High-Resolution Turbulence Measurements in the Marine Surface Layer with the Controlled Towed Vehicle (CTV) and Its Tow Aircraft during CASPER-West

The Coupled Air Sea Processes and Electromagnetic Ducting Research (CASPER) is a Multi-University Research Initiative (MURI) project funded by the Office of Naval Research (ONR) with the aim to improve our understanding of air-sea interaction processes that affect electromagnetic (EM) propagation in coastal Marine Atmospheric Boundary Layers (MABL). CASPER included two major field experiments involving research aircraft, at sea research vessels and platforms, and shore-based observation sites. 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 Controlled Towed Vehicle (CTV) and its tow aircraft the CIRPAS Twin Otter (TO) during both experiments.

In this presentation we will focus on the results obtained during CASPER-West with the CTV, the modified sea-skimming target drone we outfitted with essentially the same proven turbulence instrumentation as that of the TO. The CTV is uniquely equipped with an active height-keeping system using a controllable “wing” (elevator) to maintain its radar altitude as low as 9 m during long flux-runs while towed from the TO flying 300 m above. The CTV was mostly operated at 10 m and at several levels within the surface layer along the CASPER-West EM propagation path defined by the shore site at Pt Mugu and the R/P FLIP moored roughly 50 km offshore . The CTV also flew repeated vertical saw-tooth pattern in and out of the refractive duct 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 throughout the surface to well above the MABL.

We will briefly describe the TO and CTV instrumentation and then present air-sea interaction and boundary layer structure results from select fights along the propagation path. We will particularly focus on results obtained from flights during weak, moderate, and strong Santa Ana wind and a flight during strong northwest wind where some unusual perturbations to the environment that may be attributed to island wake effects were observed. The range-dependent EM propagation loss characteristics in these different conditions are explored by analysis of the data of a receiving antennas array on the FLIP corresponding the frequency of the emitted signals by the two X-Band oscillators on the TO and CTV.