High resolution single-Doppler observations of two tornadoes

Two tornadic storms were observed during 11 March 2000 within 15 miles of the Texas A&M University's 10-cm wavelength Aggie Doppler radar (ADRAD). The radar operated in sector scan mode with 8 sweeps covering from 0.5 to 7.2 degrees in elevation. Each volume scan took about 2 minutes to complete. The single-Doppler observations indicate that the first tornado, a F1 storm (Fig.1), developed rapidly at low levels although it was preceded by a strong mid-level mesocyclone. Colliding outflows from neighboring cells triggered a new cell in a region of cyclonic shear just beneath the mid-level mesocyclone and helped spin up the short-lived circulation. Despite dissipation of the first tornado, the region of low-level cyclonic shear persisted as the storm system propagated to the southeast. About twenty minutes after the first tornado dissipated, a strong downburst occurred just to the north of the zone of low level cyclonic shear. The outflow from this downburst helped spin up the second tornado, a F3 (Fig 2), with measured gate-gate shear in excess of 100 knots. Radar reflectivity suggests that this circulation was rain-wrapped almost immediately. Nevertheless, the tornado produced a continuous damage track during a 20 minute period. The storm then became outflow dominated and produced a bow echo that later caused additional damage but no other tornadoes. The high resolution data demonstrated the importance of low-level outflows on the development of the tornadic circulations in this storm system.