Previous cloud modeling studies have noted difficulty in producing strong, sustained deep convection in environments with convective inhibition and/or midlevel dryness when the thermal bubble technique is used to initiate convection. This difficulty is also demonstrated herein, using 113 supercell proximity soundingsmost of which contain capping inversions and some amount of convective inhibition. Instead, by using an updraft nudging initiation technique, substantially more supercells result and for a longer period. Additionally, the number of supercell-producing cases is maximized when updraft nudging is applied for only the first 15 min of cloud time near the top of the boundary layer instead of longer/shorter periods or when nudging is applied near the surface.

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.

[1] Simulations were performed using an idealized cloud model to investigate the environmental conditions influential to tornado duration and intensity. The results from twentyone tornadic simulations are presented-far more than any previous study. The results show a nearly linear relationship between storm relative environmental helicity (SREH) and simulated tornado duration. A multiple linear regression analysis was performed showing that, in addition to SREH, convective available potential energy (CAPE), convective inhibition (CIN), and precipitable water (Pwat) are useful predictor variables for simulated tornado duration. Similar results were found for simulated tornado intensity, as represented by the maximum central pressure drop.Citation: Naylor, J., and M. S. Gilmore (2012), Environmental factors influential to the duration and intensity of tornadoes in simulated supercells, Geophys.

This paper examines the nature and consequences of inner core instabilities in several intense tropical cyclones (TCs) simulated with a cloud-resolving numerical model. The initial wave growth leading to polygonal eyewalls and mesovortices in each TC is shown to closely resemble that found in a dry nonconvective vortex with the same primary circulation. Such agreement is reasonable partly because the bulk of the cloudy eyewall updraft is outside the vorticity ring in which the instability occurs. An energetic analysis of the TC instability verifies that the symmetric secondary circulation contributes relatively little to the early growth of eddy kinetic energy. On a longer time scale, isentropic potential vorticity mixing triggered by the instability irreversibly reduces the maximum wind speed of the dry vortex. In the more realistically simulated TC, moist convection modulates the mixing and regenerates the broken vorticity ring. Eventually, the maximum wind speed of the TC and the local parameters that determine its theoretical magnitude return to within a few percent of their preinstability values. It is found that the time scale for wind speed restoration is sensitive to the surface drag coefficient. The long-term effects of inner core instability on precipitation are briefly addressed.

A three-dimensional idealized cloud model was used to study the storm-scale differences between simulated supercells that produce tornado-like vortices and those that do not. Each simulation was initialized with a different Rapid Update Cycle, version 2 (RUC-2), sounding that was associated with tornadic and nontornadic supercells in nature. The focus is an analysis of vorticity along backward-integrated trajectories leading up to tornadogenesis (19 simulations) and tornadogenesis failure (14 simulations). In so doing, the differences between the nontornadic and tornadic cases can be explored in relation to their associated environmental sounding.Backward-integrated trajectories seeded in the near-surface circulation indicate that the largest differences in vertical vorticity production between the tornadic and nontornadic simulations occur in parcels that descend to the surface from aloft (i.e., descending). Thus, the results from this study support the hypothesis that descending air in the rear of the storm is crucial to tornadogenesis. In the tornadic simulations, the descending parcels experience more negative vertical vorticity production during descent and larger tilting of horizontal vorticity into positive vertical vorticity after reaching the surface, owing to stronger horizontal gradients of vertical velocity. The larger vertical velocities experienced by the trajectories just prior to tornadogenesis in the tornadic simulations are associated with environmental soundings of larger CAPE, smaller convective inhibition (CIN), and larger 0-1-km storm-relative environmental helicity. Furthermore, in contrast with what might be expected from previous works, trajectories entering the incipient tornadic circulations are more negatively buoyant than those entering the nontornadic circulations.