11jun2015 cell initiation
Overview
During the afternoon hours of 11 June 2015 several clusters of thunderstorms developed in the CSU-CHILL radar coverage area. One such cluster formed in the immediate vicinity of Loveland Colorado and produced locally flooding rainfall. The CSU-CHILL radar recorded data using continuous 1.5 deg elevation angle 360 surveillance scans with a repetition interval of ~1.6 minutes. This rapid update rate allowed the evolution of the Loveland area thunderstorm complex to be monitored in detail. As depicted in the following image, the low level convergence associated with a westward moving fine line was a significant factor in the initiation and intensification of the thunderstorm cells:
The three cells of particular interest developed in a sequential east to west manner as the fine line propagated through the Loveland area. These cells are identified in the next plot:
Reflectivity Loop
The evolution of the Loveland area thunderstorms is shown in the following image loop. The central cell (#2) intensified at a rate of ~10 dB per scan between 1939 and 1946 UTC (loop frame numbers 13 and 18).
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Differential propagation phase loop
The next loop covers the later stages of the previously-shown reflectivity sequence when the three individual thunderstorm cells merged into a single east-west oriented echo system. This merged echo complex was aligned in a radial direction with respect to the CSU-CHILL radar. This orientation maximized the differential propagation phase shift between the H and V polarized waves as they traveled an ~15 km long path through heavy rain. (i.e., propagation through the high concentration of oblate rain drops retarded the received phase of the H polarized return signals relative to the V returns.)
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Specific propagation differential phase at 2007 UTC
The final plot shows the one-way Kdp values measured in the radially-aligned echo complex at 2007 UTC. Kdp is the range derivative of the phidp profile along the radar beams. The peak Kdp values of ~3 deg / km observed at a radar wavelength of 10 cm imply that rainfall rates of ~100 mm / hr were occurring. As is typical of thunderstorm precipitation, these intense rain rates were confined to a small area located between central Loveland (point marked as LVE CTY on the plot) and the Fort Collins - Loveland Airport (KFNL AP). (These two locations are ~8 km apart.) The NWS Boulder office issued a flash flood warning for this echo system. Volunteer observers in the Community Collaborative Rain Hail and Snow (CoCoRaHS) network reported 24 hour rain totals of ~2.0 to 2.5 inches in the Loveland area. (web link)
