POST-SEASON DIRECT CURRENT MEASUREMENTS 



During the second half of the post-season 

 cruise, a series of direct current measurements 

 were made at two stations using paracliute 

 drogues (fig. 15). The parachute drogues were 

 of a relatively unsophisticated but eflfective de- 

 sign similar to one suggested by Volkmann et 

 al. (1956) (fig. 16). Reference marker buoys 

 were made using the same surface float design 

 and 50 pounds of chain as an anchor. Visibility 

 during the drogue studies was generally poor in 

 fog and rain. 



The two drogue stations were separated by 

 29 miles. Station 1 was located near 45°56'N., 

 48°32'W. in approximately 80 meters of water 

 close to the continental shelf break. Station 2 

 was further eastward in about 400 meters of 

 water near the western edge of the high velocity 

 core of the Labrador Current. These locations 

 were chosen because they were near standard 

 section A2B, and current meter data were avail- 

 able from this same area. Station 2 (45°58'N., 

 47°50'W.) was also chosen because the drogues 

 could be placed in the Labrador Current in rela- 

 tively deep water, but because of the steep bot- 

 tom gradient, it was possible to anchor the 

 marker buoy nearby in water less than 100 meters 

 deep. Three drogues were set at each station at 

 depths of 25, 50, and 75 meters, which approxi- 

 mated the depths where current can be expected 

 to act on the underwater portion of typical ice- 

 bergs encountered on the Grand Banks. The 

 measurements were made to determine how well 

 direct current measurements agreed with geo- 

 strophic values from the same area. A third 

 station 60 miles east of station 2 was cancelled 

 due to deteriorating weather after less than 3 

 hours of data were collected. Observations in- 

 cluded radar ranges and bearings to each drogue 

 and the reference marker buoy every 20 minutes, 

 as well as weatlier observations. 



Two different methods were used to compute 

 drogue velocities on the CDC 3300 computer. 

 The first method calculated the displacement and 

 angle between successive observations. The dis- 



placement was divided by the time between ob- 

 servations to get current speed. The second and 

 more complex method fitted a second or third 

 order polynomial through the x and y compo- 

 nents of the drogue displacements taken seven 

 at a time. The instantaneous velocity at the 

 midpoint of the seven observations was then de- 

 termined by differentiating the resulting poly- 

 nomial. The advantage to the curve fitting 

 method was a smoothing of any inaccuracies 

 introduced by errors in taking radar ranges and 

 bearings to the drogues and reference marker. 

 The average drogue velocities determined by the 

 two methods are shown in Table II. Agreement 

 between the two methods was much better where 

 the current speeds were higliest and the direc- 

 tions were relatively constant. Drogue speeds 

 discussed in this publication will be values from 

 the polynomial method (method 2). 



The drogues at station 1 were established in 

 an attempt to determine if the Labrador Current 

 had any influence on the current regime close to 

 Hie edge of the continental shelf. A total of 22 

 liours of observations was made at station 1. 

 All three drogues were acted upon by clockwise 

 rotary currents on which a small translatory cur- 

 rent was superposed (figs. 17 and 18). The re- 

 sultant motion of the 25 meter drogue was toward 

 305° true, while the 50 and 75 meter drogues both 

 moved toward 090° true, indicating a strong 

 shear between the 25 and 50 meter levels. All 

 three drogues completed the first Ioojd at about 

 the same time. Although there was considerable 

 change in the instantaneous current speeds (0.7 

 to 17.8 cm/s) at all three depths, the average 

 speed of the 50 and 75 meter drogues during the 

 22 hours of observations differed by only 0.1 

 cm/s. 



An attempt was made to determine if the drift 

 records would show either tidal or inertial pe- 

 riodicity by subtracting the average current ve- 

 locity components from the components of the 

 observed drogue velocity. While the results can 

 not be considered conclusive, when the apparent 



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