Probing the Surface Layer over the Gulf Stream with the Controlled Towed Vehicle: Unique High-Resolution Turbulence Measurements

We deployed  the Controlled Towed Vehicle  (CTV) on all 11 flights of the CIRPAS Twin Otter (tow) aircraft during the Coupled Air Sea Processes and EM ducting Research (CASPER) experiment off Duck, NC in the Fall of 2015. This presentation will particularly focus on three flights that were dedicated to a three-consecutive-day survey over the Gulf Stream to capture spatial and temporal variability of the SST field, wind, temperature and humidity as well as turbulent air-sea fluxes of latent heat, sensible heat and momentum from observations made as low as 8 m. These unique CTV high-resolution measurements provide new insight into the air-sea interaction physics and the scales involved in the transport of water vapor near the surface and higher above it. Data from a very accurate radar altimeter on the CTV show that near the surface the wave height is positively correlated with vertical motion of water vapor which suggests the important role of wind waves and swell in the transport mechanism. Observations from runs across the GS boundary showed a vigorous enhancement in turbulence intensity and fluxes over the warmer water even well above the surface layer.              

The 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 self-generated 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.