We apply the theory of plane-wave scattering by spheres to calculate the variance, the space-time covariance, and the temporal-frequency spectrum of the amplitude fluctuations of a light wave passing through rainfall. The development includes the effects of various sizes of raindrops and their associated terminal velocities. We show that the time-lagged covariance function of two vertically spaced sensors can provide a practical remote measurement of path-averaged rain parameters, such as drop-size distribution and rain rate, without any assumptions about the form of the drop-size distribution.
Ting-i Wang and S. F. Clifford, "Use of rainfall-induced optical scintillations to measure path-averaged rain parameters," J. Opt. Soc. Am. 65, 927-937 (1975)