which had a resultant drift angle to the left of the 

 wind was observed in a area of very weak 

 geostrophic currents that flowed opposite to the 

 wind direction. No truly dominant mode is evident 

 from the frequency distributions, but the greatest 

 number of observations fell in the 80° to 90° and 

 100° to 110° classes. 



Likewise the distributions of drift ratio frequen- 

 cies is rather even throughout the range of 0.008 to 

 0.132. Again no dominant mode can be observed. 



Since the measured wind speed varied merely 

 between 10 and 20 knots during the drift study, no 

 attempt was made to order the wind drift angles 

 and ratios by wind speed class. 



DISCUSSION 



Smith (1931) addressed the subject of current 

 and wind control drift of icebergs and considered 

 the primary forces responsible for iceberg drift 

 to be gradient currents and wind. He concluded 

 that the resultant drift is dependent upon the 

 degree to which these factors combine. In turn the 

 relative influence of each controlling force is deter- 

 mined by the proportion above and below the sur- 

 face at which the iceberg floats, the velocity and 

 duration of the wind, and the velocity and depth of 

 the gradient current. For the majority of icebergs 

 the effect of wind is least when there is a strong 

 slope or gradient current present, and maximum 

 when it is weak. The exceptions to this are the fan- 

 tastically shaped icebergs in their last stages of 

 decay which are winged or pinnacled so that they 

 offer considerable surface area for drag and lift. 



Icebergs which drift along the continental slope, 

 such as the ones studied during the 1974 Ice 

 Patrol, come under the influence of the Labrador 

 Current. This appears evident from the south- 

 southwesterly drift of these icebergs (Fig. 13). 



The only exception was iceberg No. 1 which was 

 tracked in a region of low current west of the 

 mainstream of the Labrador Current, and in 

 shallow water of 165 meters. For the majority of 

 these icebergs, then, the resultant drift is 

 controlled by the geostrophic current in the area; 

 whereas, the angle of the drift with respect to the 

 wind is a consequence of the time dependent rela- 

 tionship between the relative drag force vectors of 

 the net vertical current shear acting upon the 

 submerged portion of the iceberg, and the wind 

 drag on the above surface portion of the iceberg. 

 Since the iceberg is affected by both air and water, 

 there are two drag terms. If these drag coefficients 

 are determined from the experimental data, then 

 for a given wind velocity, the velocity of the wind 



driven surface current and the velocity of the 

 iceberg could be calculated by integrating the drag 

 forces over a time period necessary to reach 

 equilibrium. The Coriolis force works in concert 

 with the other forces, of course, to determine the 

 resultant drift, but varies only with latitude and 

 velocity. A small force associated with the slope of 

 the sea surface must be considered as it tends to 

 move the iceberg downhill. To predict iceberg drift 

 these forces must be known or at least accurately 

 approximated. 



Since the parameters routinely measured during 

 Ice Patrol are wind velocity and geostrophic 

 current, some attempt must be made to use these 

 data for iceberg drift prediction. To do this, ac- 

 curate estimates of the unknown forces must be 

 made. Efforts have been made for many years 

 (Smith, 1931; Budinger, 1960; Kollmeyer, 1965; 

 and Ettle, 1974) to characterize the effect of 

 oceanic forces on iceberg drift. Although each 

 study contributed a share to the understanding of 

 the interactions of the motive forces, none con- 

 tained all the data necessary to specify the 

 predicted drift. Furthermore, accurate iceberg 

 tracking was handicapped by a lack of precise 

 navigational equipment. The advent of satellite 

 navigation has provided the necessary position 

 determining accuracy. The 1974 Ice Patrol iceberg 

 tagging experiment had as a goal the statistical 

 sampling of several iceberg drifts and did not ob- 

 tain all the necessary force measurements either. 



To improve the U.S. Coast Guard Ice Patrol's 

 iceberg drift prediction capabilities, two 

 complimentary projects are being undertaken to 

 quantify the wind effect on icebergs. The first of 

 these is a statistical survey of iceberg dimensions 

 in an effort to relate the above surface area to the 

 submerged area. If, as Smith (1931) believed, there 

 is a characteristic ratio of height to draft (and 

 perhaps area) for each type of iceberg, then the 

 form drag might be defined for a given type of 

 berg. 



The second project entails a concentrated effort 

 on a smaller number of icebergs using two vessels 

 for the simultaneous measurement of iceberg drift, 

 wind velocities, velocities of drogues designed to 

 integrate the current over a known depth, and 

 geostrophic current velocities. 



It is hoped that these studies together will 

 provide the data base for a statistical determina- 

 tion of wind effect drift angles and speed ratios for 

 icebergs classified by their characteristic shape, 

 and for wind speed classes. 



