Geoscience Reference
In-Depth Information
covariancestatisticsat1-mistakenatfacevalue,
z
0
isabout17cm.Withthatmuch
flow disturbance from the pressure ridge, many of the assumptions underlying the
fluxdeterminationswouldbesuspect,andwewouldnormallyflagsuch dataasun-
reliable.On theotherhand,theexampleillustratesthatformdragonpressureridge
keels will constitute a significant part of the total momentum transfer between the
floeandoceanunlessthe undersurfaceisexceptionallysmooth.
Evaluatingthedragandenhancedmixingfromevenonepressureridgekeelis a
formidabletaskrequiringextensivecomputation(see,e.g.,Skyllingstadetal.2003),
and extrapolating the results to an entire heterogeneousfloe adds considerable dif-
ficulty. There is, however, a hint in the deeper current profiles in Fig. 9.1 that by
consideringwhat happensin the outer part of the IOBL, it may be feasible to infer
surface properties representative of the entire floe, the main point being that be-
causethefloemovesasarigidbody,atdepthsgreaterthanmostoftheundersurface
protrusions, the turbulence must sense some integrated impact the varying surface
conditions. In this chapter, we explore this concept with a modeling technique de-
velopedfromthe ISPOLmeasurements(McPhee2008,inpress).
9.1 Model Description
Unlike scalar conservation equations, the Ekman equation for momentum admits
a steady-state solution. A “steady” version of the Local Turbulence Closure model
(SLTC) was developed as a means of extrapolating limited measurements at par-
ticular times to deduce the structure of the entire boundary layer. The primary
assumption and simplification for the SLTC model is that turbulence adjusts in ef-
fect instantaneously to surface conditions so that the local time-dependent terms
in the conservation equations are negligible relative to the vertical exchange terms
(e.g.,formomentum
) andthatthe verticaltransportofTKE is not
a major factor in most IOBL instantiations. While these assumptions are suspect
when large inertial oscillation is present, or during rapid changes in surface flux
conditions, they nevertheless often persist for reasonably long periods, especially
when the ice cover is compact. In practice the model requires a reasonably good
description of the temperature and salinity structure of the upper ocean, and some
way of estimating friction velocity at the interface, perhaps from ice velocity or
surface wind (if ice is drifting freely). As explained below, the model utilizes an
iterative scheme that first estimates the IOBL eddy viscosity solely from surface
flux conditions. Then by Reynolds analogy, it estimates scalar fluxes using eddy
diffusivitybased on the modeled eddy viscosity. In general, these fluxes will affect
the turbulence scales and eddy viscosity, so the steady momentum is solved again
with the new eddy viscosity, fluxes are re-calculated, and so on. We demonstrated
(McPhee 1999) that using this model to simulate the time evolution of tempera-
ture and salinity in the upper ocean produced results similar to a simulation using
a second-momentclosure model (level 2
1
|
u
t
||
τ
z
−
if
u
|
2
of Mellor and Yamada 1982). The lat-
ter required forward stepping of six conservation equations, while the SLTC time
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