The spectrum of optical refractive-index fluctuations caused by temperature or salinity fluctuations in turbulent water is much different from the refractive-index spectrum of temperature or humidity fluctuations in air. This occurs because the ratio of kinematic viscosity to diffusivity is much greater for temperature and salinity fluctuations in water than for temperature and humidity fluctuations in air. In turbulent water the temperature and salinity spectra have viscous-convective ranges that extend roughly one and two decades in wave number, respectively. The consequences of these viscous-convective ranges for optical propagation in water are examined. It is found that the variance of log intensity, as a function of Fresnel-zone size, rises far above its large Fresnel-zone-size asymptotic form, and the covariance function of log-amplitude is relatively narrow when the viscous-convective range dominates its shape. If the viscous-convective range dominates the shape of the structure function of phase, then the structure function is proportional to the spacing and is less sensitive to outer scale effects than for propagation in air. Likewise, the lateral coherence length has a new asymptotic form arising from a viscous-convective range and is also less sensitive to outer scale effects than for propagation in air.
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