The Coupled Air–Sea Processes and Electromagnetic Ducting Research (CASPER) project aims to better quantify atmospheric effects on the propagation of radar and communication signals in the marine environment. Such effects are associated with vertical gradients of temperature and water vapor in the marine atmospheric surface layer (MASL) and in the capping inversion of the marine atmospheric boundary layer (MABL), as well as the horizontal variations of these vertical gradients. CASPER field measurements emphasized simultaneous characterization of electromagnetic (EM) wave propagation, the propagation environment, and the physical processes that gave rise to the measured refractivity conditions. CASPER modeling efforts utilized state-of-the-art large-eddy simulations (LESs) with a dynamically coupled MASL and phase-resolved ocean surface waves. CASPER-East was the first of two planned field campaigns, conducted in October and November 2015 offshore of Duck, North Carolina. This article highlights the scientific motivations and objectives of CASPER and provides an overview of the CASPER-East field campaign. The CASPER-East sampling strategy enabled us to obtain EM wave propagation loss as well as concurrent environmental refractive conditions along the propagation path. This article highlights the initial results from this sampling strategy showing the range-dependent propagation loss, the atmospheric and upper-oceanic variability along the propagation range, and the MASL thermodynamic profiles measured during CASPER-East.

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[1] Cross-isobath flow on continental shelves is of interest for a variety of reasons. Near Cape Hatteras, North Carolina, the transport of larval organisms, pollutants, and oceanic carbon budget constituents to and from the adjacent Albemarle and Pamlico Sounds may depend critically on cross-isobath currents. Shoreward currents in the nearsurface waters south of Cape Hatteras are documented herein, on the basis of continuous 2-year time series, encompassing all or part of three consecutive winters. Energetic shoreward currents exist $30% of the time from midfall through late spring. These currents are evident over the 20 and 35 m isobaths along a mooring line situated $40 km southwestward from Cape Hatteras. Shoreward velocities average $12 cm/s, and events persist from 0.5 to 4 days, occurring every 2.5-5 days, except in summer. These events often coincide with southwestward winds but occur under both upwelling and downwelling favorable conditions, such that Ekman veering in the surface layer does not account for the shoreward velocities. In winter the mooring line south of Cape Hatteras is frequently traversed by a strong temperature and salinity front, with light, relatively fresh, cold, stratified water on one side, and denser, more saline, warmer, unstratified water on the other. Hydrography and satellite sea surface temperature imagery help identify this front as the boundary between South Atlantic Bight and Mid-Atlantic Bight coastal shelf waters, the ''Hatteras Front.'' Flow along the Hatteras Front where it crosses the shelf appears to account for the observed shoreward currents. The along-shelf advection of the Hatteras Front may depend on both winds and Gulf Stream distance offshore.

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