DPWX/Snapshot dual-Doppler and LMA observations of a convective line: 27 June 2014

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Author: Patrick C. Kennedy

Web 27jun2014 ANOT VCHILL overview.png

CSU-CHILL reflectivity data in a low elevation angle PPI scan through a thunderstorm line. Three dimensional air motions in this echo system have been synthesized based on data collected by the CSU-CHILL and NWS KFTG radars. The locations of VHF radiation sources detected by the Northern Colorado Lightning Mapping Array (LMA) system have been combined with selected results from the CHILL - KFTG dual Doppler analysis volume.






Contents

Introduction

On 27 June 2014 a line of thunderstorms occurred in the eastern dual Doppler lobe defined by the CSU-CHILL and NWS KFTG radars. The CSU-CHILL radar was collecting various test scans in preparation for an upcoming field project. Near 2212 UTC, both radars happened to to begin volume scans within 14 seconds of each other. The resultant fairly well synchronized observations of the thunderstorm area made it feasible to synthesize the three dimensional air motions using conventional dual-Doppler analysis procedures. The thunderstorm line was also within the coverage area of the Northern Colorado Lightning Mapping Array (LMA) operated by New Mexico Tech. The following plots show selected air motion and electrification patterns observed within the 2212 - 2217 UTC time period.

Dual Doppler fields at 9 km MSL

The polar coordinate data from the CSU-CHILL and NWS KFTG radars were interpolated to a common 3D Cartesian grid using the NCAR Radx2Grid software. The gridpoint spacings were 1 km in the horizontal direction and 0.5 km in the vertical. The grid origin is located at CSU-CHILL and the grid heights are referenced to sea level. The three dimensional winds were synthesized from the gridded input data using the NCAR CEDRIC software. During the synthesis procedure, an advection correction was applied to account for the mean storm motion from 250 degrees at 13 mps. The following plot shows the Earth-relative horizontal wind fields at the 9 km MSL analysis level. This height level is in the lower portion of the anvil cloud. The horizontal winds generally diverged away from the convective echo cores. The single red contour line centered near X=+50, Y=-8 km encloses a region with specific propagation differential phase ($ K_{dp} $) values less than 0.1 deg / km on the CSU-CHILL 8.5 deg elevation PPI scan. (The height of this scan surface is approximately 9 km MSL in the vicinity of this contour.) Such negative $ K_{dp} $ areas in the upper portions of thunderstorms are due to planar ice crystals whose long axes have been rotated into a quasi-vertical orientation by strong electric fields. Sufficient concentrations of these vertically oriented ice crystals will retard the propagation of the vertically-polarized radar waves relative to horizontally-polarized waves, producing negative $ K_{dp} $.

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The black contours in the next plot mark the stronger updrafts identified by the dual-Doppler synthesis. The red negative $ K_{dp} $ contour was located in close proximity to several >10 mps updrafts. The rebounding ice crystal - graupel particle collisions that occur in these updrafts are an essential element in the noninductive charge separation process. The resultant charge separation apparently produced electric field strengths that were capable of inducing the negative $ K_{dp} $ region.

Web 27jun2014 9km updrafts anot.png

8.5 degree elevation angle PPI data

The next plot provides a more detailed view of the negative $ K_{dp} $ area. The PPI plot domain corresponds to the red square marked in the preceeding updraft plot. Since $ K_{dp} $ is based on the radial derivative of propagation differential phase ($ \psi_{dp} $), it is best viewed in polar vs. Cartesian space. The small white dots on the PPI plot are individual LMA VHF radiation sources. The plotted points occurred within +/- 30 seconds of the time when the CSU-CHILL beam passed through the negative $ K_{dp} $ area during the 8.5 deg elevation angle sweep. The heights of the LMA points were restricted to a narrow layer centered on the 9 km level (8.75 to 9.25 km MSL). The LMA activity is generally clustered around the negative $ K_{dp} $ contour. An additional area of significant LMA activity occurred in the vicinity of a developing cell located near X=78 km, Y=-5 km. (Note: the 8.5 deg elevation angle beam height is well above 9 km MSL in this developing cell.)

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Vertical cross section

The final plot shows a vertical cross section through the dual-Doppler analysis domain along the Y= -6 km plane. (The location of this plane is marked on the updraft plot shown above.) LMA points located within +/- 1 km of the cross section plane during the 2212 - 2217 UTC volume scan period are plotted as an overlay. Within the older storm centered near X=60 km, the LMA discharges occur in two general height regimes roughly centered near the 5 - 6 and 11 km heights. Since the most energetic VHF discharges are generated as negative leaders propagate into regions of positive charge (Wiens et al. JAS 2005 p. 4151-4177), these LMA source concentrations are probably related to the lower and upper positive charge regions following the tripole thunderstorm electrification model (Williams, JGR 1989 p. 13,151-13,167; see also Figure 8d in Mansell et al, JAS 2010 p. 171-194.) In the younger storm centered near X=78 km, with an echo core that is still suspended aloft, the lower positive region is not evident.


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Summary

These example results demonstrate the benefits of combining polarimetric weather radar data with the three dimensional airflows synthesized from multiple Doppler observations and the electrical discharge mappings provided by LMA technology. The full exploitation of these methods requires customized radar scanning procedures to adequately monitor the echo system's time evolution.


Acknowledgements: Prof. Bill Rison of New Mexico Tech provided the LMA data files.