Dual-polarimetric signatures of ice orientation for lightning prediction: A radar modeling study of ice mixtures at X, C and S bands

The majority of lightning-related casualties typically occur during thunderstorm initiation (e.g., first flash) or dissipation (e.g., last flash). One promising approach for lightning prediction involves the use of dual-polarimetric radar to infer the presence of oriented ice crystals in lightning producing storms. In the absence of strong vertical electric fields (E-fields), ice crystals fall with their largest (semi-major) axis in the horizontal associated with gravitational and aerodynamic forces. In thunderstorms, strong vertical electric fields (100-200 kV m^-1) have been shown to orient small (< 2 mm) ice crystals such that their semi-major axis is vertical or nearly so. After a lightning flash, the E-field typically relaxes and prior radar research suggests that ice crystals rapidly resume their preferred horizontal orientation. In active thunderstorms, the vertical E-field quickly recovers and the ice crystals repeat this cycle of orientation for each nearby flash. This change in ice crystal orientation from primarily horizontal to vertical during the development of strong vertical E-fields prior to a flash forms the physical basis for anticipating lightning initiation and, potentially, cessation. Research has shown that radar reflectivity (Z) and other co-polar back-scattering radar measurements like differential reflectivity (Z_dr) typically measured by operational dual-polarimetric radars are not as sensitive to these changes in ice crystal orientation when present in ice mixtures also containing precipitation sized ice (like small hail, graupel or aggregates) that are too large (> 2 mm) or spherical to orient preferentially with the vertical E-field.

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, φ_dp) or its range derivative, the specific differential phase (K_dp). 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, φ_dp and K_dp 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 K_dp-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 K_dp 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 K_dp ice orientation signature to various ice mixture properties and radar wavelength are explored. Since K_dp 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.