Turbulence Measurements with the Controlled Towed Vehicle in the Marine Surface Layer Under Ducting Conditions

The Controlled Towed Vehicle (CTV) uses improved towed drone technology to actively maintain via a radar altimeter and controllable wing a user-set height that can be as low as the canonical reference height of 10 m above the sea surface. After take-off, the drone is released from the tow aircraft on a ~700-m stainless steel cable. We have instrumented the 0.23 m diameter and 2.13 m long drone with high fidelity instruments to measure the means and turbulent fluctuations of 3-D wind vector, temperature, humidity, pressure, CO2 and IR sea surface temperature enabling direct calculation of eddy correlation air-sea fluxes. Data are recorded internally at 40 Hz and simultaneously transmitted to the tow aircraft via dedicated wireless Ethernet link. The CTV accommodates 40 kg of instrument payload and provides it with 250 W of continuous power through a ram air propeller-driven generator.

Manned aircraft operation at low-level boundary-layer flights is very limited. Dropsondes and UAS (Unmanned Aerial Systems) are alternates for measurements near the ocean surface. However, dropsondes have limited sensor capability and do not measure fluxes, and most present UAS vehicles do not have the payload and power capacity or the low-flying ability in high winds over the oceans. Measurements from ships and other ocean surface-based platforms are prone to flow distortions and wave induced motion. Therefore, the CTV which is essentially a non-intrusive platform fills a crucial gap between surface measurements, dropsondes, in situ aircraft, and UAS. The payload, capacity and power of the CTV makes it suitable for a variety of atmospheric research measurements. Other sensors to measure aerosol, chemistry, radiation, etc., could be readily accommodated in the CTV.

We will briefly discuss the CTV development, the engineering challenges and solutions that have been successfully implemented to overcome them and make the CTV a fully operational platform. We will present results from recent CTV flights inside the surface layer during the Coupled Air Sea Processes and EM ducting Research (CASPER) experiment off Duck, NC and the CASPER-Pilot experiment off Monterey Bay, CA.