Polarimetric radars provide variables like the specific differential phase ($K_{\mathrm{DP}}$) to detect fingerprints of dendritic growth in the dendritic growth layer (DGL) and secondary ice production, both critical for precipitation formation. A key challenge in interpreting radar observations is the lack of in situ validation of particle properties within the radar measurement volume. While high $K_{\mathrm{DP}}$ in snow is usually associated with high particle number concentrations, only few studies attributed $K_{\mathrm{DP}}$ to certain hydrometeor types and sizes. We found that at W-band, $K_{\mathrm{DP}} > 2\,^\circ\,\mathrm{km}^{-1}$ can result from a broad range of particle number concentrations, between $1$ and $100\,\mathrm{L}^{-1}$. Blowing snow and increased ice collisional fragmentation in a turbulent layer enhanced observed $K_{\mathrm{DP}}$ values. T-matrix simulations indicated that high $K_{\mathrm{DP}}$ values were primarily produced by particles smaller than $0.8\,\mathrm{mm}$ in the DGL and $1.5\,\mathrm{mm}$ near the surface.
The distinction between aggregation and riming below the DGL is important, because the latter signals the presence of super-cooled liquid water (SLW). Riming favors secondary ice production through the Hallet-Mossop process (rime splintering), which is active between $-3\,^\circ\mathrm{C}$ and $-8\,^\circ\mathrm{C}$. POLICE exploited quasi-vertical profile (QVP) data of reflectivity ($Z_{\mathrm{H}}$), differential reflectivity ($Z_{\mathrm{DR}}$), and depolarization ratio ($\mathrm{DR}$). Similar to $Z_{\mathrm{DR}}$, the variable $\mathrm{DR}$ tends to decrease in rimed snow relative to aggregated snow, but the corresponding difference in $\mathrm{DR}$ is $2-4\,\mathrm{dB}$ larger (e.g., Ryzhkov et al., 2017). Naturally, $\mathrm{DR}$ combines the information content of $Z_{\mathrm{DR}}$ and cross-correlation coefficient ($\rho_{\mathrm{hv}}$) in a single quantity. The MISPs of mean Doppler velocity ($\mathrm{MDV}$) are used to identify regions with particles falling faster than $1.5\,\mathrm{m}\,\mathrm{s}^{-1}$ and accordingly associated with riming.