Recent results on modeling the refractive-index structure parameter over the ocean surface using bulk methods

Abstract

Infrared scintillation measurements were obtained along a 7.2 km path over San Diego Bay, concurrently with mean meteorological and turbulence measurements obtained from a buoy located along the path. Bulk estimates and turbulence measurements of Cn 2 were computed from the buoy data and compared with the optical scintillation-derived Cn 2 values. Similar to the results of previous experiments, the bulk Cn 2 estimates agreed well with both the scintillation and turbulence measurements in unstable conditions, increasingly underestimated Cn 2 as conditions approached neutral, and agreed less well with scintillation and turbulence Cn 2 values in stable conditions. The mean differences between bulk Cn 2 estimates and both the turbulence and scintillation measurements when conditions were not near-neutral exhibited an air-sea temperature difference and wind speed dependence, possibly indicating that the forms of the empirical stability functions used by the bulk model are incorrect. The turbulent Cn 2 measurements from the buoy showed excellent agreement with the scintillation values in unstable conditions, but had surprisingly large differences in weakly stable conditions. This disagreement may be related to the fact that humidity fluctuations begin to increasingly influence refractive index fluctuations when the air-sea temperature difference is small and are not properly taken into account by the sonic temperature measurements. As the absolute air-sea temperature difference approaches zero the bulk Cn 2 estimates decrease much more rapidly and to much smaller values than either the scintillation or turbulence measurements. Fortunately, in such near-neutral conditions scintillation is usually small enough to have little effect on many optical system applications.