Initial dual frequency observations of rain: 17 July 2012

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CSU-CHILL reflectivity data collected at 11 cm wavelength in a PPI scan through an area of thunderstorms during dual frequency test operations. Plots of selected dual polarization data fields observed at radar wavelengths of 11 and 3 cm in an RHI scan on the 008 degree azimuth angle have been collected.



Contents

Introduction

Following the conclusion of the DC3 project, the dual frequency feedhorn was returned to service on the CSU-CHILL radar on 5 July 2012. Test operations were conducted on several subsequent days in July when rain was in the area. The data shown below was collected when an area of rain with embedded non-severe convection passed north of the radar during the early UTC hours of 17 July 2012. These are "first look" data; no detailed calibration efforts have been applied.

Reflectivity

Reflectivity data obtained at the two wavelengths (11 cm / S-Band and 3 cm / X-Band) in an RHI scan are shown below. The gate spacings at these two wavelengths are 150 and 75 m respectively. Due to the slow scan rate (1 deg / sec), the observations were oversampled with respect to the antenna beamwidths (~1.0 deg at S-Band and 0.33 deg at X-Band). The beam intervals in the recorded data were ~0.1 deg at S-Band and ~0.06 deg at X-Band. The combination of these factors gives a higher resolution, smoother image of the echo structures in the X-Band data. Reflectivity enhancement due to the onset of hydrometeor melting is apparent near the 3 km AGL level. No attenuation corrections have been applied to the X-Band data; the X-Band reflectivity levels generally decrease with respect to the S-Band values along beams that penetrate stronger echo regions.

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Differential reflectivity

The corresponding differential reflectivity ($ Z_{dr} $) plots are shown below; for reference the 30 and 45 dBZ reflectivity contours have been included. Positive $ Z_{dr} $ values are evident at and below the melting level. Differential attenuation induces a negative $ Z_{dr} $ bias generally within and down-range of the heavier rain regions (i.e., at and below 3 km AGL).

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Differential propagation phase

During propagation through oriented hydrometeors, the difference between the H and V received phases ($ \phi_{dp} $) is inversely proportional to wavelength. In the following plots, an offset has been applied to the S-Band phase data to bring its starting value (i.e., in the first few km range interval) into agreement with the X-Band data. The same color scale range is then used for both wavelengths. The X-Band $ \phi_{dp} $ accumulation through the oblate drops in the heavy rain area starting near 25 km range is approximately three times larger than the S-Band / 11 cm wavelength phase change. This increased $ \phi_{dp} $ sensitivity at X-Band also improves the ability to detect the small negative phase shifts that result when strong electric fields rotate planar ice crystals into a quasi-vertical alignment. Such a negative phase shift (green changing to blue with increasing range) "vertical ice / electric field" signature near X=33 km, Z=8 km is more clearly evident in the X-Band data.

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Summary

The dual frequency polarimetric data collected by the CSU-CHILL radar is providing reasonable-looking patterns in convective rain. More detailed data examinations as well as refinements to various attenuation correction procedures for the X-Band measurements are ongoing.