Assessment of Differential Scattering Phase and Improving estimation of Specific Differential Phase to Advance Detection of Hail and Tornado Debris

Polarimetric variables such as differential phase ₍φ_DP) and its range derivative specific differential phase (K_DP) contain useful information to improve quantitative precipitation estimation (QPE) and microphysics retrieval. K_DP in particular has become a favorable option for QPE due to its decreased sensitivity to hail contaminations and DSD variability, and its immunity to attenuation, radar miscalibration, and partial beam blockage. However, the usefulness of the current operationally utilized estimation method of K_DP is limited by measurement error and artifacts resulting from the differential scattering phase (δ). Currently, K_DP is operationally calculated by including both the scattering and propagation effects. The contribution of the differential scattering phase (δ), particularly for hail and tornado debris which scatter waves in non-Rayleigh scattering regimes, can significantly influence the φ_DP measurements and therefore negatively affect the K_DP­ estimates. Our goal is to isolate the two contributions (propagation and scattering) of φ_DP to more accurately calculate K_DP from only propagation effects.

The presence of δ when estimating K_DP can often result in unphysical behavior such as un-realistically large K_DP estimates and negative K_DP estimates in leading and trailing gates of the non-Rayleigh scattering region, respectively. These unfavorable results then propagate through all products derived using estimated K_DP giving way to unreasonably high QPE and negative rainfall rates. Neglecting the presence of δ within non-Rayleigh scattering regimes has led to the adoption of incorrect terminology regarding signatures seen within current operational K_DP estimates. As an example, the term ‘K_DP foot’ is often used to describe a signature of enhanced K_DP corresponding with a region of the supercell often associated with the presence of hail. This terminology can lead to a misinterpretation regarding the physical nature of the region such that the signature is associated with enhanced liquid water content. These issues highlight the necessity for processing techniques that separate the δ from the differential propagation phase (Φ_DP) such that K_DP can be accurately and correctly estimated, and precipitation microphysics may be ascertained from these components independently.

We propose a new processing method to estimate both K_DP and δ which was ignored previously. Linear Programming (LP) has been shown to effectively avoid the δ component thus maintaining monotonic profiles of Φ_DP and nonnegative, unbiased K_DP­ profiles within rain regions. By applying the LP technique specifically to the rain regions of Rayleigh scattering along a radial profile, accurate estimates of differential propagation phase, specific differential phase and differential scattering phase can be retrieved within regions of both Rayleigh and non-Rayleigh scattering. We apply this methodology to cases of reported hail and tornado debris and compare the LP results to the operationally utilized least-square-fit (LSF) estimates. This allows us to show the potential use of the differential scattering phase signature in the detection of hail and tornado debris as well as illustrate the component’s impact on LSF estimates which have led to the adoption of the incorrect terminology mentioned previously.