# Varying outflow boundary motion: 11 September 2012

Low-level outflow from rain showers caused the deformation and re-orientation of a boundary layer echo line on 11 September 2012. Time lapse image loops of this evolution have been assembled.

## Introduction

A series of low elevation angle PPI scans was conducted during the afternoon of 11 September 2012 as a synoptic scale trough approached the area from the west. A boundary layer echo line was located ~50 km north of CSU-CHILL at the start of the observations; this fine was moving towards the south. An area of rain showers subsequently developed ~60 km northeast of the radar. Low level outflow from these showers caused a large portion of the boundary layer echo line to change orientation by ~90 degrees.

## Reflectivity loop

The following image sequence was made from a series of PPI scans at an elevation angle of 1.0 deg. The echo line of interest initially had a west-southwest - east-northeast orientation. Locally-enhanced outflow from the rain showers located approximately 60 km to the northeast of the radar rotated the echo line into a northwest - southeast orientation. The white "X" ~35 km north of the radar marks the approximate pivot point for this rotation. Additional, patchy echo areas are also visible moving towards the northeast in the pre-trough synoptic flow.

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## Differential reflectivity loop

Differential reflectivity levels were very high (+7 to +9 dB) in the boundary layer echo line. These values are significantly more positive than the ~ +3 - +4 dB magnitudes generated by large raindrops. The distinctly flattened cross-sectional shapes presented by flying insects are responsible for the extremely positive Zdr values observed in the echo line.

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Between the start and end times in the loop, inbound (negative) radial velocities overspread much of the northeastern azimuth quadrant within a range of ~30 km.

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## RHI images

The following two RHI scan were taken on an azimuth of 239 degrees before the PPI image loop sequence. Due to the initially dry boundary layer, only a few areas of virga extended below the general mid-level cloud base.

This RHI azimuth was oriented into the synoptic-scale wind flow within the mid-level echo region; this caused the radial velocities observed in this layer to be negative in sign.

The final two RHI panels were obtained from a scan done on an azimuth of 332 degrees after the end of the PPI image loop sequence. At this later time, the base of the mid-level echo had lowered and the precipitation shafts were beginning to reach the ground. Localized reflectivity enhancement occurred in the "bright band" near 2.8 km AGL where the descending frozen hydrometeors began to melt.

The melting level was also indicated by a reduction in the correlation between the H and V co-polar signal returns ($\rho_{HV}$). The co-existence of fully-melted rain drops with irregularly-shaped, incompletely-melted ice particles produces a diverse population of hydrometeors in the radar pulse volume that lowers $\rho_{HV}$ (Giangrande et al, JAMC May, 2008).

## Summary

A variety of echo motions were observed in the low elevation PPI scans collected on 11 September 2012. Developing rain showers produced localized outflows that perturbed the motion of a boundary layer echo line that was initially drifting southward with the ambient low-level flow. Higher-altitude echoes were carried north-eastward by the mid-tropospheric flow ahead of an approaching trough. Data processing techniques that involve simple definitions of echo motion can be compromised under these conditions.

## References

• Bringi, V.N., Chandrasekar, V. (2001). Polarimetric Doppler Weather Radar: Principles and Applications (pp. 490-513). ISBN-13: 978-0521623841