PhiDP variability in clutter

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A-scope plot of dBZ and phidp vs. range

Conceptual Summary

The differential propagation phase signal generally displays the least noisy behavior in precipitation echoes where Rayleigh scattering conditions prevail. In contrast, differential propagation phase shift upon scattering (delta), typically produces large phidp fluctuations when ground targets are being illuminated. The A-scope plot above shows a ray trace taken from a 0.5 degree PPI sweep made by the CSU-CHILL radar. The reflectivity values are plotted in red; phidp is in green. The first ~35 km range interval includes clutter, after which convective rain echo is encountered. The reduced phidp fluctuations in the rain echo are apparent.

This characteristic allows the standard deviation of phidp to be used to discriminate between meteorological and non-meteorological echo classes. Shown below are three versions of the PPI scan from which the A-scope ray was taken. The initial reflectivity plot shows both the thunderstorm echo and the close range clutter area. The second plot shows the standard deviation of phidp calculated over 3 gate by 3 beam subregions. The large standard deviations outside of the precipitation echo are quite evident. The final plot shows the original reflectivity field thresholded to remove the data when the standard deviation of phidp exceeds 20 degrees. (An additional despiking and local filling step was also done to further clean up the image.) The information provided by polarimetric radar data is of considerable use in separating meteorological echoes from other target types. Raw dBZ plot Standard deviation of PhiDP plot Filtered dBZ plot

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References

  • Alexander V. Ryzhkov, Terry J. Schuur, Donald W. Burgess, Pamela L. Heinselman, Scott E. Giangrande and Dusan S. Zrnic. 2005: The Joint Polarization Experiment: Polarimetric Rainfall Measurements and Hydrometeor Classification. Bulletin of the American Meteorological Society: Vol. 86, No. 6, pp. 809-824.