Dual-polarimetric signatures of ice orientation for lightning prediction: A radar modeling study of ice mixtures at X, C and S bands
Prior research has demonstrated that oriented ice crystals cause significant propagation effects that can be routinely measured by most dual-polarimetric radars from X-band (3 cm) to S-band (10 cm) wavelengths using the differential propagation phase shift (often just called differential phase, φdp) or its range derivative, the specific differential phase (Kdp). Advantages of the differential phase include independence from absolute or relative power calibration, attenuation, differential attenuation and relative insensitivity to ground clutter and partial beam occultation effects (as long as the signal remains above noise). In research mode, φdp and Kdp have been used to anticipate initial cloud electrification, lightning initiation, and cessation.
However, few studies have explored the effects of ice mixtures on these ice crystal orientation signatures. Preliminary radar model results suggest that masking (or conversely artificial enhancement) of Kdp-based ice crystal orientation signatures in a strong E-field can be caused by the presence of larger precipitation sized ice whose orientation is actually unrelated to the E-field (i.e., dominated by gravitational and aerodynamic forces). An example of this type of precipitation ice would be graupel. Hence, before Kdp can be used reliably as an indicator of electrical activity and lightning potential, its sensitivity to variability in hydrometeor properties in ice mixtures must first be ascertained.
In this study, we develop an idealized model of ice particle size, shape, orientation and dielectric in order to simulate dual-polarimetric radar parameters in various idealized scenarios of ice crystals responding to a strong vertical E-field within a mixture of precipitation sized ice particles (e.g., aggregates, graupel, or small hail) that do not respond to the E-field. The sensitivity of the Kdp ice orientation signature to various ice mixture properties and radar wavelength are explored. Since Kdp is proportional to frequency, the ice orientation signatures should be more obvious at higher (lower) frequencies (wavelengths). As a result, radar simulations at three precipitation radar wavelengths (S, C, X band) are conducted.