The genesis of a strong and long-lived tornado observed during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) in Goshen County, Wyoming, on 5 June 2009 is studied. Mobile radar, mobile mesonet, rawinsonde, and photographic data are used to produce an integrated analysis of the evolution of the wind, precipitation, and thermodynamic fields in the parent supercell to deduce the processes that resulted in tornadogenesis. Several minutes prior to tornadogenesis, the rear-flank downdraft intensifies, and a secondary rear-flank downdraft forms and cyclonically wraps around the developing tornado. Kinematic and thermodynamic analyses suggest that horizontal vorticity created in the forward flank and hook echo is tilted and then stretched near the developing tornado. Tilting and stretching are enhanced in the developing low-level circulation as the secondary rear-flank downdraft develops, intensifies, and wraps around the circulation center. Shortly thereafter, the tornado forms. Tornadogenesis does not proceed steadily. Strengthening, weakening, and renewed intensification of the tornado are documented in photographic, reflectivity, Doppler velocity, and dual-Doppler fields and are associated with, and shortly follow, changes in the secondary rear-flank downdraft, convergence, location of the vortex relative to the updraft/downdraft couplet, tilting and stretching near and in the developing tornado, and the evolution of total circulation.

Dual-Doppler wind synthesis and ensemble Kalman filter analyses produced by assimilating Doppler-on-Wheels velocity data collected in four tornadic supercells are examined in order to further understand the maintenance of tornadoes. Although tornado-scale features are not resolved in these analyses, larger-scale processes involved with tornado maintenance are well represented. The longest-lived tornado is maintained underneath the midlevel updraft within a zone of low-level horizontal convergence along a rear-flank gust front for a considerable time, and dissipates when horizontally displaced from the midlevel updraft. The shortest-lived tornado resides in a similar zone of low-level convergence briefly, but dissipates underneath the location of the midlevel updraft when the updraft becomes tilted and low-level convergence is displaced several kilometers from the tornado. This suggests that a location beneath the midlevel updraft is not always a sufficient condition for tornado maintenance, particularly in the presence of strongly surging outflow. Tornadoes in two other storms persist within a band of low-level convergence in the outflow air (a possible secondary rear-flank gust front), suggesting that tornado maintenance can occur away from the main boundary separating the outflow air and the ambient environment. In at least one case, tilting of horizontal vorticity occurs near the tornado along the secondary gust front, as evidenced by three-dimensional vortex line arching. This observation suggests that a relatively cold secondary rear-flank downdraft may assist with tornado maintenance through the baroclinic generation and tilting of horizontal vorticity, despite the fact that parcels composing them would be more negatively buoyant than the preceding outflow air.

Fine-resolution single-and dual-Doppler data were collected in the tornadic region of a supercell storm intercepted by two Doppler-on-Wheels radars on 30 April 2000 near Crowell, Texas. Eleven dual-Doppler analyses characterize the 2D and 3D near-surface wind fields associated with a tornado during a 13-min period. An interesting evolution of the low-level rotation is observed. Initially concentric "tornado" (ϳ500 m wide) and "tornado-cyclone" (ϳ2 km wide) radar velocity couplets make a transition into a solitary intermediate-sized (ϳ750 m wide) circulation that widens and makes a further transition into a two-celled multiple-vortex structure with an asymmetric distribution of vertical vorticity. The asymmetry and eventual disruption of the multiple-vortex structure may have been partially controlled by locally strong outflow winds that affect the convergence fields in its vicinity. A smaller (ϳ500 m wide) tornado embedded in a broad area of rotation is subsequently observed. The dual-Doppler wind fields are also used to characterize aspects of the storm-scale flow. Locally surging outflow winds result in a double rear-flank gust front structure. The tornado and tornado-cyclone are completely surrounded by outflow at all observation times and air parcels traced within the inflow to the storm rise along the gust front rather than enter the tornado near the ground.

During the International H2O Project, mobile radars collected high-resolution data of several 0.5–2-km-wide vertically oriented vortices (or misocyclones) along at least five mesoscale airmass boundaries. This study analyzes the properties of the misocyclones in three of these datasets—3, 10, and 19 June 2002—to verify findings from finescale numerical models and other past observations of misocyclones and to further the understanding of the role that they play in the initiation of deep moist convection and nonsupercell tornadoes. Misocyclones inflect or disjoint the swath of low-level convergence along each boundary to varying degrees depending on the size of their circulations. When several relatively large misocyclones are next to each other, the shape of low-level convergence along each boundary is arranged into a staircase pattern. Mergers of misocyclones are an important process in the evolution of the vorticity field, as a population of small vortices consolidates into a smaller number of larger ones. Additionally, merging misocyclones may affect the mixing of thermodynamic fields in their vicinity when the merger axis is perpendicular to the boundary. Misocyclones interact with linear and cellular structures in the planetary boundary layers (PBLs) of the air masses adjacent to each boundary. Cyclonic low-level vertical vorticity generated by both types of structures makes contact with each boundary and sometimes is incorporated into preexisting misocyclones. Intersections of either type of PBL structure with the boundary result in strengthened pockets of low-level convergence and, typically, strengthened misocyclones.

The dynamical processes responsible for the intensification of low-level rotation prior to tornadogenesis are investigated in the Goshen County, Wyoming, supercell of 5 June 2009 intercepted by the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). The circulation of material circuits that converge upon the low-level mesocyclone is principally acquired along the southern periphery of the forward-flank precipitation region, which is a corridor characterized by a horizontal buoyancy gradient; thus, much of the circulation appears to have been baroclinically generated. The descending reflectivity core (DRC) documented in Part I of this paper has an important modulating influence on the circulation of the material circuits. A circuit that converges upon the low-level mesocyclone center prior to the DRC's arrival at low levels (approximately the arrival of the 55-dBZ reflectivity isosurface in this case) loses some of its previously acquired circulation during the final few minutes of its approach. In contrast, a circuit that approaches the low-level mesocyclone center after the DRC arrives at low levels does not experience the same adversity.An analysis of the evolution of angular momentum within a circular control disk centered on the low-level mesocyclone reveals that the area-averaged angular momentum in the nearby surroundings of the low-level mesocyclone increases while the mesocyclone is occluding and warm-sector air is being displaced from the near surroundings. The occlusion process reduces the overall negative vertical flux of angular momentum into the control disk and enables the area-averaged angular momentum to continue increasing even though the positive radial influx of angular momentum is decreasing in time.