The 29 June 2000 Supercell Observed during STEPS. Part I: Kinematics and Microphysics

This is a two-part study that addresses the kinematic, microphysical, and electrical aspects of a severe storm that occurred in western Kansas on 29 June 2000 observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) field campaign. In this first part, polarimetric and Doppler radar data are used along with a simple particle growth model to examine the evolution of the kinematic and microphysical properties of the storm from its earliest developing phase through its mature and dissipating phases. During its severe stage, the storm exhibited frequent positive cloud-to-ground lightning strikes, very large (∼5 cm) hail, and a tornado. Doppler-derived winds, radar reflectivity, and hydrometeor classifications from the polarimetric data over a nearly 4-h period are presented. It is shown that updraft velocity and vertical vorticity had to reach magnitudes of at least 10 m s−1 and 10−2 s−1 and occupy major portions of the storm before it could produce most of the observed severe storm characteristics. Furthermore, the establishment of cyclonic horizontal flow around the right flank of the updraft core was essential for hail production. Most of the largest hail grew from near millimeter-sized particles that originated in the mid- to upper-level stagnation region that resulted from obstacle-like flow of environmental air around the divergent outflow from the upper part of the updraft. These recycling embryonic particles descended around the right flank of the updraft core and reentered the updraft, intermingling with other smaller particles that had grown from cloud base along the main low-level updraft stream.