1 8 . 5 Hail reflectivity signatures from two adjacent WSR - 88 Ds : carrier frequency and calibration issues

1. Introduction National Weather Service (NWS) operates a network of about 160 Weather Surveillance Radars WSR-88D, where D stands for Doppler and the numbers indicate the year of production. A frequency band from 2700 to 3000 MHz has been assigned for the radar carrier frequencies (S-band) corresponding to the wavelengths from 10 to 11.1 cm. Carrier frequencies of adjacent WSR-88D radars are offset to reduce signal interference. Changes in carrier frequencies slightly change radar parameters such as the antenna beamwidth, waveguide losses, and receiver sensitivity. An automatic calibration procedure, running on all radars, brings reflectivity values to the same level with accuracy of 1 dB. The radar calibration procedure is based on basic engineering principles and assumes same scattering properties of weather echoes. One of the missions of the WSR-88Ds is precipitation measurement. The maximal stable size of raindrops is 6 mm which is small compared to the wavelength, i.e., 10 cm, so that the Rayleigh approximation for scattering properties are often used for rain. Sizes of hailstones can be a few centimeters and the Rayleigh approximation cannot be used in calculation of their scattering properties. For spherical hailstones, Mie theory is used. It follows from the theory that the radar cross section is an oscillating function of the diameter and wavelength so that radar backscattering cross sections and correspondingly reflectivities are different at different wavelengths. This is frequently called the resonant effect highlighting strong oscillations of scattering cross sections as functions of size or wavelength. This effect is used for hail detection with a two-wavelength radar, 3-and 10-cm, i.