High-Temporal Resolution X-Band Polarimetric Radar Analysis of the 20 May 2013 Moore, Oklahoma Supercell during Tornadogenesis and Tornado Intensification

On 20 May 2013, a long-track violent EF-5 tornado impacted Moore, Oklahoma and surrounding areas. Due to the relatively high frequency of severe weather in this urban region, there exists an extensive network of radars that were operating in close proximity to the supercell throughout its lifecycle. This network of radars consists of mainly experimental and operational NEXRAD WSR-88Ds: 1) Twin Lakes, Oklahoma (KTLX), 2) Norman, Oklahoma (KOUN), and 3) Norman, Oklahoma (KCRI). Additionally, PX-1000, a transportable, X-band, dual-polarimetric radar, was operating at the University of Oklahoma Westheimer Airport in a 2.6° single-elevation PPI scanning strategy, providing updates every 20 s from 19:14:59 to 20:39:51 UTC. While previous studies have focused on the mature period of the tornadic supercell (Burgess et al. 2014; Kurdzo et al. 2015), there has been a lack of studies detailing the early evolution of the supercell.

The objective of this study is to analyze the dual-polarization signatures to detail storm-scale processes that occurred over short time periods between (i) the early evolution of the supercell to tornadogenesis, ~19:30 UTC to 19:56 UTC, and (ii) from tornadogenesis through tornado intensification, ~19:56 UTC to 20:08 UTC. These processes are extracted using the high-spatiotemporal resolution data from the PX-1000 to examine low-level processes during tornadogenesis and intensification. A comparative analysis using contemporaneous observations from the nearby WSR-88Ds, and PX-1000 are presented. The evolution of the Z_DR arc and K_DP foot structure, vertical extent of the Z_DR and K_DP columns, tornado debris signature characteristics, radial velocity signatures, and hydrometeor classification are investigated as evidence of precursors to tornadogenesis and the subsequent rapid intensification.

Key findings include the presence of erratic tornado debris signatures observed as early as ~15 minutes prior to tornadogenesis. Radial velocity analyses are presented to determine if low-level rotation was sufficient to meet tornado criteria during this period and examine the transient behavior of this signature. Trends in the vertical extent of Z_DR and K_DP columns are also analyzed to infer the presence and robustness of mid-level updrafts. A Z_DR column split was evident in the same period as the erratic tornado debris signatures, further suggesting that there may have been transient intensifications in shallow layers prior to tornadogenesis. Additionally, trends in Z_DR and K_DP foot spatial structure, magnitudes of peak values, and the separation of these signatures are analyzed for the periods before and after tornadogenesis. Hypotheses of the influence of a storm merger as well as hail versus debris fallout are also discussed.