Forced Heat Convection 



Forced convection is primarily 

 dependent upon the relative 

 temperature between iceberg and 

 the water flowing past the iceberg. 

 The cluster's average daily melt 

 due to forced convection usingjl^, 

 was approximately IScnVday with 

 average values for individual 

 icebergs ranging from 12cnVday 

 (#787) to 21 cm/day (#785). The 

 melt rate as a function of T^ 

 averaged 47cm/day greater; with 

 daily differences ranging from 

 +17cm/day (#784/20 June) to 

 +69cm/day (both #747 and #784 

 on 18 June). These differences 

 were associated with surface 

 temperatures which were about 

 0.9°C warmer than the averaged 

 temperature for the first ten meters 

 of the water column. 



Combined Effect in Using Sea 

 Surface Temperature 



By using sea surface temperature 

 to derive the relative temperature 

 term, the waterline loss could be 

 overestimated by 20 to 80cm/day. 

 This error represents summer 

 conditions (surface warming). 

 Errors for the period f^arch 

 through mid-May should be 

 significantly smaller. Although the 

 error associated with summer sea 

 surface temperatures appears 

 significant, it is an order of magni- 

 tude less than the sum of all 

 modelled deterioration processes. 



Subsurface temperature values 

 can be requested from operational 

 data centers; however, the quality 

 of the analyses are highly depend- 



ent upon the availability of 

 bathythermograph data. More 

 daily observations occur in the IIP 

 region for surface than for subsur- 

 face temperature. Additionally, 

 the thermal stmcture of the water 

 column depicted by data center 

 products often have an accuracy 

 no better than the temperature 

 differences between surface and 

 near surface values (Clancy et al, 

 1987). Using a subsurface 

 temperature profile would mean 

 substituting a known small bias in 

 melt rate for errors which could 

 vary as described. It would also 

 mean a two- to four-fold increase 

 in data input. For these reasons, 

 IIP could not justify requesting and 

 using subsurface temperature 

 fields from operational data 

 centers. 



RELATIVE VELOCITY 



Forced convection is also a 

 function of relative velocity be- 

 tween the iceberg and the sur- 

 rounding water column. The 

 model equates relative velocity to 

 the difference between iceberg 

 drift and the IIP historical current 

 in the iceberg's vicinity. The wind- 

 induced component of the current 

 is ignored. 



Melt rates for forced convection 

 using relative velocities derived 

 from different current inputs were 

 compared. The shallow-drogued 

 drifter was assumed to represent 

 the velocity of the water mass 

 between the surface and the 40m 

 thermocline, that portion of the 

 water column which contributed 

 most to iceberg deterioration. The 



relative velocity between each 

 iceberg and the following were cal- 

 culated as inputs to the model: 

 shallow- and deep- drogued 

 drifters, and the MP's "master 

 historical current field velocity, 

 which for the entire study area 

 was 160°Tat 23cm/sec. The 

 deep-drogued drifter represents 

 real-time current data which, when 

 available, is used to "modify" the 

 historical current. (Summy et al, 

 1983) Sea surface temperature 

 was used to compute the relative 

 temperature term. The melt rates 

 derived using the deep-drogued 

 drifter and "master" current as 

 inputs were compared to the melt 

 rate derived from the shallow- 

 drogued drifter input. 



The average daily melt due to 

 forced convection, using iceberg 

 drift relative to the shallow- 

 drogued drifter, was estimated at 

 59cm/day with average values for 

 individual icebergs ranging from 

 55cnVday (#785) to 62cm/day 

 (#747). The melt rate as a func- 

 tion of iceberg drift relative to the 

 deep-drogued drifter ranged from 

 25cm/day slower (#747/19 June) 

 to 38cm/day faster (#784/20 

 June). Using the "master histori- 

 cal current, the melt rate ranged 

 from 53cm/day slower (#747/19 

 June) to 63cnrVday faster (#784/20 

 June). 



These differences in melt equated 

 to velocity differences between the 

 shallow.-drogued drifter and the 

 deep-drogued drifter (i.e. the 

 "modified"' current) and between 

 the shallow-drogued drifter and 

 the "master current of +/- 9cm/sec 



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