Impacts of Vertical Nonuniform Beam Filling on the Observability of Secondary Ice Production due to Sublimation

Quasi-vertical profiles (QVPs) of polarimetric radar data have emerged as a powerful tool for studying precipitation microphysics. Various studies have found enhancements in specific differential phase K_dp in regions of suspected secondary ice production (SIP) due to rime splintering. Similar K_dp enhancements have also been found in regions of sublimating snow, another proposed SIP process. This work explores these K_dp signatures for two cases of sublimating snow using nearly collocated S-and Ka-band radars. The presence of the signature was inconsistent between the radars, prompting exploration of alternative causes. Idealized simulations are performed using a radar beam-broadening model to explore the impact of nonuniform beam filling (NBF) on the observed reflectivity Z and K_dp within the sublimation layer. Rather than an intrinsic increase in ice concentration, the observed K_dp enhancements can instead be explained by NBF in the presence of sharp vertical gradients of Z and K_dp within the sublimation zone, which results in a K_dp bias dipole. The sever-ity of the bias is sensitive to the Z gradient and radar beamwidth and elevation angle, which explains its appearance at only one radar. In addition, differences in scanning strategies and range thresholds during QVP processing can constructively enhance these positive K_dp biases by excluding the negative portion of the dipole. These results highlight the need to con-sider NBF effects in regions not traditionally considered (e.g., in pure snow) due to the increased K_dp fidelity afforded by QVPs and the subsequent ramifications this has on the observability of sublimational SIP.