those from the Terminal Doppler Weather Radar (TDWR) and the Integrated Terminal Weather System (ITWS) at locations where all systems are available (Cole, 1992; Cole and Todd, 1994). An analysis was undertaken at Orlando (MCO) and Dallas/Ft. Worth (DFW) International Airports to assess the accuracy of wind shear alerts produced by LLWAS-NE and the TDWR/LLWASNE integration algorithm. Identifying improvements that can be made to either system is important, as LLWAS-NE alert information is anticipated to be integrated with ITWS in an ITWS/LLWAS-NE integration algorithm. As currently specified, the ITWS/LLWAS-NE integration algorithm will work the same as the TDWR/LLWAS-NE version. The ITWS/LLWAS-NE algorithm is an area where additional work is necessary to ascertain if the integration parameters should be modified to account for performance differences between the ITWS and TDWR algorithms. We suggest that ongoing assessment of the LLWAS-NE should use both LLWAS-NE data and TDWR base data, when possible. Comparing both data sets also will facilitate optimization of LLWAS-NE parameters used in the computation of the alerts.