Articles/An RHI scan sequence through thunderstorm outflow: 27 July 2013

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Author: P. C. Kennedy


27jul2013 GF overview anot.png

A fine line echo associated with a thunderstorm gust front approaches the CSU CHILL radar site shortly after 00 UTC on 27 July 2013. RHI scans on an azimuth of 55 degrees were repeated at ~1 minute intervals as the gust front passed the radar site. Loops of the 3 GHZ reflectivity and radial velocity data images collected in these RHI scans have been assembled.


Contents

Introduction

Near 00 UTC on 27 July 2013, an area of severe thunderstorms passed ~30 km northeast of the CSU-CHILL radar site. A well-defined fine line echo was observed along the leading edge of the surface-based outflow generated by the downdrafts in the storms. The following two-panel NCAR SOLO program plot shows the general situation as depicted by the the S-Band data. The strong inbound (negative in sign) radial velocities advanced the leading edge of the outflow past the radar location in the next 15 minutes.

27jul2013 SOLO GF anot.png

Shortly after this PPI data was collected, the radar scanning was changed to a two sweep (#1 up; #2 down) pattern to reduce the cycle time between individual sweep repetitions to slightly less than one minute. The following image sequences were made from the downward moving sweeps that were done on an azimuth angle of 055 degrees.

Reflectivity loop

Within the main portion of the thunderstorm echo, the reflectivity levels and the maximum heights to which they extend decreased with time. This is due to the combined effects of the storm's weakening with time and to its moving south of the fixed RHI azimuth. At shorter ranges near the ground, the fine line echo associated with the leading edge of the outflow approaches and passes over the radar site during the image loop's time period. Behind the fine line echo, near a height of 2.5 km AGL, wave patterns developed on the upper surface of the cold, dense outflow airmass.


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Radial velocity loop

The strongest inbound flow (negative velocities coded with purple colors) occured near the surface within the cold outflow airmass. The overall depth of the outflow airmass decreases with time. The shallowness of thunderstorm outflows makes them impossible to accurately sample at long (~50 km or longer) radar ranges where atmospheric propagation effects raise the minimum pulse volume height well above the Earth's surface.


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Gust front area reflectivity loop

The final loop was made from the reflectivity data in the immediate vicinity of the reflectivity maximum at the leading edge of the outflow airmass. In a detailed analysis of gust front lifecycles, Wakimoto (MWR August 1982) found that this leading edge fine line echo was due to scatterers (varying mixtures of rain drops and insects) that were collected in the convergent horizontal roll circulation along the leading edge of the outflow.

27jul2013 GF head anot.png

The time lapse of RHI scans through the gust front gives an indication of the low-level lifting that frequently occurs above the leading edge of gust front echoes. Areas affected by moving gust fronts are favored regions for the development of new convection (Wilson and Schreiber, MWR December, 1986).


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Summary

RHI scans conducted at high time resolution are useful for observing the evolution of both gust fronts and their parent thunderstorms.

References