Analysis of the Integrated Terminal Weather System (ITWS) 5-NM Product Suite

Currently, the Integrated Terminal Weather System (ITWS) demonstration systems display six-level precipitation data produced by 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. Since the ASR-9 is a fan beam radar, the data represent the average precipitation within the vertical column. As documented in previous studies, this sensor can significantly underestimate the precipitation intensity and areal coverage due to precipitation processing limitations and hardware failures. In particular, small storms that develop or track over the airport can be underestimated or missed entirely. 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 data at each grid point are based on the maximum reflectivity value within the vertical column. This sensor can overestimate the precipitation intensity near the surface due to bright band contamination and the composite processing technique. 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 and will also serve as a backup to the ASR-9 product. Since the TDWR is located off-airport, this sensor should be less likely to underestimate the precipitation intensity over the airport. Also, the TDWR one-minute precipitation 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 paper discusses the capabilities and limitations of the 5-nm products (precipitation, storm motion/extrapolated position, and storm cell information) based on an analysis of data collected by the Memphis (MEM) and New York City (NYC) ITWS demonstration systems. Specifically, a quantitative analysis of the ASR-9 and TDWR products will be reported since the update rates for these two radars are similar. Although some deficiencies remain with the 5-nm products, overall they provide beneficial safety and planning information and should be evaluated in 2000 at the ITWS sites.

* This work was sponsored by the Federal Aviation Administration. The views expressed are those of the authors and do not reflect the official policy or position of the United States Government. † Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Air Force.