MABL Vertical Structure and Air–Sea Interaction during CASPER-East and CASPER-West and Implications on Electromagnetic (EM) Wave Propagation

The propagation of electromagnetic (EM) waves in the lower Marine Atmospheric Boundary Layer (MABL) is highly dependent on the vertical gradient of the index of refraction of the air. The atmospheric index of refraction depends mainly on humidity and temperature whose profiles and vertical gradients can deviate from those of standard atmosphere under certain atmospheric conditions and associated air-sea interaction processes such as evaporation. This may result in the formation of layers with negative vertical gradients of the modified refractivity that cause trapping and ducting of EM energy and extend its propagation well beyond the horizon. It is then crucial to obtain comprehensive and concurrent characterizations of the propagation environment and the propagation loss and understand the air-sea interaction processes at play in order to make better predictions of EM propagation. This is essentially the goal of the ONR-funded Coupled Air Sea Processes and Electromagnetic Ducting Research (CASPER) project which included two major field experiments involving at sea research vessels and platforms, shore-based observation sites and research aircraft. CASPER-East was conducted in the fall 2015 off Duck, NC and CASPER-West was conducted in the fall 2017 off Pt Mugu, CA.

We successfully obtained high-resolution turbulence and air-sea interaction measurements concurrently with EM propagation loss measurements using the NPS/CIRPAS Twin Otter (TO) aircraft and its Controlled Towed Vehicle (CTV) during both experiments. The CTV is a ~2-m long modified sea-skimming target drone we outfitted with essentially the same state of the art turbulence instrumentation as that of the TO. The CTV is uniquely equipped with an active height-keeping system that maintains its radar altitude as low as 10 m during long flux-runs while towed from the TO flying 300 m above. It is ideally suited for operation in the lowest part of the MABL where most of the abnormal EM propagation takes place.

The CTV was mostly flown at 10 m and at several other levels within the surface layer along the EM propagation path. It also flew repeated vertical saw-tooth pattern in and out of the surface layer and deeper saw-tooth pattern were flown by the TO that together provided a quasi-instantaneous high-resolution depiction of the vertical structure of the propagation environment. The fine turbulence structure and air-sea fluxes were obtained from long CTV “flux” legs within the surface layer.

After a brief description of the CTV instrumentation and capabilities, we will present results from select cases obtained from both CASPER-East and CASPER-West flights depicting different atmospheric and EM propagation conditions.