Improving bulk Cn2 models for over-ocean applications through new determinations of the dimensionless temperature structure parameter

Abstract

The performance of imaging and laser systems can be severely degraded by atmospheric turbulence, especially for near-horizon propagation paths. Having the ability to predict turbulence effects from relatively easily obtained measurements can be useful for system design and feasibility studies, and for real-time optimization of optical systems for the current environment. For this reason, so-called ‘bulk’ models have been developed that can estimate turbulence effects through the refractive index structure parameter (Cn2) from mean near-surface meteorological and sea surface temperature measurements. Bulk Cn2 models are directly dependent upon empirically determined dimensionless functions, known as the dimensionless structure parameter functions for temperature and humidity. In this paper we attempt to improve bulk optical turbulence model performance by determining new over-ocean forms for the dimensionless temperature structure parameter (fT). During 2005-2006 atmospheric propagation experiments were conducted in the Zuniga Shoals area near San Diego to examine the impact of environmental conditions on low-altitude electro-optical propagation above the ocean surface. As part of this experiment the Naval Postgraduate School (NPS) deployed its flux research buoy along the propagation path. The measurements obtained on the NPS buoy enabled fT values to be obtained and new functions to be determined. These new functions differ greatly from those presented in the past, in that the new fT values asymptote towards very high values as the stability approaches neutrality. The dependence of the new fT function on the stability parameter in stable conditions was also different from that previously proposed. When these new functions were inserted into the NPS bulk Cn2 model, the resulting values agreed much better with directly measured turbulent Cn2 values in unstable conditions, but in stable conditions the new function actually made the agreement worse.