Geoscience Reference
In-Depth Information
Chapter 9
The Steady Local Turbulence Closure Model
Abstract: A fundamental problem in boundary-layer physics is extrapolating
limited measurements to a general description of the mean velocity and scalar
properties, along with their Reynolds fluxes including values at the immediate
boundary. For the atmospheric surface layer, extensive research has been devoted to
methods relating relatively simple measurements to fluxes. Central to this approach
is characterizing surface roughness for momentum and scalar variables. Typically, a
tower is deployed with two or more levels of instrumentation and the surface fluxes
are estimated either from the mean measurements across the tower using some form
of the Monin-Obukhov dimensionless gradients (e.g., Businger et al. 1971; Andreas
and Claffey 1995), or from a combination of mean gradients and fluxes, determined
either by direct covariance or by spectral techniques (e.g., Edson et al. 1991).
In the IOBL, this is much less straightforward for a variety of reasons. First, in
contrast to the upper sea-ice surface, variation in the underice morphology often
occupies a significant fraction of the entire boundary layer. If the IOBL scales with
about 1/30 of the atmospheric boundary layer, a pressure ridge with a 1-m sail and
5-6-m keel presents completely different aspects to the respective boundary lay-
ers. In general, for the IOBL parameterization problem, many of the surface-layer
assumptions (constant stress, stress and mean velocity collinear with no direction
change, etc.) are clearly inappropriate.
As illustrated in Chapter 8, it is sometimes possible to solve a time-dependent
numerical PBL model with given initial conditions, letting it evolve in time as the
forcing fields change. Given a suitable time series of observations at a particular
location, to the extent that the model can reproduce the observed characteristics
(say mixed layer temperature, salinity, depth), the model will provide a reasonably
accurate description of the overall exchanges across the OBL. This depends on both
having realistic initial conditions and a reasonably accurate time series of forcing
fields (e.g., wind or ice velocity, conductive heat flux in the ice, etc.). In many cases,
observations are scattered in both time and location (for example, stations taken
from a ship or airplane during a regional survey), and one would like to produce a
“snapshot” of the OBL structure, to estimate fluxes at the surface or near the base
of the mixed layer.
