Idealized Model Simulations to Determine Impacts of Storm‐Relative Winds on Differential Reflectivity and Specific Differential Phase Fields

Abstract

As clouds begin to precipitate, there are several different microphysical processes that can change the drop size distribution (DSD) as the drops fall out, such as evaporation, breakup, and coalescence. The DSDs can also evolve due to hydrometeor size sorting arising from differential sedimentation. Terminal velocities of raindrops increase with increasing size (e.g., Beard, 1976; Brandes et al., 2002; Foote & du Toit, 1969; R. Gunn & Kinzer, 1949) so that larger drops fall through a given layer more rapidly than smaller drops. Larger drops will reach the surface before smaller drops, so there will be an initial sorting of raindrops by size with progressively smaller drops located higher above the surface. However, after some time (assuming steadystate conditions aloft), the smaller drops will reach the surface and there will no longer be a separation of different drop sizes. This transient size sorting from initial differential sedimentation lasts on the order of 5-10 min (Kingfield & Picca, 2018). There are mechanisms that will maintain this size sorting beyond this initial transient effect, as described in Kumjian and Ryzhkov (2012), hereafter K12. One of these mechanisms is an updraft, where only larger raindrops with sufficiently large terminal velocities can overcome the strength of the updraft and descend to the surface. Another mechanism, and the focus of this study, is storm-relative flow located in the layer through which the drops descend. These storm-relative winds are the fundamental mechanism for this type of size sorting (Dawson et al., 2015, hereafter D15) and not shear or storm-relative helicity (SRH) as previous studies had suggested (e.g., K. Gunn & Marshall 1955; Kumjian & Ryzhkov, 2009). Larger drops with faster fall speeds fall through a sorting layer more quickly compared to smaller drops with slower fall speeds. The decreased time in a sorting layer leads to the larger drops being advected a shorter distance downwind

Using the Supercell Polarimetric Observation Research Kit (SPORK) to Examine a Large Sample of Pretornadic and Nontornadic Supercells

“…Much recent work has focused on examining the unique dual-pol signatures found in supercell storms (Romine et al 2008; Ryzhkov 2008, 2009;Van Den Broeke et al 2008;Kuster et al 2019;Homeyer et al 2020;Van Den Broeke 2016, 2020). Several of these signatures have exhibited potentially useful differences between severe and nonsevere storms and between tornadic and nontornadic supercells, including differential reflectivity (Z DR ) columns (VDB20, Kuster et al 2019Kuster et al , 2020, inferred hailfall area (VDB16, VDB20), and the separation between areas of enhanced Z DR and specific differential phase (K DP ) (Crowe et al 2012;Loeffler and Kumjian 2018;Loeffler et al 2020;Homeyer et al 2020). A brief description of each of these signatures and the potential connections between their characteristics and a given supercell's hazard production is given below.…”

“…Size sorting of rain and melting hail by the storm-relative wind often leads to a pronounced area of enhanced Z DR along the edge of the storm's forward flank, known as the Z DR arc (e.g, Ryzhkov 2008, 2009;Dawson et al 2014Dawson et al , 2015, as well as a pronounced horizontal separation between the Z DR arc and an area of enhanced K DP in the storm core known as the K DP foot (Romine et al 2008;Crowe et al 2010Crowe et al , 2012Loeffler and Kumjian 2018;Loeffler et al 2020). While the maximum value of Z DR in the Z DR arc was initially thought to be correlated to the strength of low-level storm-relative helicity in a supercell's inflow (Kumjian and Ryzhkov 2009) and could be of use in identifying storms that are more likely to be tornadic, subsequent work revealed that Z DR arc size and intensity tend to differ little between tornadic and nontornadic storms (VDB20).…”

“…However, the separation between the Z DR arc and K DP foot has shown more promise in distinguishing between tornadic and nontornadic storms. Loeffler and Kumjian (2018) found that a vector drawn from the centroid of the K DP foot to the centroid of the Z DR arc (referred to as the separation vector) in tornadic nonsupercell storms tended to be closer to perpendicular to storm motion for a given vector length when low-level SRH was higher. In a larger sample of supercells (n = 116), Loeffler et al (2020) found that the separation vector was closer to perpendicular to storm motion in tornadic storms than in nontornadic storms.…”

“…However, the latter still required manual intervention in cases where a Z DR column was composed of multiple areas of enhanced Z DR . For low-level signatures of size sorting, Loeffler and Kumjian (2018) built a semi-automated algorithm to calculate the separation vector between areas of enhanced Z DR and K DP in nonsupercell storms, that was then applied to supercells by Loeffler et al (2020).…”

“…Question 2 focuses on conducting a novel analysis of how dual-pol signatures vary in different environments. The relationship between Z DR arc and K DP -Z DR separation signature characteristics and low-level wind field characteristics has been examined (e.g., Ryzhkov 2008, 2009;Dawson et al 2014Dawson et al , 2015Loeffler andKumjian 2018, 2020); however, comparatively little is known about how these and other dual-pol signatures vary with other environmental parameters outside of the small sample of storms examined by VDB16. To answer these questions, Section 2 of this paper describes the design of SPORK and its validation against manually calculated dual-pol metrics from the dataset used by VDB20.…”