Turbulence Measurements with the Controlled Towed Vehicle and its tow Aircraft during CASPER East

During the Coupled Air Sea Processes and Electromagnetic Ducting Research (CASPER) experiment conducted in the fall 2015 off Duck, NC we successfully obtained high-resolution turbulence and air-sea interaction measurements from 11 flights using the Controlled Towed Vehicle (CTV) and its tow aircraft the CIRPAS Twin Otter (TO). The CTV is a modified sea-skimming target drone we equipped with essentially the same proven turbulence instrumentation as that of the TO. The CTV is capable of active height-keeping via controllable “wing” (elevator) and is capable to maintain radar altitude as low as 9 m above ocean surface while towed from the TO at 300 m above. This setup also allows simultaneous similar measurements from the TO at 300 m directly above the CTV. The drone is carried under the tow aircraft for take-off and landing, and released via the automated cable/winch system housed inside the aircraft. The main advantage of the CTV is the direct measurement of means and fluctuations of meteorological parameters and associated fluxes at the canonical air-sea interaction height of 10 m. While air-sea fluxes divergence is traditionally thought to be negligible between 30 m (lowest flight level of the TO) and 10 m and can be approximately corrected, the use of the CTV removes all assumptions. More importantly, the mean wind is directly measured at 10 m without resort to over-land extrapolation parameterizations derived from MOST. The CTV was mostly operated at 10 m and at several levels within the surface layer along the CASPER Ship-Ship and Ship-Shore East-West EM propagation path. The CTV also flew vertical saw-tooth pattern in and out of the refractive duct and deeper saw-tooth pattern were flown by the TO. We will briefly describe the TO and CTV instrumentation and then present air-sea interaction and boundary layer structure results from flights along the East-West EM propagation path. We will particularly focus on results obtained from 3 flights that were coordinated with the two CASPER research vessels operations on 3 consecutive days over the Gulf Stream (GS). SST jumped by about 8 °C across the GS boundary causing large fluctuations in air humidity and temperature (and hence refractive index) and significantly enhanced wind speed, turbulence and air-sea fluxes over warm side. Horizontal variability of the SST and dynamics of the GS boundary were revealed by measurements from the CTV downward-looking IR pyrometer obtained from low level lawn-mowing mapping patterns.