A NUMERICAL STUDY OF A ROTATING DOWNBURST

Previous studies have revealed that convective storms often contain in tense small-scale downdrafts, termed ''downbursts,'' that are a signif icant hazard to aviation. These downbursts sometimes possess strong ro tation about their vertical axis in the lower and middle levels of the storm, but studies of how this rotation is produced and how it impact s downdraft strength are lacking. In this study a three-dimensional cl oud model was used to simulate a rotating downburst based on condition s observed on a day that produced rotating downbursts. It was found th at rotating downbursts may occur when the direction of the wind shear vector in the middle levels of the troposphere varies with height. In the early stages of the convective system, vertical vorticity is gener ated from tilting of the ambient vertical shear by the updraft, result ing in a vertical vorticity couplet on the flanks of the updraft. Late r, the negative buoyancy associated with precipitation loading causes the updraft to collapse and to be eventually replaced by a downdraft d ownshear of the midlevel updraft. When the direction of the vertical s hear vector varies with height, a correlation may develop between the location of the vertical vorticity previously produced by the updraft at midlevels and the location of the developing downdraft. This mechan ism causes downbursts to rotate cyclonically when the vertical shear v ector veers with height and to rotate anticyclonically when the vertic al shear vector backs with height. The rotation associated with the do wnburst, however, does not significantly enhance the peak downdraft ma gnitude. The mechanism for the generation of vorticity in a downburst is different from that found for supercell downdrafts, and, for a give n vertical shear vector, downbursts and supercell downdrafts will rota te in the opposite sense.