In Situ Measurements of Optical Turbulence in the Marine Atmospheric Boundary Layer

The optical refractive index of clear air is a function of temperature, pressure, specific humidity, and optical wavelength. The turbulent fluctuations of the optical refractive index and their effects on optical observables are collectively referred to as “optical turbulence”. According to the classical Obukhov-Corrsin-Tatarskii theory of scalar turbulence, optical turbulence is parameterized in terms of the optical refractive-index structure parameter C_n², which is a weighted sum of the temperature structure parameter C_T², the specific-humidity structure parameter C_q², and the temperature-humidity cross-structure parameter C_Tq. These structure parameters, however, characterize only the inertial-convective subrange, where the 3D wave-number spectrum follows a k^-11/3 power law and where (based on Taylor’s frozen-turbulence hypothesis) the frequency spectrum follows an f^-5/3 power law. The classical theory, however, does not account for inhomogeneity, anisotropy, a finite outer scale and for inner-scale effects associated with finite scalar diffusivities and with a finite kinematic viscosity.

In this paper, we present and discuss characteristics of optical turbulence obtained from fast-response temperature and humidity measurements collected onboard the research vessel FLIP during the month-long CASPER-West campaign in October 2017. The figure shows, as an example, the temperature spectrum estimated from measurements collected with a single ultrasonic anemometer-thermometer (“sonic”) between 2200 UTC and 2300 UTC on 01 Oct 2017. We evaluate and discuss, on the basis of measurements by means of multiple, vertically spaced sonics and fast-response hygrometers, the variations of the temperature, humidity, and refractive-index spectra and structure parameters with respect to height, shear and stratification. We use the Monin-Obukhov similarity theory as the guiding conceptual framework.