Quantifying the Relationship Between the ZDR Arc Signature and Low-Level Vertical Wind Shear

The ZDR arc signature found in supercell storms is hypothesized to be related to size sorting of hydrometeors by the low-level vertical wind shear. Previous studies have suggested a relation between the low-level wind shear and the spatial offset between the ZDR arc and the storm's core. The alignment of the ZDR arc's axis relative to the storm-motion vector may be an indicator of enhanced low-level storm-relative helicity. In addition, observations generally reveal that the presence of the ZDR arc is most notable leading up to the occlusion of the low-level mesocyclone, which may precede tornadogenesis in tornadic storms. Thus, the appearance of this signature could be a compelling precursor to tornadogenesis and potentially may have applications widely used by operational meteorologists.

With the upgrade to polarimetric capabilities of the U.S. NEXRAD network complete, the ability to relate and quantify the relationship between the ZDR arc and low-level wind shear in supercells in a variety of locations is now possible. Observations from polarimetric WSR-88D radars are analyzed to pursue the following objectives: 1) Develop a preliminary method for objectively locating the ZDR arc in supercells; 2) Quantify the offset between the ZDR arc and the storm's high-reflectivity core; 3) Investigate the alignment of the ZDR arc and the storm-motion vector and its relation to the storm's observed tornadic history; and 4) Relate these radar-observed quantities to the low-level vertical shear as inferred from soundings.