Appendix C 

 Iceberg Deterioration Model 



By Lieutenant lain Anderson 



Introduction 



The ICEPLOT computer program used by the 

 International Ice Patrol (IIP) to predict the posi- 

 tions of reported icebergs depends on iceberg 

 size. It is well known that icebergs drifting 

 south in the IIP operating area (between 

 40°N-52°N and 57°W-39°W) deteriorate. The 

 need for an iceberg deterioration model has ex- 

 isted for some time. Previously, IIP used a 

 hand-held calculator deterioration scheme that 

 was based solely on sea surface temperature. 

 In 1980, Coast Guard Research and Develop- 

 ment Report No. CG-D-62-80, "Theoretical 

 Estimate of the Various Mechanisms Involved 

 in Iceberg Deterioration in the Open Ocean" 

 (White, et al., 1980) was completed. The R&D 

 Center report discussed some of the physics in- 

 volved in buoyant convective, wind forced con- 

 vective, insolation, and wave erosion melting. 

 All equations and figures referenced in this 

 paper refer to this R&D Center report. 



In order to make use of the equations in the 

 R&D Center report, real-time environmental in- 

 formation (sea surface temperature (SST), wave 

 height, and wave period) for the IIP operating 

 area had to be obtained. The required en- 

 vironmental information was obtained from the 

 Fleet Numerical Oceanography Center (FNOC), 

 Monterey, California on a one degree latitude 

 by two degree longitude grid. An evaluation of 

 this data is discussed later. 



The Model 



The two planned uses of the deterioration 

 model are to change the size of the iceberg as it 

 melts so the berg could be drifted more ac- 

 curately and to remove the iceberg from the list 

 of active bergs when it has completely melted. 

 Until more evaluations of the model have been 

 completed, the model will be used only to 

 "flag" bergs that have accumulated a "melt" 

 greater than 175% of their original length. 

 Presently the model is run once a day. 



The output from the deterioration model is 

 presented in an easy-to-interpret form. The 

 form is a number that represents the percen- 

 tage of the original length that has been melted 



by the model. The model requirements are: 



- any assumptions made are to be conser- 

 vative; i.e., they will cause the berg to melt 

 more slowly. 



- the model can be integrated into the ex- 

 isting ICEPLOT computer package. 



- equations used are to be relatively simple 

 to program and produce an answer accurate 

 within the error limits of the inputs. 



Each size of iceberg is assigned a 

 characteristic length (Table C-1). The lengths 

 assigned, except for the large bergs, are those 

 used by IIP to classify icebergs. The length 

 used for the large iceberg was chosen arbitrari- 

 ly. "Melting" each iceberg reduces the length 

 of the iceberg. This is the method used in the 

 R&D Center report. The percent melt output of 

 the model is the total length removed from the 

 melting iceberg divided by the characteristic 

 length of the berg multiplied by 100 (to make 

 the output in percent). The four melting pro- 

 cesses programmed are discussed below in 

 order of increasing importance (Table C-2). 



Table C-1 



Size 



Growler 

 Small 

 Medium 

 Large 



Characteristic 

 Length 



16 m 



60 m 



122 m 



225 m 



Characteristic Iceberg Lengths 

 Used in Deterioration Model 



67 



