Doppler and reflectivity measurements at two closely-spaced frequencies

Closely-spaced dual frequency weather radar data can be gathered using a single broad-band power amplifier and antenna. This is an attractive feature for observation platforms that are limited with respect to mass or available power. Center frequencies at or near 35 GHz with bandwidths on the order of 5% give relatively strong differential signals. The data, moreover, are more directly related to the median mass diameter of the rain and snow than are the differential signals at lower frequencies. The differential mean Doppler at frequencies where non-Rayleigh scattering effects are significant may also hold promise in that the differential measurement is insensitive to the mean air and platform motion and dependent only on the terminal velocity of the drops. Together with the mean Doppler velocity, the mean differential velocity may provide estimates of the radial component of the mean air motion along with the median drop diameter of the raindrop size distribution. To test the concept, zenith-looking measurements from the EDOP radar have been made using simultaneous transmission and reception at frequencies of 9.1 and 10 GHz. Comparisons of the median mass diameter derived from the differential-frequency reflectivity signal and from a disdrometer indicate that the approach may be feasible for center frequencies at X-band and above.