Propagation of electromagnetic (EM) waves from radar and communication devices at sea can be significantly affected by the low-altitude atmospheric refractive conditions. Characterizing the spatial and temporal variability of refractivity is thus crucial to many important Navy and civilian applications. Surface layer models based on Monin-Obukhov Similarity Theory (MOST), such as the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM), can be used to estimate the vertical refractive structure of the evaporation duct (ED) from measured or forecasted environmental parameters, assuming a horizontally-homogeneous stratified atmosphere. As air and sea surface temperature are two critical inputs to NAVSLaM, the rapid variation in sea surface temperature (SST) and air-sea temperature difference in the Gulf Stream region provides a relatively controlled environment for investigating the utility of NAVSLaM under inhomogeneous ducting conditions. In this paper, EM propagation in the ED over the Gulf Stream is investigated using measured data from the Coupled Air-Sea Processes and Electromagnetic-ducting Research (CASPER) East Coast experiment conducted off the coast of Duck, NC, during October-November of 2015. Measurements of the one-way propagation loss between a transmitter and receiving array as a function of the range were performed during ship crossings of the Gulf Stream. Concurrent and co-located meteorological and oceanographic measurements were also collected within the marine atmospheric boundary layer (MABL), including air temperature, SST, relative humidity, air pressure and wind speed to be used as inputs to NAVSLaM. The environmental conditions in the region of the Gulf Stream are used to model the low-altitude refractivity profiles as a function of height and range, which are input into the Advanced Propagation Model (APM) for predicting the EM propagation. Of particular focus is the effect of the rapid change in SST across the boundary between the warmer Gulf Stream and the colder surrounding ocean. EM measurements are compared with the simulated propagation loss based on the range-dependent refractivity profile predicted from NAVSLaM, as well as a best-fit Paulus-Jeske ED model. Both stable and unstable conditions encountered during the CASPER East experiment are investigated.

The growth of the marine internal boundary layer (MIBL, height ℎ𝑖 ) with the shore-normal distance 𝑥, is a topic of continuing interest because of its applications in coastal dispersion, offshore wind farm siting, coastal air-sea fluxes and in evaporative ducting. Available data on MIBL are only scarce, given the difficulty to measurements and variability of coastal winds. During Coupled Air-Sea Processes and Electromagnetic Research (CASPER)-West campaign, from September 27 to October 27, 2017, an array of instrumentation was deployed on the Californian coast at Point-Mugu and on the research vessel Sally Ride. A state-of-art triple Doppler Lidar system was used to map the flow field up to 500 m in height above sea level and to a distance of ~ 4 kilometers from the coast with a spatial resolution of 30 m. Triple Doppler Lidar provides simultaneous vertical profiles of all three velocity components in the atmospheric boundary layer. A 20 m flux tower was used to characterize the atmospheric surface layer, especially its stability. The temporal variability of MIBL growth measured by the triple Doppler Lidar near the coast every 15 minutes were synthesized by its offshore wind direction and atmospheric stability. Over 5 unique case-studies of MIBL offshore development provided some useful general conclusions on the MIBL growth. Measurements also from a state-of-art motion-stabilized coherent Doppler Lidar and microwave radiometer on Sally Ride also showed the growth of offshore internal boundary layer several kilometers from the coast.