A Numerical Investigation of a Supercell Tornado: Genesis and Vorticity Budget

Abstract

The mechanism of supercell tornadogenesis and its vorticity budget are investigated by means of a highresolution (horizontally uniform grid size of 70 m) numerical simulation of the Del City storm, which occurred in Oklahoma, USA, on May 20, 1977. After 50 min of the simulation, a meso-low, which is generated by nonlinear interaction between the storm updraft and vertical wind shear associated with both environmental and storm-induced horizontal flow, develops at around 1.8 km above ground level (AGL). The meso-low acts to strengthen the underlying updraft, and generates a low-level mesocyclone via the tilting of horizontal vorticity associated with the environmental wind and that generated by baroclinic processes. In turn, a pressure depression associated with this low-level mesocyclone generates an updraft exceeding 43 m s À1 at 1.5 km AGL. In addition, small-scale vortices (pretornadic vortices) develop along a gust front. When the low-level updraft strengthened, one of these pretornadic vortices located immediately beneath the updraft shows a rapid growth into a major tornado. As the tornado develops, a downward pressure gradient force associated with an intense rotation of the tornado strengthens a tornado-scale downdraft on its north side. The developed downdraft compresses the vertical vortex of the tornado, eventually resulting in its dissipation.We also performed a vorticity budget analysis along a typical air-parcel trajectory. Air parcels in the mature tornado vortex originate mainly from the northwest in a layer between 10 and 500 m AGL. Vertical vorticity within the mature tornado is initially produced during descent via tilting of the horizontal vorticity, which is enhanced by stretching of the vortex tube.

An Investigation of the Goshen County, Wyoming, Tornadic Supercell of 5 June 2009 Using EnKF Assimilation of Mobile Mesonet and Radar Observations Collected during VORTEX2. Part I: Experiment Design and Verification of the EnKF Analyses

“…19). The majority of the parcels en route to the low-level mesocyclone in the EnKF analyses originate near the ground, many ascending to z $ 1500 m AGL before descending into their positions in the ring at 700 m. Qualitatively similar parcel paths have been inferred near the time of tornado or mesocyclone maturity from past observations or modeling studies (e.g., Brandes 1981;1984;Wicker and Wilhelmson 1995;Wakimoto and Liu 1998;Dowell and Bluestein 2002;Mashiko et al 2009;Noda and Niino 2010;Dahl et al 2012). The trajectories are computed within the storm-relative reference frame (using a mean midlevel updraft motion) to minimize temporal interpolation errors caused by the motion of storm features.…”

Uncertainties in Trajectory Calculations within Near-Surface Mesocyclones of Simulated Supercells

“…These ''inflow trajectories'' are quite prevalent in the literature (Rotunno and Klemp 1985;Davies-Jones and Brooks 1993;Wicker and Wilhelmson 1995;Adlerman et al 1999;Atkins and St. Laurent 2009;Mashiko et al 2009;Noda and Niino 2010). A widely used approach to address this problem, both in models and dual-Doppler analyses, is the backward integration of trajectories initialized within the nearsurface vortex.…”

Future Change of Tornadogenesis-Favorable Environmental Conditions in Japan Estimated by a 20-km-Mesh Atmospheric General Circulation Model

“…The lower threshold of CIN (30 J kg -1 ) was the 10th percentile value, and the higher threshold (400 J kg -1 ) was the 90th percentile value in all tornado events (not shown). The lower threshold value was applied as well, because tornado events almost never occurred when the CIN was very small, probably because low-level dry air, which reinforces downdrafts and cold outflow associated with supercell storms, is an important factor that contributes to tornadogenesis (Markowski and Richardson 2010;Noda and Niino 2010). The results showed that the characteristics of the distributions of FSC future/ present ratios were very similar to those without the CIN condition (Figs.…”