Kinematic and Dynamic Aspects of the Harrah Tornadic Storm Analyzed from Dual-Doppler Radar Data

Abstract

The paper deals with a diagnostic study of the three-dimensional kinematic and dynamic structure of the Harrah tornadic storm. Wind fields were computed from data collected over a 28 min interval in four dual-Doppler radar volume scans during a tornado occurrence. Associated with this storm, there was a strong low-level inflow jet supplying warm, moist air to a tilted updraft, a mid-level vortex doublet, and downdrafts on both storm flanks. The left flank downdraft intensified, undercut the tilted updraft, and formed a gust front along the right rear flank. The gust front propagated cyclonically around the mesocyclone. Calculations of divergence and vorticity showed that in the middle troposphere, the updraft nearly coincided with cyclonic vorticity approximately 10?2 s?1. The low-level tornado cyclone was between the horizontally sheared inflow-outflow region. A mechanism for producing and intensifying this vorticity and downdraft structure is presented on the basis of calculations of the tilting and divergence terms of the vorticity equation. The gross features of the mid-level vortex doublet were simulated by a potential flow model. The storm translational motion is discussed in terms of this model and a balance of drag, momentum and rotational forces. The force due to vertical transport of low-level momentum in the updraft is important in counteracting the large rightward force due to rotation. In an appendix, sources of errors in the wind computations are discussed in terms of the assumptions of the statistical interpolation and vertical motion calculation. The scales of motion resolved in the analysis are larger than approximately 4 km due to the interpolation and grid-filtering used.