Relative Influence of Sea State and Mean, Turbulent, and Heterogenous Refractivity on X-Band Propagation

In predicting electromagnetic (EM) wave propagation within the marine atmospheric surface layer (MASL), it is common to assume steady homogenous conditions. However, discrepancies between predicted and measured propagation remain, which could be due in part to turbulent fluctuations of the refractive index, spatially heterogenous evaporative ducting environments, and mischaracterization of the rough ocean bottom boundary. To better understand the relative importance of these contributors, this study explores the sensitivity of X-band propagation to parameters describing sea-state conditions and refractive environments, including turbulent fluctuations and spatially heterogeneous conditions. This study employs the extended Fourier amplitude sensitivity test (eFAST) to compute sensitivity indices that evaluate the leading-order effects and effects due to nonlinear interactions between these refractive and sea state parameters on a parabolic wave equation (PWE) EM wave propagation simulation. The parameters are delineated and evaluated in three atmospheric stability regime experiments: stable, neutral, and unstable, and consider both trapping and nontrapping propagation conditions. Parameter sensitivity indices are ranked for each experiment to examine the relative effects of the parameters on X-band propagation prediction. The results show that in neutral and stable regimes, mean evaporation duct characteristics have the greatest impact on propagation beyond the geometric horizon (BGH), while in unstable conditions, turbulence also plays a significant role. Additionally, in the lowest 10 m of the atmosphere, forward scattering from the rough sea surface has the greatest effect on propagation predictions regardless of atmospheric stability.