APPENDIX B 



OCEANOGRAPHIC CONDITIONS 

 LT J. J. MURRAY, U.S. Coast Guard 



BACKGROUND 



The 1979 season witnessed significant changes 

 to the Ice Patrol mode of operation. In February 

 1979 a new computer model to predict the drift of 

 icebergs was accepted as operational by 

 Commander, International Ice Patrol (CUP) and 

 was used throughout the season. The IIP current 

 file used in this model was also updated to include 

 information obtained since its establishment in 

 1964. Perhaps most significantly, traditional 

 standard oceanographic surveys were replaced 

 with satellite tracked buoys as the primary method 

 of measuring currents and verifying the IIP 

 current file. 



A NEW ICEBERG DRIFT PREDICTION MODEL 



During the 1979 ice season a new computer 

 model was used to predict the drift of icebergs. The 

 drift model, termed IBERG, was developed at the 

 Coast Guard Oceanographic Unit during the fall of 

 1977, and was tested during the 1978 ice season. 

 The model was then integrated into the opera- 

 tional Ice Patrol system and accepted for 

 operational use in February 1979. 



IBERG forecasts the drift of an iceberg by solv- 

 ing differential equations of motion that express 

 the forces acting on the iceberg. The forces 

 included are the Coriolis force, the wind drag, the 

 water drag, and a gravitational component due to 

 the slope of the sea surface. The equations are 

 solved by a fourth order Runge-Kutta method 

 using a variable time step to insure a convergent 

 solution. 



The primary improvement over past modeling 

 efforts is in the calculation of the water drag, 

 which is usually the dominant forcing term. The 

 water drag is proportional to the iceberg cross- 

 sectional area and the square of the velocity of the 

 ice relative to the water. The water current used is 

 the sum of a mean value and a wind driven compo- 

 nent that is derived from a solution to a time 



dependent Ekman equation using a 96 hour wind 

 history. The average Ekman current in each of 

 four layers of the water column is determined. The 

 water drag in each layer is calculated and the 

 results are summed to yield the total drag over the 

 iceberg. This approach allows different sizes of 

 icebergs to be modeled by varying the cross- 

 sectional area in each layer. Small icebergs float in 

 the near surface layer which is strongly affected by 

 the local wind, while large icebergs have much of 

 their area below the wind-influenced layers. Seven 

 iceberg sizes are allowed in the operational form of 

 IBERG. 



A continuing evaluation of IBERG is being 

 conducted. Observed iceberg drift tracks are 

 compared to model generated paths to estimate 

 the model accuracy. Initial results suggest that the 

 primary limitation to accurate forecasts lies in the 

 inputs to the model, most notably the current and 

 wind fields used in the drag calculations. 



IIP CURRENTS 



The IIP current file extends from 40°N to 52°N 

 and from 39 °W to 57° W. Within this area are two 

 regions. The currents are best known in the first 

 region in the proximity of the Labrador Current 

 where standard oceanographic surveys were 

 conducted under the auspices of CUP every season 

 from 1934 to 1978 (except during World War II). 

 Usually 2 or 3 surveys were made each season. 

 These surveys yielded a large data base which was 

 used in 1964 (Soule, 1964) to compile "normal" 

 dynamic topography charts representing the 

 average dynamic heights observed up to that time. 

 From these charts normal geostrophic velocities 

 were calculated. The normal values are useful 

 because the general oceanic circulation in the Ice 

 Patrol area is similar from year to year. The 

 normals were reviewed and updated in 1976 

 (Scobie and Schultz, 1976) and it is basically these 

 updated currents which were used during the 1979 

 season. The second region is composed of all other 



B-1 



