Apparent Drop Shapes and Estimation of Rainfall by Polarimetric Radar

Abstract

The knowledge of the exact shape of falling raindrops is essential for the retrieval of rainfall rate from measurements with polarimetric weather radar. Polarimetric radar parameters depend only on the drop shape. Despite the large number of studies performed in the past, no common parameterisation of the shape of falling raindrops is available yet. Equilibrium shapes have been determined by Pruppacher and Pitter (1971) or Beard et al. (1989). However, falling raindrops tend to oscillate, rotate, and tumble. All those motions affect the polarimetric radar parameters like differential reflectivity (ZDR) or differential propagation phase (KDP). Both parameters are commonly used to provide a better estimation of rainfall rate, than it would be possible with a classical Z-R relation. In the present study a simple oscillation model and canting angle model are used to simulate raindrops with various shapes. The scattering amplitudes at C-band frequencies are estimated and are recursively used to estimate an apparent drop shape of falling raindrops. Besides reflectivity weighting of different oscillation modes, this approach also considers deviations from Rayleigh scattering. The latter is relevant at C-band frequencies for raindrops with axis dimensions larger than about 3 mm. The apparent drop shape is also compared to polarimetric radar measurements.