Appendix C 



MODIFICATIONS 



TO ICE PATROL'S 



MEAN CURRENT 



DATA BASE 



by 



DONALD L MURPHY 



WALTER E. HANSON 



ROSS L. TUXHORN 



INTRODUCTION 



In 1989, the Interna- 

 tional Ice Patrol (IIP) began 

 an extensive program to re- 

 view and modify the mean cur- 

 rent data base that it uses to 

 predict iceberg drift. This re- 

 view was made in response to 

 problems identified by IIP 

 watch-standersandthe results 

 of research sponsored by the 

 Atmospheric Environment 

 Service (AES) of Canada. 



In recent years, Ice 

 Patrol watch-standers have 

 encountered problems in two 

 regions of the IIP operations 

 area: the offshore branch of 

 the Labrador Current and the 

 coastal waters near the island 

 of Newfoundland (Figure 

 C-1). Repeated sightings of 

 icebergs moving southward 

 in the offshore branch south 

 of Flemish Pass showed that 



Page 76 



the llPdrift model was predict- 

 ing a southward movement 

 far in excess of the observed 

 iceberg movement. Fre- 

 quently, iceberg resightings 

 required the watch-standers 

 to intervene and move the 

 icebergs back to the north, in 

 effect slowing the southward 

 progress of these icebergs. 

 In a study sponsored by 

 AES (FENCO, 1987) current 

 meter and drifting buoy data 

 were compared to the mean 

 values in this region. They 

 found that the magnitudes of 

 mean values in IIP data base 

 were 2.5 times greater than 

 the observations. 



Along the Newfound- 

 land coast, the IIP current data 

 base had some areas where 

 no mean currents were speci- 

 fied, particularly Notre Dame 

 Bay, which is off the north- 

 eastern coast, and the coastal 

 waters off southern Newfound- 

 land. When icebergs entered 

 these regions, HP's iceberg 

 drift model calculated only 

 wind-related effects, i.e., the 

 mean current terms became 

 zero. As a result, the model 

 calculations showed that ice- 

 bergs drifting into these re- 

 gions tended to accumulate 

 there. Reconnaissance 

 showed no such accumula- 

 tions of icebergs. 



The primary goal of 

 this report is to document the 



changes that were made to 

 the IIP current data base dur- 

 ing 1989-1990. In addition, 

 this report summarizes the 

 history of HP's mean current 

 field and documents the 

 changes that have occurred 

 since it was established. 



HOW THE IIP CURRENT 

 DATA BASE IS USED 



Ice Patrol's iceberg 

 tracking operation relies 

 heavily on the use of its ice- 

 berg drift model (Mountain, 

 1980). Although aerial ice re- 

 connaissance is the most ef- 

 fective means to survey the 

 operations area to locate ice- 

 bergs, continuously monitor- 

 ing the iceberg threat from the 

 air is not practical. For ex- 

 ample, Hanson (1989) re- 

 ported that during a 30-day 

 test period in June 1 988, neariy 

 50% of the icebergs in the IIP 

 operations area were seen 

 only once. This occurred de- 

 spite an unusually large num- 

 ber of air patrols flown during 

 the period. There were 30 

 patrols, 17 by the Canadian 

 Ice Patrol and 13 by IIP. How- 

 ever, during the period the ice- 

 berg limits encompassed over 

 1x10^ km2 of the ocean's 

 surface and the coverage of 

 each flight was less than 1 0% 

 of this area. 



