Polarimetric Radar Rainfall Estimation at X-band: Evaluation of Attenuation Correction

The most commonly used polarimetric radar measurements in rainfall estimation are the reflectivity factor, usually at horizontal polarization (Zh), differential reflectivity (Zdr) and specific differential propagation phase (Kdp). Based on the above three measurements, a number of algorithms have been derived in the literature to estimate rainfall (see Bringi and Chandrasekar, 2001 for a summary of references). These algorithms have been derived assuming a particular shape size relation for raindrops. The mean shape versus size relation is crucial for deriving algorithms that use Zdr and Kdp. At low rainfall rates, the radar rainfall algorithms that use Kdp at S-band have relatively large measurement error. However Kdp directly scales with frequency, and has a higher dynamic range at X band. This feature makes Kdp at X band more useful at low rainrates (Matrosov et al., 2002). Along with the advantages, X band measurements come with their own problems; the significant ones being, attenuation, differential attenuation and phase shift on backscatter. Testud et al. (2000) extended the attenuation correction method used in space borne radars to ground polarimetric radars. The “rain profiling” technique uses parameterization of the specific attenuation with respect to reflectivity of the form b h aZ A = where A is specific attenuation at horizontal polarization, Zh is the reflectivity and a, b are the parameters. The parameter b was fixed whereas the parameter a is changed for each path. Assuming a constant a along a propagation path is equivalent to assuming a constant intercept parameter Nw in the drop size distribution. Chandrasekar et al. (2002) compared S band and X band estimate of rainfall and showed that (in presence of measurement errors) for a given accuracy that can be achieved at S band, the same can be achieved at about 1/3 the rainfall rate at X-band, which is a direct result of frequency scaling. This paper investigates the attenuation correction aspect of rainfall estimation at X-band specifically the attenuation correction process and accuracies. 2. POLARIMETRIC RADAR MEASUREMENTS AND RAINFALL ALGORITHMS