In Situ and Radar Observations of the Low Reflectivity Ribbon

During the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) field campaign, a previously undocumented anomaly was captured by both Doppler on Wheels (DOW) and UMASS X-Pol 3cm radars in at least six separate instances. This feature was named the Low Reflectivity Ribbon (LRR) (Wurman et al. 2012; Snyder et al. 2013), most consistently characterized by depressions in reflectivity (Z_H) and differential reflectivity (Z_DR) through a narrow region extending from the intersection of the ‘hook' appendage and forward flank regions of a supercell.

This study synthesizes kinematic and polarimetric observations made by DOW and UMass X-Pol radars during VORTEX2 with measurements taken by Texas Tech University's “StickNet” observing network. StickNet data have been used to establish, for the first time, a consistent thermodynamic signature associated with the LRR at the surface by matching concurrent StickNet observations with the radar observed position of the LRR. Both single and dual-Doppler analyses of radial velocity are used to examine the two- and three-dimensional structure of the flow within the LRR. Polarimetric radar variables are used to gain information regarding the hydrometeor characteristics which are responsible for the microphysical representation of the LRR and the processes which cause those hydrometers to be present. Special consideration is given to the vertical distribution of traditional and polarimetric variables as well as the progression of radar variables correlating with the temporal evolution of the LRR.

Three separate cases have provided the opportunity to directly measure the surface thermodynamics of the LRR. These observations are consistent in demonstrating that the LRR is associated with a localized minimum in equivalent potential temperature (Θ_e). This study confirms the previous polarimetric observations of the LRR and introduces new information regarding the vertical structure of these variables. For example, the presence of Z_DR and ρ_hv rings aloft have been identified (Kumjian and Ryzhkov 2008), which suggest that the main supercell updraft overlaps the LRR. Radar observations will also be compared to previous polarimetric studies in order to determine what types of hydrometeors reside within the LRR. Additionally, this study presents novel dual-Doppler derived objective analyses involving the two-dimensional winds and implied three-dimensional circulations within the LRR. Finally, a case is presented which illustrates how a changing single Doppler kinematic field correlates with an evolving LRR. A combination of thermodynamic, kinematic, and microphysical observations will allow for a unified theory regarding the processes occurring within the LRR to be formulated.