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1. Introduction
As a result of many observational and numerical weather prediction studies, much is known about downbursts (e.g., Fujita and Byers 1977; Fujita and Wakimoto 1983; Straka and Anderson 1993; Wakimoto 2001) and the environments that typically support them (e.g., Caracena et al. 1983; Srivastava 1985; Atkins and Wakimoto 1991). Wilson et al. (1984) used radar observations to define a downburst as any near-surface divergent signature with a velocity differential (hereafter DeltaV) of 10 m s−1 or higher. To aid in our ability to forecast downbursts, previous radar-based studies (e.g., Isaminger 1988; Wakimoto and Bringi 1988; Roberts and Wilson 1989; Smith et al. 2004) identified downburst precursor signatures that tend to develop prior to damaging winds at the surface. These signatures include descending reflectivity cores (DRCs) and midlevel radial convergence (hereafter midlevel convergence) observed by Isaminger (1988) in Tennessee and Alabama and Roberts and Wilson (1989) in Colorado. Many radar-based studies (e.g., Eilts 1987; Isaminger 1988; Wakimoto and Bringi 1988; Mahale et al. 2013) used relatively slow (3–5 min) volumetric update times, which prevented them from observing trends in precursor evolution. Therefore, they identified precursor signatures or focused on their characteristics rather than quantifying precursor evolution. An exception is Roberts and Wilson (1989), who used 2.0–2.5-min volumetric data collected during the Joint Airport Weather Studies Project (McCarthy et al. 1982) to produce time–height trends of DRCs and midlevel convergence for several downburst-producing storms in Colorado. However, they did not examine trends relative to near-surface downburst evolution (e.g., intensification) or look at differences between severe and nonsevere downbursts.
To address the need for rapid sampling of severe weather, including downbursts, the National Severe Storms Laboratory and partners developed the National Weather Radar Testbed Phased Array Radar (NWRT PAR; hereafter PAR) in Norman, Oklahoma (Forsyth et al. 2005). Heinselman et al. (2008) took advantage of PAR’s rapid (1 min or less) volumetric update time to study several severe thunderstorms, including one that produced a severe downburst (i.e., winds 25 m s−1 or higher). They observed rapid evolutions of downburst precursor signatures, including a reflectivity core that elongated and descended in only 7 min (i.e., a DRC) and midlevel convergence that developed in association with the DRC. Newman and Heinselman (2012) also observed the...