Rapid-Update Radar Observations of Downbursts

High-temporal resolution radar data collected by the National Weather Radar Testbed Phased Array Radar (NWRT PAR, hereafter PAR) provides unique insights into rapidly evolving weather phenomenon, such as downbursts. On 14 June 2011 and 10 July 2013, the PAR sampled downburst-producing storms within 75 km of the radar. The 14 June 2011 multicell thunderstorm produced three severe (winds greater than 25 m s^-1) downbursts and two non-severe downbursts, while the 10 July 2013 multicell thunderstorm produced two severe and two non-severe downbursts. In all but one instance, the PAR sampled the complete evolution of precursor signatures associated with the downbursts. In addition, a dual-polarimetric Weather Surveillance Radar 1988-Doppler (WSR-88D) research radar (KOUN) collected volume scans on both days. This data provided insight into the hydrometeor composition of each downburst's high reflectivity core.

An in-depth analysis of radar data and environmental conditions from these two case studies yielded results about the evolution of a variety of precursor signatures and their capability to distinguish between severe and non-severe downbursts. All severe downbursts and a majority of the non-severe downbursts displayed descending high reflectivity cores. The upper extent of the core generally exhibited rapid descent only minutes prior to the maximum intensity of the sampled near-surface winds. This high reflectivity core descent likely represents the collapse of a thunderstorm updraft pulse and subsequent strong downdraft, which ultimately leads to a downburst. Midlevel (2-7 km above radar level) convergence provided the most reliable precursor signature to severe downbursts in each case. During the 14 June 2011 event, all severe downbursts were accompanied by an increase in the magnitude of midlevel convergence, which reached a clear maximum minutes prior to the occurrence of strong winds at the surface. In addition, only weak midlevel convergence accompanied the non-severe events. Midlevel convergence magnitude also provided a reliable precursor for severe downbursts on 10 July 2013, though strong midlevel convergence also accompanied one of the non-severe downbursts. This particular non-severe downburst, however, did exhibit maximum sampled Doppler velocities similar to this case's severe downbursts. Therefore, midlevel convergence magnitude served as the most effective discriminator between severe and non-severe downbursts in both cases.

Dual-polarization data were also analyzed in order to determine information about hydrometeor characteristics and microphysical processes, such as melting, associated with each downburst's high reflectivity core depicted by the PAR. These data snapshots allowed for the identification of various dual-polarization precursor signatures such as Z_DR “holes” and troughs. In these cases, dual-polarization precursor signatures were detected for all severe and non-severe downbursts. An author will discuss the key results of this ongoing study.