Atmospheric precipitation, similar to turbulence (and together with it), causes significant intensity fluctuations (σ23). They also result in characteristic peculiarities of the intensity fluctuation spectrum [W(ƒ)]. The measurements were carried out along the 130–1310-m path. The He-Ne laser generated at λ = 0.6328 µm. The beam diffraction parameter Ω= kα20/L (k = 2π/λ is the wavenumber, α0 is the effective beam radius). The measurements were made in collimated, divergent, and focused beams. The receiver’s diameter was 0.1 mm. It was noticed that, in precipitation (snowfall, rain) regardless of the laser beam parameters, the turbulence properties are mainly observed in the low-frequency region and those of precipitation in the high-frequency region. In weak precipitation the spectrum had two maxima, and at heavy precipitation it had its hydrometeoric maximum at frequency ƒr in the range of several kilohertz. In heavy rains in the region of low frequencies ƒ < ƒr the spectrum was described by the dependence W(ƒ) ~ ƒ with satisfactory accuracy. In rain at ƒ > ƒr the spectrum decreased as W(ƒ) ~ ƒ-α. In snowfall at ƒ > ƒr the following dependence was observed: W(ƒ) ~ l-βf. The theoretical conclusion ƒr ~ V/d, where V is the terminal rate and d is the mean particle size, was quantitatively verified. On the 130-m path the experimental values of the normalized variance (σ23) are described by the dependence σ23 = A + Nτ, where τ is the optical depth of precipitation. The coefficient N in the divergent beam depends on the particle sizes and increases from 0.3 to 0.8 when increasing the maximum size of particles from 0.1 to 3 cm, respectively. The estimates of turbulence σ2T and snowfall σ2c contributions to the measured variance σ23 were made assuming that they are additive (i.e., σ23 = σ2T + σ2c). When the maximum diameter of the snowfall particles was <5 mm and τ = 0.4–0.5, the empirical dependence of σ2c = -0.07 + 0.37 logΩ for Ω values of 0.3–30 was obtained. Measurements of the scattered radiation in the snowfall (Ω = 54, L = 130 m) were carried out at the receiver’s angular distance 10-4 rad from the beam axis; σ23 at τ > 0.2 was saturated at the leve of ~0.85. Here W(ƒ) had its maximum in the region of a few kilohertz. We concluded that the high-frequency region of the intensity fluctuation spectrum in the divergent laser beam had the largest information content. A focused beam was preferable for studying the turbulence in precipitation.
© 1988 Optical Society of America
A. F. Zhukov, M. V. Kabanov, and R. Sh. Tsvyk, "Temporal fluctuations of laser beam radiation in atmospheric precipitation," Appl. Opt. 27, 578-583 (1988)