Geoscience Reference
In-Depth Information
Chapter 8
LTC Modeling Examples
Abstract: This chapter explores several features of the IOBL by combining ob-
servations and modeling based on
local turbulence closure
as incorporated into a
numerical model described in Chapter 7. The intent is to elucidate certain features
of the response of the upper ocean to variations in forcing that require consideration
of the time dependence of the physical conservation equations.
First, we show that an interesting series of upper ocean measurements at the
SHEBA site near the time of maximum insolation, when there was a clearly dis-
cernible diurnal signal in both temperature and downward turbulent heat flux at two
measurement levels, can be adequately simulated. However, the simulation makes
sense only if solar radiation penetrating the compact ice cover is significantly greater
than has been typically assumed in the past.
Next is a simulation of events observed in late summer at the SHEBA site, when
there was energetic inertial motion of the ice and upper ocean. Inertial oscillation
nearly always implies strong shear in the upper part of the pycnocline, and early
models of mixed-layer evolution (e.g., Pollard et al. 1973; Niiler and Kraus 1977)
related the rate of mixed-layer deepening (“entrainment velocity”) to a Richardson
number involving the inverse square of the velocity of a uniform slab of water (vol-
ume transport divided by mixed-layer depth). In the slab model of Pollard et al.
(1973), for example, the velocity was inertial and any deepening was confined to
the first half inertial period unless the inertial velocity increased. This was an unre-
alistic limitation and much effort was devoted to elaborating how entrainment would
take place at the base of the mixed layer, while still retaining the simplicity of con-
stant temperature, salinity, and velocity in the mixed layer (and the shear that this
implied at the mixed-layer/pycnocline interface). Our initial measurements from the
AIDJEX Pilot Experiment demonstrated convincingly that the IOBL was not “slab-
like” but exhibited definite and predictable shear in the IOBL. McPhee and Smith
(1976, their Figs. 8.11 and 8.12) included an example during a storm where on the
second day, the Ekman layer was confined to levels well above the obvious pycn-
ocline established by stronger forcing on the first day. Nevertheless, it remains an
article of faith among many oceanographers that inertially oscillating slabs are a
primary mechanism by which mixed layers remain mixed. In Section 8.2 we look at
this from the perspective of a model forced with different boundary conditions.
