Posted by Huai-Min Zhang on May 15, 2001 at 10:13:19:
Julie et al.:
This is a nice poster on model/data intercomparison using
features and statistics, and nice to learn that a model
resolution of 0.1 degrees or higher is necessary to catch the
features in the North Atlantic. My comments/questions are on
the Lagrangian statistics.
The method you used to compute the Lagrangian statistics
(diffusivity and time and length scales) is from Davis (1987, 1991)
approximate transport theory. A key restriction on the validity
of the Davis model (Davis, 1987, eq. 2.5) is that the mean velocity
lateral gradient/shear can be neglected within the particle
displacement considered. In other words, it assumes that the
scale over which the mean velocity varies is much larger than
the eddy scales. This might be true in the ocean interior, but
not generally so in dynamic front regions such as in the GS and
NAC, as discussed in our paper using subsurface isopycnal floats
data (Zhang et al., 2001, JGR-Ocean, in press). Apparently you
are aware of this issue, as you pointed out in your
Appendix B and in Poulain (2001). As Paula Perez Brunius pointed
out/asked, a 2^o by 2^o box binning smears out the mean NAC
structure. Smith et al. (2000) used smaller bin size and resulted
in a mean NAC structure which is similar to our isopycnal float
observation using a 0.5 degree bin. Our experiments show that
larger (than 1 degree) bin size for the Eulerian mean velocity
results biased (larger) Lagrangian statistics. My question is how
sensitive your results to the bin size for the Eulerian mean
velocity? It would be nice if you have a chance to look at this.