Geoscience Reference
In-Depth Information
Even when a relatively complete set of measurementsexists, we are often faced
with the sampling problem of extrapolating measurements made at a few location
(which because of operational considerations, are often biased toward relatively
smooth ice) to a general description for the entire surrounding ice field, which in
turn might be appropriate to characterizing a grid cell in a numerical model (this
is sometimes referred to as the “scaling up” problem). An example from ISPOL
(McPhee 2008, in press) serves to clarify this problem. The floe with which we
drifted north in the western Weddell Sea comprised a conglomerate of several dif-
ferentice typesincludingheavily ridged portions,relativelythin (
1m) regionsof
first year ice, plus reasonablysmooth regionsof multiyear ice about 2m thick. For
most of the projectthe turbulence mast was located under ice of the last type, with
the undersurface in the immediate vicinity quite smooth, but with pressure ridges
and the floe edge within the first 100m or so from the site. Toward the end of the
drift phase of the project (on December 25) the ice floe split, forcing relocation of
the turbulence mast, which for the last week of the project was located under thin
iceneara smallpressureridge.
During the first deployment, we consistently observed a substantial increase in
turbulentstresswithdepthacrossthe6mspanoftheturbulencemast(seeFig.9.10
ofMcPhee2008),whichweinterpretedasthedeepersensorspickingupturbulence
generated by large undersurface features some distance away. This phenomenon
has often been observed in other projects as well, typically where the mast was
locatedundersmoothice,buttherewereroughnessfeatureswithinadistancegiven
roughly by the ratio of mean velocity to scale velocity
∼
times the depth of
theturbulencesensormeasuredfromtheinterface(MorisonandMcPhee2001).So,
for example,a TIC 2m below the boundarymight sense roughnessfeatureswithin
about30m,whereasturbulencemeasured4mlowermightrespondtoundersurface
protrusions up to 100m away. This rule of thumb seemed to hold reasonably well
forSHEBA aswellasISPOL(McPhee2002).
For the short deployment at the end of the project, a mast with two clusters, 1
and 3m below the ice undersurface, respectively, was initially placed so that the
predominant tidal flow would approach from the north or south across relatively
smooth ice, and parallel to a small pressure ridge situated to the west. Soon after
deployment,however,thefloerotatedsothatifthecurrentsensedbythemastcame
from the northeast, the keel was directly upstream from the turbulence mast, with
large impacton flow in the upperfew meters. The installation includedan acoustic
Dopplerprofilerthatprovidedhigh-resolutioncurrentprofilesfromabout10to30m
depth. Two examples from this second installation are shown in Fig. 9.1: one with
flow approachingthe mast across smooth ice, and the second with relative flow al-
mostdirectlyacrossthepressureridgekeel.Intheformercase,thecurrentstructure
shows a reasonably well developed Ekman spiral, with friction velocity at 1m of
about 5mms
−
1
, and from direct application of the LOW to the dimensionless ve-
locity, we infer a surface roughness of about 0.8mm. When flow approaches from
across the keel, the hodograph from ADP data in the range from 10 to 30m again
exhibitstheexpectedEkmanturning,butnowcurrentsattheTICdepthsareasmall
fraction of the deeper currents, indicating flow blockage. If
u
∗
estimated from the
(
u
/
u
∗
)
