Publications

MIT Lincoln Laboratory is being sponsored by the Federal Aviation Administration (FAA) to develop and test the Terminal Doppler Weather Radar (TDWR) wind shear surveillance system. As part of this program Lincoln has developed algorithms for automatically detecting microbursts, or thunderstorm outflows using the radial velocity data gathered from a single TDWR. Output from the detection algorithms will be used to warn aircraft of microburst hazards. While the success in automatically detecting microbursts using the Lincoln Laboratory microburst detection algorithm has been encouraging, one issue which continues to cause concern is microburst asymmetry. Asymmetry, or aspect angle dependence, in microbursts refers to outflows that have a divergent surface outflow strength or extent what varies depending on the aspect (or viewing) angle of the radar. The TDWR detection algorithms utilize input from a single Doppler radar; therefore, an asymmetric microburst may be underestimated or go undetected if the radar is viewing the event from an aspect angle where the strength of the outflow is weak. Additionally, the size and location of the event may be distorted when the outflow extent is significantly asymmetric. Most of the present outflow modeling and detection methods are based on the assumption of axial symmetry both in the strength and extent of outflows. Asymmetry in microbursts, therefore, is a major concern for TDWR microburst detection performance. Past work by Wilson et al. and Eilts has indicated that some microbursts are highly asymmetric, for at least a portion of their lifetime. However, this previous work has been limited in scope to single "snap-shots" of the microbursts, generally at their peak outflow strength. Strength asymmetries from these previous studies indicated asymmetry ratios (maximum over minimum strength) ranging from 1.3:1 to as high as 6:1. None of the studies dealt with shape (or extent) asymmetries. This paper describes the results from a detailed study of 96 individual observations from 27 microburst events. Measurements were taken to determine both the strength and extent of each microburst at multiple aspect angles. The data clearly show that microbursts, on average, have maximum strengths and extents which are 1.9:1 and 1.5:1 asymmetric, respectively.

From 1984 to 1986, Lincoln Laboratory under the sponsorship of the Federal Aviation Administration (FAA) collected wind shear measurements in the southeastern United States using a pulsed Doppler radar. The major emphasis of the measurement program and subsequent analyses is the development and testing of algorithms that will enable the Terminal Doppler Weather Radar (TDWR) to provide wind shear warnings to the aviation community by detection and tracking gust fronts and microbursts. An important phase of the program involves determining appropriate scan strategies and algorithms to detect other radar measurable features which precede or accompany the surface outflows of microbursts. The detection of features aloft such as convergence, rotation, divergence, storm cells, and descending reflectivity cores may permit advanced recognition of the wind shear while it is less than 10 m/s. In this report a microburst on 24 August 1986 in Huntsville is analyzed with single and dual-Doppler techniques to assess microburst precursors, asymmetry, and forcing mechanisms which could be used for futute algorithm development. The microburst producing storm formed within a moist adiabatic, unstable air-mass with weak wind shear at low to mid-levels of the atmosphere. Rotation, convergence, divergent tops, and a descending core were detected prior to the outflow attaining a divergence of 10 m/s. This storm is similar to other Huntsville microburst producing cells in exhibiting upper-level divergence prior to the initial microburst outflow. Previous analyses of wind shear in Denver and Oklahoma did not discuss divergent tops as a possible microburst precursor. However, its relation to storm severity and hailstorm intensity has been reported by Witt and Nelson (1984) and NEXRAD Program Office (1985). In this case-study, the 3-dimensional microburst detection algorithm provided an early declaration of the event while the radial velocity differential was less than 10 m/s.