The Radar “Three-Body Scatter Spike”: An Operational Large-Hail Signature

Abstract

Recently, a rare radar artifact called the ?flare echo? or ?three-body scatter signature? has been examined by several researchers. Here, this midlevel storm signature is called the ?three-body scatter spike? (TBSS) and is examined in detail for some severe storms scanned by operational WSR-88Ds. The TBSS is a generally 10?30-km long region of echo aligned radially downrange from a highly reflective (>63 dBZ) echo core. It is found almost exclusively aloft and is characterized by low reflectivity and is usually characterized by near-zero or weak inbound velocities. Spectrum widths are very broad and often noise like. The aforementioned research concluded that it is caused by non-Rayleigh radar microwave scattering (Mie scattering) from a region of large hydrometeors; most likely large, wet hail. This conclusion is supported and expanded upon. WSR-88D data are presented concerning a storm attended by a TBSS that produced giant (>5 cm) hail and violent surface winds. In this case, the three-body signature was found in midlevels, down radial from a 65-dBZ reflectivity core in the storm echo overhang. It was most pronounced during 2.5-cm surface hail occurrence and preceded by ?25 min ?baseball-size? hail. The signature is shown in base reflectivity and velocity products and in cross sections of reflectivity, velocity, and spectrum width. TBSS theory of production, its appearance, and characteristics are discussed as well as its operational use. Other examples are also examined briefly. Time and location of surface severe weather reports are also examined and related to three-body scattering. The operational interpretation and application to the severe storm warning program are emphasized. It is suggested that the TBSS is a sufficient but not necessary condition for large hail detection. Although all storms examined in this study produced hail >6.5 cm, it is concluded that surface hail of at least 2.5-cm diameter should be expected with artifact-bearing storms. It is further shown that recognition of the TBSS can provide a warning lead time because it typically precedes the largest surface hailfall (and very often violent surface winds) by 10?30 min.