Comparison of Objective Supercell Identification Techniques Using an Idealized Cloud Model

Abstract

The accuracy, reliability, and skill of several objective supercell identification methods are evaluated using 113 simulations from an idealized cloud model with 1-km horizontal grid spacing. Horizontal cross sections of vorticity and radar reflectivity at both mid-and low levels were analyzed for the presence of a supercell, every 5 min of simulation time, to develop a ''truth'' database. Supercells were identified using well-known characteristics such as hook echoes, inflow notches, bounded weak-echo regions (BWERs), and the presence of significant vertical vorticity.The three objective supercell identification techniques compared were the Pearson correlation (PC) using an analysis window centered on the midlevel storm updraft; a modified Pearson correlation (MPC), which calculates the PC at every point in the horizontal using a small 3 km 3 3 km analysis window; and updraft helicity (UH). Results show that the UH method integrated from 2 to 5 km AGL, and using a threshold value of 180 m 2 s 22 , was equally as accurate as the MPC technique-averaged from 2 to 5 km AGL and using a minimum updraft threshold of 7 m s 21 with a detection threshold of 0.3-in discriminating between supercells and nonsupercells for 1-km horizontal grid spacing simulations. At courser resolutions, the UH technique performed best, while the MPC technique produced the largest threat scores for higher-resolution simulations. In addition, requiring that the supercell detection thresholds last at least 20 min reduced the number of false alarms.

Roles of wind shear at different vertical levels: Cloud system organization and properties

“…At the higher levels at 8–10 km, a clockwise circulation is produced corresponding to the divergence (Figure c). The updraft helicity (UH) is greater than 60 m 2 s −2 at 08:00 UTC, also suggesting a high vorticity system and supercellular‐like system [ Naylor et al , ].…”

Environmental factors influential to the duration and intensity of tornadoes in simulated supercells

“…Idealized simulations were performed using the Bryan cloud model [ Bryan and Fritsch , 2002]. The model setup was identical to that of Naylor et al [2012a], except for the following: 100 m grid spacing (vertical and horizontal) within a 120‐km × 120‐km × 20‐km domain; a large time step of 1 s for advection and a small time step of 0.167 s. Convection was initiated with an updraft nudging technique similar to that used by Ziegler et al [2010] and Naylor et al [2012a, 2012b]for the first 900 s of cloud time over a 10‐km × 10‐km × 3‐km spheroid centered at z = 1.5 km. Each idealized simulation was initialized with one of the 113 RUC‐2 proximity soundings associated with significantly tornadic (EF2 or greater) supercells from Thompson et al [2003].…”

Convective Initiation in an Idealized Cloud Model Using an Updraft Nudging Technique

“…The model setup follows Naylor et al (2012), which includes a single moment, bulk ice microphysics parameterization (Gilmore et al 2004) with default parameters; a model grid that moves FIG. 1.…”

“…180 m 2 s 22 for 20 min; following Naylor et al 2012). This method is used to determine both supercell duration and supercell intensity (time average of domain maximum updraft helicity) based on model output available every 60 s during the simulation.…”

“…This method is used to determine both supercell duration and supercell intensity (time average of domain maximum updraft helicity) based on model output available every 60 s during the simulation. Naylor et al (2012) showed that the UH technique has a small (roughly 5%) false alarm rate from successive nonsupercell mesocyclones that jointly exceed the 20-min temporal criteria.…”