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energetic inertial motion and a relatively shallow pycnocline, inertial shear may
play a relatively minor role in upward mixing. For the SHEBA circumstances, the
pycnoclineisverystrong.Forweakerstratificationnearthebaseofthemixedlayer
(as in the Southern Ocean), inertial shear may be significantly more important in
themixingprocess.
8.2.3 Short-Term Velocity Prediction
Developmentofthecomplexdemodulationtechniqueforanalyzingsea-icedriftde-
scribed in Section 2.5 (see also McPhee 1988) was spurred by a practical task of
predictingwhereice floestrackedbyradarinthe vicinityofexploratoryoildrilling
platforms in the Beaufort Sea might drift over the next few days. Inertial oscilla-
tionswereaprominentfeatureintheradartrackedtrajectoriesofnearbyfloes,with
rapidly changing directions and speeds, so that depending on where the tracking
pickedupintheinertialloop,thefloemightbeheadeddirectlytowardtheplatform
athighspeed,thenashorttimelatergoinginaquitedifferentdirection.Extrapolat-
ingfuturedriftfromashorthistory ofobserveddriftthusrequiredconsiderationof
theinertialmotion.
The problem of starting a model at a particular time for short term predictions
is related to the fact that a simple harmonic oscillator (e.g., equation (2.22)), when
forced from rest impulsively will oscillate continuously about a steady state that it
neverreaches. In reality, of course, the ice/IOBL system is not frictionless, yet it is
clear from records like Fig. 8.5 that oscillations can persist for several days. In the
SHEBA examples of Sections 8.2.1 and 8.2.2, the model was started from a time
when wind and ice drift velocity were nearly zero, so impulsive initial forcingwas
not much of an issue. But suppose we wished to predict ice motion starting early
on day 263, when the wind is high but forecast to diminish, and inertial motion is
large. To highlight the problem, we drive the ice/IOBL system with the observed
wind, initialized with
T/S
structure as observed at time 263.125, and started from
rest (all velocities zero). Results for drift velocity (Fig. 8.16) show that although
inertial oscillations are generated in the model, they are substantially out of phase
withtheobservationsandwouldbeoflittleuseinactuallyestimatingwherethefloe
wouldbein ashorttime.
To address this problem the IOBL model was initialized by (i) solving a steady
version of the model (described in detail in Chapter 9), and then (ii) adding to the
steady solution for velocitiesin the mixed layer, the inertial componentof velocity
from the complex demodulation record synthesized from the GPS positions (this
assumes thatthe iceandIOBL areoscillatingin phase).Inthis case (Fig.8.17),the
firstfewinertialcyclesaremuchcloserto theobservations.
