An area of non-severe, rather ordinary, thunderstorms moved east from the foothills of the Rocky Mountains during the afternoon hours of 6 September. During the first portion of the CSU-CHILL radar data collection period, a continuous series of 4 sweep RHI volumes were started. To maximize the length of time that the echoes remained in the RHI planes, the azimuth angles were selected so that they were generally along the echo motion direction as estimated from NWS KDEN image loops. Following this period of RHI scanning, a two elevation angle PPI sector scan covering the southern azimuth sector was established. Both scan regimes captured the evolution of several aspects of the convection.
X-band RHI loops on the 286 degree azimuth
The sequence of echo patterns observed in the first RHI sweep (azimuth 286 degrees) showed an echo core whose centriod descended with time as it approached the radar. The time step between the images is ~3 minutes.
Reflectivity loop
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Radial velocity loop
The final frame of the radial velocity loop shows the appearance of of divergence pattern centered near the 19.5 km range. At this time, the inbound (negative) and outbound (positive) radial velocity features are only separated by 2 - 3 km in range. This meets the definition of a microburst (under 4 km horizontal scale).
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Differential reflectivity (Zdr) loop
The most positive Zdr values appear as the reflectivity core descends and vertically elongates below the melting level. (The melting level is visible as the thin, enhanced positive Zdr layer near 2.5 km AGL). This is likely due to development of oblate hydrometeors that are initially partially melted ice particles that transform into large raindrops as they descend into the increasingly warm temperatures. The combined concentration, preferred oblate shape, and increasingly large refractive index value causes differential attenuation to become significant towards the end of the loop. This is manifest as the negative Zdr "shadow" that appears down-range of the rain shaft.
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Specific differential propagation phase (Kdp) loop
Patterns of positive Kdp provide another indication of the volumetric concentration of oblate hydrometeors. The highest one-way Kdp values appear ~1 km below the melting level in the fourth frame (2108 UTC). The reduction in Kdp below this maximum may be due to drop breakup from collisions, hydrodynamic instability, etc. that reduce the concentration of large, more oblate drops. It is also likely that the drop trajectories did not remain within the RHI scan plane.
