The maintenance of tornadoes observed with high-resolution mobile Doppler radars

High-resolution dual-Doppler wind syntheses and ensemble Kalman filter data assimilation experiments performed using Doppler on Wheels velocity data collected in four tornadic supercells are examined in order to determine how the evolution of storm structure affects the maintenance of tornadoes. Tornado maintenance can occur as long as the tornado remains connected to the horizontal convergence located along the rear-flank gust front. The tornadoes dissipate when this connection is severed. The strongest and longest-lived tornado examined in this study maintains a connection to the primary rear-flank gust front for a considerable time. The weakest and shortest-lived tornado maintains only brief contact with the gust front. Rather than perishing while completely surrounded by outflow air (as is shown in past conceptual models of tornadic supercell storms), the two tornadoes with a moderate intensity and duration are sustained by convergence and vertical vorticity provided by secondary rear-flank gust fronts. It is speculated that these tornadoes do not achieve a greater intensity or a longer duration because of the transient nature of the secondary gust fronts to which they are connected.;The strongest and longest-lived tornado dissipates when it becomes far removed from the mid-level updraft. At this time, the mid-level updraft is fueled by near-surface inflow that rises straight upward. However, the weakest and shortest-lived tornado dissipates beneath the mid-level updraft that primarily ingests near-surface inflow located several kilometers to its east. This observation suggests that having a tornado located beneath the primary updraft is not always a sufficient condition for maintenance, particularly in the presence of strongly surging outflow winds.;The three-dimensional structure of vortex lines along the rear-flank gust fronts is consistent with the tilting of baroclinically-generated horizontal vortex rings surrounding a negatively buoyant downdraft. When the tornadoes are completely surrounded by outflow air, tilting along the primary rear-flank gust front does not generate cyclonic vertical vorticity at the tornado. Instead, tilting of the horizontal vortex lines by the secondary gust fronts more directly supplies the mature tornadoes with low-level rotation. This observation suggests that relatively cold secondary surges of outflow may assist with tornado maintenance despite a greater negative buoyancy. Thermodynamic data retrieved in the two data assimilation cases suggest that a warming of the outflow appears to be symptomatic of changes in the rear-flank downdraft that adversely affect tornado maintenance.