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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.

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Studies of small cumulus clouds in Florida using X-and S-band radar (3-and 10-cm wavelengths) reveal both hydrometeor and Bragg scattering signals. Turbulent mixing between cloudy and drier environmental air can produce centimeter-scale variations in refractive index that can lead to strong mantle echoes around the sides and tops of the clouds. When the environmental air is exceptionally dry, the S-band Bragg scattering signals are as strong as 10 dBZ at cloud boundaries, with weaker echoes in the cloud cores where hydrometeor scattering is also present. The Bragg signal at S-band is typically about 19 dB stronger than that at X-band, as expected from theory. However, there is in many cases an unexplained, Bragg-like return from the clouds at Sband that correlates with the X-band echo but is only about 10 dB stronger. The X-band echo is often dominated by backscattering from the cloud droplets, and shows adiabatic ascent within the cloud cores fairly often up to at least 1 km above cloud base. In these cases, the radar echo profiles can be used to estimate the adiabatic droplet concentration, given rough knowledge of the cloud-base height and temperature. The first precipitation shafts often occur before the cloud tops reach the 0ЊC level, are narrow, and probably consist of low concentrations of drops several millimeters in diameter.

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