Rapid Sampling of Radar Precursor Signatures Associated with Downbursts in Central Oklahoma on 14 June 2011

High-temporal resolution radar data collected by the National Weather Radar Testbed Phased Array Radar (NWRT PAR) provides unique insights into rapidly evolving weather phenomenon, including downbursts. On 14 June 2011, the NWRT PAR sampled a long-lived multicell thunderstorm with midlevel rotation that produced a series of downbursts just west of the I-35 corridor. During data collection, the storm produced three severe (winds greater than 25 ms-1) downbursts and two non-severe (one very weak and the other just below severe status) downbursts. The NWRT PAR captured the complete evolution of precursor signatures associated with all of these downbursts. In addition, a dual-polarimetric Weather Service Radar 1988-Doppler (WSR-88D) research radar (KOUN) collected volume scans of the same storm providing insight into the hydrometeor composition of each descending reflectivity core.

In-depth analysis of the radar data and environmental conditions 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 the strong non-severe event displayed clearly descending high reflectivity cores. The upper extent of the core, defined by the 65 dBZ isosurface in this case, exhibited clear and rapid descent only minutes prior to the maximum intensity of the near surface winds. This rapid descent of the high reflectivity core likely represents the collapse of a thunderstorm updraft and subsequent strong downdraft, which ultimately leads to a downburst. The weak non-severe downburst also possessed a descending reflectivity core, but the lower extent of the core exhibited only slow descent (compared to the severe events), while the upper extent of the core descended very little, remaining nearly constant throughout the duration of the downburst. Midlevel convergence provided the most reliable precursor to severe wind producing downbursts. All severe downbursts were accompanied by increasing midlevel convergence, which reached a clear maximum in magnitude minutes before strong surface winds occurred. Compared to the severe events, the magnitude of the midlevel convergence was lower for the strong non-severe event and almost nonexistent for the weak non-severe event, making midlevel convergence an effective discriminator between damaging and non-damaging downbursts in this case. Other signatures considered include, reflectivity notches, three-body scatter spikes, and the magnitude of storm-top divergence associated with the thunderstorm complex. Of these, reflectivity notches served as the most reliable precursor signature as one was located near the high reflectivity core of each severe event, but not the weak non-severe event. Three-body scatter spikes preceded downburst development by up to twenty minutes, but were not associated with every severe downburst's high reflectivity core, while the magnitude of divergence at storm top served to diagnose the overall intensity of the thunderstorm complex rather than indicating the potential for damaging downbursts.

Dual-polarization data were also analyzed to determine the general hydrometeor characteristics of each downburst, severe and non-severe, observed in the NWRT PAR data. This added data provided snapshots of microphysical information and processes, such as melting, coinciding with the evolution of each downburst depicted by the NWRT PAR. Dual-polarization precursor signatures, such as Zdr “holes” and troughs, were detected for each severe and non-severe downburst. No clear differences in the occurrence of these signatures existed between the severe and non-severe downbursts in this case. This result likely arises from the coarser temporal resolution (approximately five minutes) of the KOUN data, further illustrating the importance of rapid sampling capabilities. The primary author will discuss the key results of this ongoing study.