Doppler radar investigation of tornadic and lightning producing storms in northeast Colorado
Date
1996-05
Authors
Lucci, Richard M., author
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Abstract
Kinematic, microphysical, and electrical characteristics of two multi-cell thunderstorms observed in northeast Colorado were investigated using single- and dual- Doppler radar and cloud-to-ground (CG) lightning data. These particular thunderstorms were chosen because of their generation of weak short-lived tornadoes and CG lightning dominated by positive flashes. Storm data was collected using the multiparameter, CSUCHILL, and the National Center for Atmospheric Research (NCAR) Mile High Doppler radars. Various radar analysis methods were employed in an effort to infer origins of rotation, tornado formation mechanisms, and the storm structure responsible for the observed lightning patterns. Single-Doppler techniques included multiparameter measurements, calculations of azimuthal shear, histogram analyses, a study of constant altitude and vertical cross-section data, and an examination of the unedited reflectivity and radial velocity fields. Dual-Doppler data was used to synthesize the three-dimensional wind field. The synoptic, meteorological conditions in which these thunderstorms developed were found to be similar to those referred to as a High Plains "day 2" scenario. In the first case study, a thunderstorm cell initially produced moderate rates of negative CG lightning and then ceased CG activity as the cell underwent rapid intensification. This cell later produced three positive CG flashes upshear of the main reflectivity core and generated a weak tornado (gustnado) along its outflow boundary. Dual-Doppler data revealed intense updrafts in excess of 40 m s·1• The positive flashes and initial generation of the shear-induced vorticity were found to be concurrent with the formation of hail induced downdrafts. An azimuthal shear feature associated with the formation of the gustnado was tracked and no apparent link was found to the mid-level mesocyclone. The thunderstorm of the second case study produced persistent dominant CG flashes of positive polarity. A column of positive differential reflectivity (Zdr) was used to infer the presence of a strong updraft containing supercooled droplets which contributed to the formation of a large ice and hail mass aloft A unique hail curtain extended from this region to the surface, along the edge of the main updraft, producing an intense region of horizontal shear which spawned an observed tornado. The existence of a well organized and intense updraft was believed to be the primary ingredient necessary to produce the observed lightning patterns and weak tornadoes not associated with a midlevel mesocyclone for both case studies.
Description
May 1996.
Also issued as author's thesis (M.S.) -- Colorado State University, 1996.
Also issued as author's thesis (M.S.) -- Colorado State University, 1996.
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Subject
Doppler radar -- Colorado
Thunderstorms -- Colorado
Severe storms -- Colorado