Measurements of the water-following capability of holey-sock and TRISTAR drifters

Since 1985, a number of measurements have been made in deep water to determine the water-following characteristics of mixed layer drifters with both holey-sock and TRISTAR drogues at 15 m depth, The measurements were done by attaching two neutrally buoyant vector measuring current meters (VMCMs) to the top and the bottom of the drogues and deploying the drifters in different wind and upper ocean shear conditions for periods of 2-4 h. The average velocity of the VMCM records was taken to be a quantitative measure of the slip of the drogue through the water, observed to be 0.5-3.5 cm s(-1). The most important hydrodynamic design parameter which influenced the slip of the drogue was the ratio of the drag area of the drogue to the sum of the drag areas of the tether and surface floats: the drag area ratio R, The most important environmental parameters which affected the slip were the wind and the measured velocity difference across the vertical extent of the drogue. A model of the vector slip as a function of R, vector wind and velocity difference across the drogue was developed and a least squares fit accounts for 85% of the variance of the slip measurements. These measurements indicated that to reduce the wind produced slip below 1 cm s(-1) in 10 m s(-1) wind speed, R > 40. Conversely, if the daily average wind is known to 5 m s(-1) accuracy, the displacement of the R = 40 drifter can be corrected to an accuracy of 0.5 km day(-1).