The power spectrum of the phase difference fluctuations between two IR beams propagating side by side is strongly influence by rain along the path. According to our measurements over a 1.37-km path in convective rainfall averaging 18 mm/h, the power spectrum of phase difference fluctuations in rain extends over the 300-Hz to 2-kHz band, and it is as much as 18 dB above the no-rain spectrum at ~600 Hz during periods of heavier rain (>30 mm/h). The shape of the spectrum is different at different stages of the storm probably because of changing drop size distributions. A simple dimensionless parameter derived from the power spectrum of phase difference fluctuations is 0.186 ± 20% for a rain rate of 41 (mm/h) km ± 25%, but this value may depend on the chopped transmitter waveform. The simple optical design and data analysis procedure allow power spectra to be obtained from a single signal channel. We conclude that measurement of phase difference fluctuations in 10.6-µm propagation should allow the path-averaged rainfall rate to be inferred over path lengths of at least 1.5 km if a properly calibrated system is used.
R. L. Schwiesow, R. E. Cupp, and S. F. Clifford, "Phase difference power spectra in atmospheric propagation through rain at 10.6 µm," Appl. Opt. 24, 4516-4524 (1985)