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.
© 1985 Optical Society of AmericaFull Article | PDF Article