Hail reflectivity signatures from two adjacent WSR-88Ds: carrier frequency and calibration issues

Carrier frequencies of adjacent WSR-88D radars are offset to reduce interference. The carrier frequencies occupy a band 2700 to 3000 MHz (S band) with the corresponding wavelengths of 12 to 10 cm. An automatic calibration procedure, running on all radars, brings reflectivity values to the same level assuming that the intrinsic reflectivity of the scattering media is the same for this narrow frequency band. In this paper, the latter assumption is investigated using T-matrix calculations for oblate droplets, spherical and non-spherical hailstones, and graupel. It is shown that reflectivity difference measured in rain at two close wavelengths is less than 0.5 dB and thus can be neglected. In the presence of hail, on the other hand, reflectivity differences can reach 10 dB due to resonant effects upon scattering. Reflectivity values, measured at different carrier frequencies within thunderstorm cores, can differ if hail is present. Carrier frequency and calibration issues should be taken into consideration when comparing reflectivity values measured in thunderstorms with adjacent WSR-88Ds.

Radar data are analyzed from KOUN and KCRI WSR-88D radars located in Norman OK. The two radars are separated by about 300 m and operate at slightly different frequencies. Statistics for rain and hail reflectivity values are presented. In strong reflectivity hail cores, reflectivity values from KOUN, 2705 MHz, are larger than reflectivity values from KCRI, 2995 MHz. This observation is in accordance with theory.

A possible method of hail detection, based on these observations, is discussed. The influence of resonant effects on polarimetric parameters measured at different carrier frequencies are also presented in light of future polarization of the WSR-88D systems.