The Sensitivity of Polarimetric Radar Observables to Melting Hail

Polarimetric radar has great potential to improve our knowledge on the physics of hail melting, where a combination of the four polarimetric radar observables, i.e., reflectivity (Zh), differential reflectivity (Zdr), specific differential phase (Kdp) and copolar correlation coefficient (ρhv), provides a specific insight. In this study, we investigate how polarimetric observables respond to the process of hail melting by performing a series of sensitivity studies. In particular, the dependence on hail size distribution and the atmospheric state is assessed.

A 1D hail shaft model with spectral bin microphysics is used taking into account the processes of sedimentation, melting and shedding. For the initial hail particle size distribution at model top, a three parameter gamma distribution is assumed,

N(D) = N0*D^μ*exp(-λ*D)

where N0 is the intercept, μ the shape and λ the slope parameter.

We assume an idealized atmospheric profile with the 0°C level located at 3 km (AGL). Relative humidity decreases linearly from the melting level to ground surface, while the temperature increases linearly towards the ground surface. Polarimetric observables are then simulated by using polarimetric radar forward operator, assuming that hail particles are mixed-phase oblates and their scattering is calculated by the T-matrix method.

The results show that, the process of hail melting influences substantially both the magnitude of polarimetric observables and their peak heights, where a maximum or minimum of polarimetric observable is located. For variations in the atmospheric profiles, the influence of temperature is much stronger, compared to relative humidity. A higher temperature accelerates the hail melting processes and results in a shift of the peak height towards higher altitudes. As expected, the hail size distribution plays a rather significant role. Hail water path and N0 determine the magnitude of polarimetric observables, while μ has a strong impact on the peak heights of polarimetric observables since it determines whether small or large hail particles dominate the polarimetric radar signals. With larger hail particles, the peak heights move closer to the ground surface. Negative Kdp and Zdr are also found with the presence of large hails. Further work will deal with realistic hail cases, in order to improve the accuracy of hail retrieval algorithms and validate hail parameterization schemes in models.