Development of hail and heavy rain signatures in a thunderstorm: 29 June 2007

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Contents

Introduction

During the early hours (UTC) on 29 June 2007, the CSU-CHILL radar collected low elevation angle PPI scans over a narrow azimuth sector that was centered on an area of developing thunderstorms. This scanning procedure gave a volume scan cycle time of slightly less than one minute. The following image loops present a sequence of 10 0.5 degree elevation angle PPI images. During this time period, the differential reflectivity field develops a near 0 dB local minimum when hailstones descend into the scan plane.

Reflectivity (dBZ)

Reflectivity levels exceeding ~60 dBZ begin to cross the 45 km range ring in the 5th image frame.


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Differential Reflectivity ($ Z_{dr} $)

In concert with the advancing 60 dBZ echo core noted above, an area of ~0 dB $ Z_{dr} $ values also starts to cross the 45 km range range in the 5th image frame. The quasi-random orientations of the hailstones act to equalize the received H and V co-polar signal levels. $ Z_{dr} $ is an expression of the reflectivity-weighted mean axis ratio of the illuminated scatterers. Thus, the arrival of the highly reflective tumbling hailstones lowers the $ Z_{dr} $ from the positive values associated with oblate raindrops to ~ 0dB in the hail region.


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Specific Differential Propagation Phase ($ K_{dp} $)

Despite the onset of hail, the specific differential propagation phase ($ K_{dp} $) values do not increase appreciably along the 45 km range ring. This is due to $ K_{dp} $'s sensitivity to oriented, non-spherical hydrometeors (i.e., large raindrops), vs. its insensitivity to randomly oriented, nearly spherical particles like hailstones. Because of this behavior, $ K_{dp} $-based rain rate estimators generally give more accurate results than reflectivity-based rain rate estimators when hail is present.


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