Casebook 25 June 2005

CSU-CHILL Casebook: 25 June, 2005

During the afternoon hours of 25 June 2005, a group of thunderstorms developed in the southeast azimuth quadrant of the CSU-CHILL radar. The leading edge of the low level outflow from these storms produced a gust front that propagated into the boundary layer airmass ahead of the storms. Insects were fairly evenly distributed within the undisturbed regions of the boundary layer. The insect concentrations were enhanced by the convergent air motions along the gust front. The distinctly flattened shapes of the insects gives highly positive $Z_{dr}$ values (> ~6 dB). In contrast, the maximum $Z_{dr}$ values in rain are typically ~4 dB. These two $Z_{dr}$ regimes can be seen in the thunderstorm RHI scan that was taken at 2221 UTC. (See VCHILL example below.) In this RHI plane, a narrow rain shaft was falling into the gust front at a range of 35 km. The rain shaft produced a reduced $Z_{dr}$ intrusion into the insect echo along the gust front.

example in VCHILL

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