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by the presence nearby of open water remained,but analysis of aerial photography
indicatedthatanyopenwater“upstream”oftheoceanmeasurementswasfarenough
away to have had little impact. The apparent discrepancy between the modeling
examplehereandearlierestimatesoflighttransmittanceiniceappeartoberesolved
byrecentworkofLightetal.(2008).Theyreportextinctioncoefficientsforbareand
ponded ice at SHEBA that are substantially smaller than previous estimates, and
that3-10timesasmuchsolarradiationpenetratestheicecoverthanispredictedby
currentglobalcirculationmodels.
8.2 Inertial Oscillations in Late Summer, SHEBA
By mid September at the SHEBA station in 1998, the ice cover was relatively
compact, the well mixed layer had cooled to within few centikelvins of freezing
but remained relatively shallow, as evidenced by strong inertial oscillations during
muchofthemonth(Fig.8.5).Usually,thepresenceofstronginertialoscillationsig-
nals that the internal ice stress gradient is small enough that ice is in a “free-drift”
state, i.e., wind driven. We chose a period during 14-22 September 1998, as a sort
of “modeling laboratory” to look as different aspects of the upper ocean response
(both modeled and observed) during a period of significant inertial oscillation. At
0900 on 14 September (257.375), the wind was still and ice drift speed near zero.
Over the next five days, ice drift speed rose rather steadily to about 0
3ms
−
1
,fol-
lowingwind closely atslightly over2%of thewind magnitude(Fig. 8.5c).Late on
day262, winddroppedquickly,as did mean icedrift;however,a fairly strongtrain
ofinertialoscillationscontinuedforseveraldays.
.
8.2.1 Wind Forced Model
The first LTC model exercise (SEP 14A) was initialized with the SHEBA profiler
3-h average
T/S
data in the upper 61m of the water column, and driven by wind
stress for the period 257.375-265, obtained by applying a drag coefficient,
c
10
=
0
002,totheobserved10-mwindattheprojectofficetower,andusingthedynamic
boundary condition for stress (Section 7.2.3). During this period shortly after the
start of freeze-up,the temperature gradientin the lower part of the ice was slightly
positive,indicatingthat evenin relativelythin ice at the end of the meltseason, the
downward“freezingwave”hadnotyetreachedtheice base.Thiswasincorporated
into the model as a
downward
interface heat flux averaging about
.
6Wm
−
2
.
Since there was not a lot of open water, for simplicity, we assumed that the long
wave radiative loss from open water would roughly cancel incoming short wave
gain.Undersurfaceroughnesswassetat0.048masabove.
Modeled versus observed surface velocity (Fig. 8.6) demonstrates reasonable
simulation of both the mean and inertial components of ice velocity, including
−
.
0
