Detection of Bragg scatter using S-band polarimetric weather radar: Theoretical and experimental considerations

Weather radar operating at S-band (10 cm) is capable of detecting Bragg scatter, which results when gradients in the refractive index are produced by homogeneous and isotropic turbulence that has spatial scales within the inertial subrange and equal to half the radar wavelength. For polarimetric weather radars, Bragg scatter is expected to produce values of differential reflectivity near zero and correlation coefficient near one. The NCAR S-band polarimetric weather radar (S-Pol) was operated in February 2014 as part of a multi-institutional and multi-sensor boundary-layer research experiment called LATTE (Lower Atmospheric Thermodynamics and Turbulence Experiment). The bulk of the experiment was conducted at the NOAA Boulder Atmospheric Observatory (BAO) located along the Colorado Front Range and approximately 13 km from S-Pol. By conducting the experiment in February, the likelihood of receiving biological scatter from insects was minimized.

The sensitivity of S-Pol was maximized for LATTE by increasing the transmit power, increasing the pulse length, and increasing the number of samples. Range Height Indicator scans were made along a radial corresponding to the location of the BAO. Signals from S-Pol indicative of Bragg scatter were detected under a variety of atmospheric conditions.

Observations from S-Pol have been compared with the backscattered power recorded using the NCAR 449-MHz (67 cm) spaced antenna, multiple-frequency wind profiling radar (WPR), which was operated at the BAO during LATTE. Moreover, several radiosondes were launched from the BAO site while both S-Pol and the WPR were operating. In this presentation we discuss the theoretical framework needed to compare the relative contributions of Bragg and Rayleigh scatter for these two radars; the role of polarimetry when isolating Bragg scatter from other sources of echo power; and the implications of these findings for both research and operational atmospheric communities.