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of
against u providesconvincingevidencethatforlowvaluesofstress,itisoften
theplanetaryratherthanthegeometricscalethatgovernsturbulence,evenrelatively
closeto theboundary.
λ
5.2 The IOBL with Stabilizing Boundary Buoyancy Flux
As discussed in Section 4.2.3, in compact pack ice it is rare to encounter sustained
conditionswhereboundarybuoyancyfluxhasmajorimpactonice/oceandragchar-
acteristics or near surface turbulence scales. 1 This is particularly true for manned
research stations, which are typically sited with an eye for survivability. The 1984
MarginalIceZoneExperiment(MIZEX)northofFramStraitinthe GreenlandSea
was an exception. For this ship-supported drift, we hoped to encounter a range of
conditionselucidatingthebehaviorofpackiceasitencounterstheopenocean.Af-
ter drifting with a multiyear floe for about three weeks in late June and early July
withoceanconditionsfairlyrepresentativeoftheArcticsummericepack,northerly
winds blew the station southward across a front that marked the edge of an eddy
previously visible in satellite imagery (Morison et al. 1987). Water temperature in
the well mixed layer rose from near freezing to well over a degree above freezing
as we crossed the front early on year day 191 (Fig. 5.8a). In Fig. 5.8, boxes mark
two 6-h average blocks of data for which the friction velocity at 7m was nearly
identical, but the temperatureand heat flux were quite different.Average w spectra
for the two cases (Fig. 5.9) are similar in shape and magnitude except that for the
melting (warm) regime there is a significant shift toward higher wave number. As
shown, this implies that
λ peak is smaller, with the ratio being about 0.6. The verti-
cal structure of both friction velocity and
λ peak for the two samples is illustrated in
Fig.5.10.
This is, of course, an almost singular sample, yet it provides a very useful tem-
plate for looking at turbulence scales. The mean value of turbulentheat flux at 7m
for 6h centered about time 191.125 is about 475Wm 2 , but this is not apriori
representative of the basal heat. That can be estimated, however, from the “three-
equationsolution”forinterfacefluxquantitiesdiscussedindetailinChapter6,using
measured T
S ,and u ,providinga(waterequivalent)basalmeltrateofabout2.5cm
overthe6-hperiodfromheatfluxattheinterfaceofabout350Wm 2 .This,inturn,
implies a buoyancy flux
,
10 7 m 2 s 3 , and from (4.26),
w b 0
2
×
η
0
.
65.
Since the planetaryscale
(
u 0
/
f
)
in each case is aboutthe same, the ratio of scales
intheouterlayershouldgoas
η .
An independent estimate of buoyancy flux may be made from the measured u
and
3
(
=
/ λ
)
and TKE dissipa-
tion rate calculate via (5.3), as graphed in Fig. 5.11. As discussed below, we often
find that in the outer layer, shear production exceeds dissipation. We found, for
λ
peak by considering the TKE production
P s
u
peak
1 This does not pertain to dynamics in the outer part of the IOBL where buoyancy associated with
quitemodest meltrates can significantlylimitthedepth towhichturbulent mixingpenetrates.
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