Five Scales of Airflow Associated with a Series of Downbursts on 16 July 1980

Abstract

A series of destructive windstorms on 16 July 1980 in a 50 km (30 mi) wide zone from Chicago to Detroit was surveyed both from the air and the ground. In spite of the initial suspicion of 10?20 tornadoes in the area, the nature of the windstorms was confirmed to be downbursts and microbursts characterized by multiple scales of airflows with their horizontal dimensions extending tens of meters to hundreds of kilometers. An attempt was made to estimate the wind speed based on three types of airborne objects: a 180 kg (390 lb) chimney, a 1000 kg (one ton) corn storage bin, and lumber from damaged roofs found inside downburst areas, obtaining the maximum wind speed of 63 ± 10 m -1(140 ± 25 mph). A total of $500 million damage reported was caused by thunderstrom-induced non-tornadic storms which affected very large areas. SMS/GOES pictures showed that the parent cloud was oval-shaped with its lifetime in excess of 12 h. The overshooting areas enclosed by the ?66°C isotherms shrunk rapidly at the onset of the Chicago-area downbursts, indicating that the downbursts began when overshooting activities subsided. This variation of the overshooting features, however, does not necessary imply a direct physical link between the collapsing top and the downbursts at the surface. This paper presents cloud-top features and wind effects on the ground with no attempt to relate them on the basis of conceptual models currently available.