The purpose of the Tropical Air–Sea Propagation Study (TAPS), which was conducted during November–December 2013, was to gather coordinated atmospheric and radio frequency (RF) data, offshore of northeastern Australia, in order to address the question of how well radio wave propagation can be predicted in a clear-air, tropical, littoral maritime environment. Spatiotemporal variations in vertical gradients of the conserved thermodynamic variables found in surface layers, mixing layers, and entrainment layers have the potential to bend or refract RF energy in directions that can either enhance or limit the intended function of an RF system. TAPS facilitated the collaboration of scientists and technologists from the United Kingdom, the United States, France, New Zealand, and Australia, bringing together expertise in boundary layer meteorology, mesoscale numerical weather prediction (NWP), and RF propagation. The focus of the study was on investigating for the first time in a tropical, littoral environment the i) refractivity structure in the marine and coastal inland boundary layers; ii) the spatial and temporal behavior of momentum, heat, and moisture fluxes; and iii) the ability of propagation models seeded with refractive index functions derived from blended NWP and surface-layer models to predict the propagation of radio wave signals of ultrahigh frequency (UHF; 300 MHz–3 GHz), super-high frequency (SHF; 3–30 GHz), and extremely high frequency (EHF; 30–300 GHz). Coordinated atmospheric and RF measurements were made using a small research aircraft, slow-ascent radiosondes, lidar, flux towers, a kitesonde, and land-based transmitters. The use of a ship as an RF-receiving platform facilitated variable-range RF links extending to distances of 80 km from the mainland. Four high-resolution NWP forecasting systems were employed to characterize environmental variability. This paper provides an overview of the TAPS experimental design and field campaign, including a description of the unique data that were collected, preliminary findings, and the envisaged interpretation of the results.

Shallow surface radio frequency (RF) trapping layers or ducts form in stable internal boundary layers (SIBL) when warm dry air flows offshore over colder and more humid sea surfaces. Depending on wind speed and land sea temperature difference, these surface ducts can exist for hundreds of kilometers offshore and trap radar energy in layers below 100m above sea level. This paper describes the performance of notional shore based S, C and X band radars during a SIBL event. The three dimensional (3D) verified refractivity field is modeled by the Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS ®). The radar performance is modeled by the Advanced Refractive Effects Prediction System (AREPS).

Recent advances in mesoscale numerical weather prediction (NWP) models have supported four-dimensional (4D) radio-frequency (RF) propagation modeling in challenging heterogeneous refractive marine environments. Numerical weather prediction models typically provide a vertical profi le of refractivity every 1 km to 3 km horizontally in the domain of interest for each hour in a 48-hour forecast. Due to surface roughness and turbulence constraints, these profi les extend from the stratosphere to within 5 m to 10 m of the sea's surface. Because of strong evaporation at the sea's surface, signifi cant impacts on RF system performance can be induced by refractivity gradients in the fi rst 10 m above sea level (ASL). Historically, the lower-layer refractivity profi les have been calculated using Monin-Obukhov-Similarity-(MOS) based turbulence models. This dualmodel approach requires a robust technique for blending on the order of 3 10 profi les per forecast hour without creating non-physical refractivity artifacts. This paper describes a zero-order-closure turbulent-fl ux technique for blending numerical weather prediction and Monin-Obukhov Similarity refractivity profi les, and presents the results of a multi-wavelength data-comparison process.

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