Tornado Formation from Downdraft Interaction In the FACE Mesonetwork

Abstract

A tornado observed on 15 June 1973 in the FACE surface mesonetwork was studied on the mesoscale and cloud scale. Downdrafts from two preexisting cumulonimbi, initially 80 km apart, met along a north-south line in the center of the mesonet 30 min before tornado formation. Fed by the convergence of flow from the outflows of the predecessor cumulonimbi, a line of deep cumuli formed and developed rapidly. A tornado was observed as it dropped from this cumulus line. When the tornado dissipated 10 min later, heavy precipitation was reaching the surface and new outflow began to spread from the now vigorous cumulonimbus that had spawned the tornado. The life cycle of the tornado and a period of 90 rain surrounding its occurrence studied in detail from observed surface winds, radar reflectivity and surface rain gage data. The evolution of the parent cloud and tornado in a tropical thermodynamic environment with local forcing, weak shear and winds, and a potentially unstable sounding contrasts with the conditions that accompany large-scale forcing of the parent clouds in which extratropical tornadoes are found. The 850?200 mb wind shear of <2 m s?1 was the weakest found over many summers of FACE at Miami and was the only unique environmental parameter detectable on the day when the tornado formed. The similarity of the 15 June FACE tornado to Florida waterspout life cycles is noted.