Re: [ferret_users] dynamic height calculation near coast

["William S. Kessler" <william.s.kessler@xxxxxxxx>]

On 25 Jul 2011, at 6:47 AM, golla nageswararao wrote:

>     The dynamic_height.jnl in ferret is not taking into  
> consideration less than the level of no motion i.,e near coast no  
> computation. But how to compute the dynamic height near to coast by  
> appending the deepest profile in offshore region along the same  
> latitude ? Anyone tried doing in ferret....

Computationally this is straightforward:
- Find dynamic height *relative to the surface*; carry the calculation  
to the bottom of each profile.
- Find geostrophic shear *relative to the surface* from all adjacent  
DH profiles,
   down to the deepest common level of all adjacent pairs.
- Adjust all the ug profiles to have a value of zero at the chosen  
reference level,
   or at the bottom where shallower.

This raises important issues, however. Is it physically reasonable to  
assume that the bottom velocity is zero on a sloping bottom where an  
offshore current exists? Very likely not: On a coast with a strong  
alongshore boundary current, it is probable that the current continues  
inshore above the shallow bottom as well. But a zero-at-bottom  
assumption will produce zero current at some intermediate depth where  
the offshore current may be large, with resulting horizontal shear.  
The more realistic assumption would be that the boundary current  
continues to flow with a similar profile close to the coast; i.e. the  
bottom value should not be zero. One can try adjustments like carrying  
the offshore value to the nearest inshore point at the same depth, but  
except on straight coasts this leads to ambiguities at curves and  
corners.

==> There is no correct way to do this calculation without further  
information about the offshore structure of the current. Any  
assumption carries strong implications for the result!

BK