Low-Level ZDR Signatures in Supercell Forward Flanks: The Role of Size Sorting and Melting of Hail

Abstract

The low levels of supercell forward flanks commonly exhibit distinct differential reflectivity (Z DR ) signatures, including the low-Z DR hail signature and the high-Z DR ''arc.'' The Z DR arc has been previously associated with size sorting of raindrops in the presence of vertical wind shear; here this model is extended to include size sorting of hail. Idealized simulations of a supercell storm observed by the Norman, Oklahoma (KOUN), polarimetric radar on 1 June 2008 are performed using a multimoment bulk microphysics scheme, in which size sorting is allowed or disallowed for hydrometeor species. Several velocity-diameter relationships for the hail fall speed are considered, as well as fixed or variable bulk densities that span the graupel-tohail spectrum. A T-matrix-based emulator is used to derive polarimetric fields from the hydrometeor state variables.Size sorting of hail is found to have a dominant impact on Z DR and can result in a Z DR arc from melting hail even when size sorting is disallowed in the rain field. The low-Z DR hail core only appears when size sorting is allowed for hail. The mean storm-relative wind in a deep layer is found to align closely with the gradient in mean mass diameter of both rain and hail, with a slight shift toward the storm-relative mean wind below the melting level in the case of rain. The best comparison with the observed 1 June 2008 supercell is obtained when both rain and hail are allowed to sort, and the bulk density and associated fall-speed curve for hail are predicted by the model microphysics.

A Finescale Radar Examination of the Tornadic Debris Signature and Weak-Echo Reflectivity Band Associated with a Large, Violent Tornado

“…Dual-polarization observations have identified several previously unobserved features including the differential reflectivity (Z DR ) column, Z DR arc, r hv rings, and tornadic debris signature (TDS), among others (e.g., Illingworth et al 1987;Jameson et al 1996;Ryzhkov et al 2005;Romine et al 2008; Kumjian and Ryzhkov 2008;Palmer et al 2011;Snyder et al 2013;Dawson et al 2014;Kumjian et al 2014;Snyder and Bluestein 2014;Snyder et al 2015). Of the features listed above, the evolution of the TDS as manifest by the copolar cross-correlation coefficient (r hv ) is examined here in detail.…”

The Anatomy and Physics of ZDR Columns: Investigating a Polarimetric Radar Signature with a Spectral Bin Microphysical Model

“…Figure 17 compares the simulated lag correlation with that observed during a supercell that occurred on 1 June 2008 in Oklahoma. This storm is selected because it was sampled with a polarimetric WSR-88D in ''rapid scan'' mode Dawson et al 2014). The peak lag correlation is found after about 23 min in the observations (r lag ' 0.8), although to facilitate comparison with the HUCM-simulated storm it has been shifted left by 8.8 min.…”

Characteristics of flash initiations in a supercell cluster with tornadoes

“…8a2, b2, c2, and d2). The size-sorting hypothesis, in which strongly veering wind shear in supercell environments advects raindrops from their source positions, was believed to result in a region of large drops along the southern edge of the forward flank downdraft (FFD) (e.g., Ryzhkov, 2008, 2009;Dawson et al, 2014).…”