DPWX/Gust front observations: 7 May 2017.

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Author: Patrick C. Kennedy



7may2017 2243 ovrview 360 anot.png

CSU-CHILL S-band reflectivity data collected in a 2.3 degree elevation angle PPI scan just after a strong gust front passed the radar site. Time lapse loops of selected PPI and RHI scan data collected as the gust front moved away from the radar have been prepared.




Contents

Overview

The collection of radar observations of the low-level outflows from thunderstorms was one of the objectives of Prof. S. van den Heever's C3LOUD Ex (CSU Convective Cloud Outflows and UpDrafts Experiment). During project operations on 7 May 2017, a dissipating area of thunderstorms located to the southwest of the radar generated a well-defined gust front that began to affect the radar site near 2240 UTC. After the initial surface wind shift, wind speeds increased dramatically, with the automated weather observing equipment at the Greeley airport recording a peak wind from 220 degrees at 45 knots at 2301 UTC. Convergence along the leading edge of this outflow concentrated the weak boundary layer scatterers (primarily insects), producing an enhanced reflectivity fine line echo that was detectable by the CSU-CHILL radar. (Surface dust and debris raised by the strong winds also contributed to the radar returns). The following image loop shows a sequence of low elevation angle PPI sector scans that were done as the fine line echo propagated rapidly away from the radar towards the northeast.


PPI scan reflectivity


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PPI scan radial velocity

One frame of the corresponding radial velocity data is shown in the next plot. To show the full extent of the radial velocity patterns, no thresholding has been applied to the data. The annotations in the plot show the area where the outbound (positive in sign) radial velocities have exceeded the 27.5 mps unambiguous measurement limit set by the radar's wavelength and pulse repetition frequency. In this "folded" velocity region, the recorded radial velocities become negative in sign (towards the radar) with high magnitudes near the measurement limit.


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The complete four frame radial velocity loop is shown next. The image frames were taken from sweep number 2 of the PPI sector volume scans. The maximum outbound velocities occur in the areas where folding (indicated by purple and blue colors) appeared. The existence of folded radial velocities in S-band data collected at near-surface beam heights indicates a significant possibility of wind damage.


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RHI scan reflectivity

RHI scans where done between each of the PPI sector volume scans that were the source of the previous image loops. The next loops were assembled from the RHI scans that were taken on the 060 deg azimuth angle. This azimuth intercepted some of the highest radial velocities. In the following reflectivity loop, the narrow, surface-based echo maximum associated with the gust front can be seen moving rapidly away from the radar. The ~2 km vertical extent of this echo is typical of the head structure at the leading edge of thunderstorm cold pools.


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RHI scan radial velocity

The corresponding radial velocities are shown in the final loop. While the depth of the positive / outbound radial velocity layer undergoes some localized variations, there is an overall tendency for the depth of the out-flowing cold pool to decrease with time.


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