Understanding Evaporation Ducts on Turbulent Eddy Scales

Evaporative Ducts (ED) are common refractive features that exist due to the persistent/strong vertical moisture gradient inherent to the lowest 10s of meters of the marine atmospheric surface layer (MASL). The ED plays a large role in the propagation of surface based radar/communications systems due to the ducting of electromagnetic (EM) waves. Previous studies have characterized ED structure using mesoscale and surface layer models which are based on Monin-Obukhov similarity theory (MOST). As a result, only the spatially/temporally averaged mean ED structure has been historically examined. Conversely, this study focuses on ED variability occurring over turbulent energy containing eddy (ECE) scales by utilizing large eddy simulations (LES) of the MASL. We examine the use of both Eularian and Lagrangian LES domains to analyze mean and turbulent ED structure in both range independent and range dependent situations, respectively. This innovative approach reveals that the LES-resolved refractivity perturbations are directly linked to MASL large eddy dynamic and thermodynamic processes. In the thermally unstable MASL, significant turbulent ED variability is noted, with regions of increased ED heights associated with convective updrafts and positive moisture perturbations. In contrast, the thermally stable MASL is shown to exhibit significantly less ED variability over ECE-scales. Since current surface layer models have difficultly calculating ED properties in the thermally stable MASL, utilization of LES is helpful to gain understanding of the ED in these conditions. A conceptual model of turbulent ED variation is proposed to describe the relationship between MASL dynamics/thermodynamic processes, state variable perturbations, and refractive variations.