An analysis of microburst characteristics related to automatic detection from Huntsville, Alabama and Denver, Colorado
During 1986 and 1987-8, Lincoln Laboratory, under the sponsorship of the Federal Aviation Administration (FAA), collected Doppler radar measurements in Huntsville, Alabama and Denver, Colorado, respectively. These field programs focused on developing and evaluating an automated wind shear detection system that would provide timely warnings of hazardous low-altitude wind shear events to pilots in the airport terminal area. Two previous projects in Denver (JAWS and CLAWS) documented the ability of a pulsed Doppler radar system to detect wind shear near an airport. In the last two decades, there have been 27 aircraft accidents or incidents at least partially attributed to this phenomenon. According to the National Transportation Safety Board, the most hazardous form of wind shear to aviation is the microburst, first identified by Fujita (1981). A microburst is an outflow of downdraft winds from a convective cloud which exhibits a strong divergent pattern near the surface. The radial velocity differential (delta V) must be greater than or equal to 10 m/s over a distance of 4 km or less to be classified as a microburst. In this paper, microburst measurements from the TDWR testbed are analyzed to characterize and compare the type of outflows in an environment with a typically dry sub-cloud layer (Denver) and a typically moist sub-cloud layer (Huntsville), and to relate these characteristics wo observable radar features being used in the Terminal Doppler Weather Radar (TDWR) system for microburst detection. Section 2 describes the primary radar used in the data collection program. Section 3 contrasts microburst characteristics from the two locales. Evidence is presented which suggests that the reflectivity and intensity of the outflow are important to the performance of the microburst detection algorithm, while the frequency and intensity of features aloft may provide for an earlier declaration of a microburst. In section 4, key microburst characteristics from Huntsville and Denver are summarized in relation to the automatic detection process.