Airborne Polarimetric Doppler Phased Array Weather Radar: Quantification of Doppler Spectrum width and Beam broadening

The essential parameter, the width of Doppler spectra, quantifies the atmosphere's dynamics at the radar resolution volume scale. This spectrum width is shaped by several factors, including viewing geometry, non-uniform volume filling of the pulse emitted by the radar, and internal atmospheric conditions such as wind shear and turbulence (Nastrom, 1997). Historically, seminal studies quantified how these effects broaden the Doppler spectrum within a pulse resolution volume (Sloss and Atlas, 1968). The principle behind these broadening effects is rooted in the validity of the radar equations under specific atmospheric and operational environments, and these studies provide a foundational understanding of robust Doppler weather radar measurements.

Regarding Doppler and polarimetric measurements, Advanced Electronically Scanned Arrays (AESA) introduce complexities due to beam-broadening effects away from the radar's bore sight. Further, aircraft motion introduces additional perturbations into Doppler spectrum measurements. For a specified number of in-phase and quadrature-phase (I/Q) samples taken by the radar system, the quality and reliability of both Doppler and polarimetric measurements are closely tied to the Signal-to-Noise Ratio (SNR), as well as the Doppler spectrum width.

The phased array radar (PAR) equation is pivotal in quantifying beam broadening, an essential parameter for radar performance. AESA's capability for rapid scanning and adaptive beamforming without mechanical rotation presents a significant leap in improved temporal and spatial sampling of weather echo. In the context of an airborne polarimetric phased array radar (APAR), the impact of aircraft motion can be especially pronounced. The motion introduces notable Doppler spectral broadening, typically ranging from 0.5 to 1.5 m s-1, which is contingent upon the aircraft's velocity and the tilt angle of the radar beam. This broadening effect complicates the interpretation of radar data, necessitating sophisticated algorithms and corrections to mitigate its influence on the measured variables.

The interplay between phased array radar equations, AESA technology, aircraft motion, and the associated Doppler and polarimetric measurements demands a meticulous approach to interpreting APAR observations. Presentations will delve into bias and statistical error in Doppler and polarimetric measurements, highlighting the importance of understanding these effects to optimize radar scans and data interpretation.

References:

Nastrom, G.D., 1997: Doppler radar spectral width broadening due to beamwidth and wind

shear. Annales Geophysicae, 15 (6), 786-796.

Sloss, P.W., and D. Atlas, 1968: Wind shear and reflectivity gradient effects on Doppler radar spectra, J. Atmos. Sci., 25, 1080-1089.