| 21 Aug 2026
Potential use of radar slant-linear depolarization for measuring rainfall microphysics
Abstract. Single-transmit, dual-receive millimeter-wave radars can be attractive for compact and scanning precipitation observations, but they do not necessarily provide the conventional horizontal/vertical dual-polarization variables used to constrain raindrop size distributions. This study evaluates whether a fixed slant-linear basis can turn the weak backscattering anisotropy of oblate raindrops into a useful liquid-rain observable. In the proposed geometry, a +45° transmitted polarization and co-/cross-slant receive channels project the difference between horizontal and vertical co-polar backscattering amplitudes into an orthogonal-slant return. We use explicit T-matrix scattering calculations, normalized-gamma and observed drop size distributions, and controlled neural-regression diagnostics to quantify the information content and observability of the resulting slant-linear depolarization ratio (SLDR). For the baseline Ka-band gamma ensemble, the reflectivity (Z+)–SLDR space separates characteristic drop size and concentration that are ambiguous under reflectivity alone. With 1 dB SLDR uncertainty, adding SLDR reduces the independent-test RMSE of mass-weighted mean diameter (Dm) from 0.672 to 0.243 mm and reduces the RMSE of log-transformed rain rate (log10R) from 0.163 to 0.055. The same sign of improvement persists for Nanjing 2DVD, EPFL HyMeX, NASA IFLOODS, and variable-shape gamma checks. Sensitivity tests show that random SLDR uncertainty degrades retrievals gradually, whereas relative co-/cross-slant gain bias and polarization leakage define stronger practical constraints. Through the controlled forward-simulation information-content tests, SLDR is identified as a physically distinct measurement coordinate for liquid-rain microphysics. The calibration and detectability conditions are also defined for a field test with suitable single-transmit, dual-receive radars.
This paper addresses the limitation that single-transmit, dual-receive millimeter-wave radars cannot obtain conventional H/V dual-polarimetric observations, and systematically evaluates the potential of slant-linear depolarization ratio (SLDR) for retrieving rainfall microphysics. A comprehensive information content analysis is performed based on T-matrix scattering simulations, multiple raindrop size distribution datasets, and neural network regression diagnostics to clarify the physical origin of SLDR, quantify retrieval errors, and provide observability constraints for field observations. The research topic targets the practical engineering limitations of compact millimeter-wave cloud radars, with well-defined scientific questions, rigorous physical derivations, and a complete design of simulation experiments. The results possess favorable theoretical value and provide important theoretical support for subsequent field observation campaigns.