Momentum terms from velocity and SSH

[Rodrigo Duran]

Hi,

Consider the momentum primitive equations in the following form:

If we define pressure at the sea surface as gravity times sea-surface height times density, it is straightforward to compute all terms in the momentum equations, except the horizontal and vertical diffusion of momentum, by only using the velocity and SSH fields from HyCOM output, through standard numerical differentiation. 

My questions are:

Would the momentum-term computation be accurate? Or would it be unreliable because HyCOM equations are in a different form due to the generalized vertical coordinate? 

Are the vertical and horizontal diffusion coefficients available anywhere? Does someone know an order-of-magnitude estimate of their relative importance in the Loop Current area? It seems to me the vertical diffusion at the surface would dominate due to wind stress. 

Thanks very much.

Rodrigo. 

[Alan Wallcraft]

The interpolation in both the horizontal and the vertical makes computation of terms that involve differentiation inaccurate.

The best approach would be to use native-grid "snapshot" archives (archv files), but we don't have any of them on-line.  We do have some daily mean archives (archm files), and these are probably your beat bet although I'm not sure haw the mean effects the terms.  See http://data.hycom.org/datasets/GLBb0.08/expt_93.0/data/archm/

The horizontal diffusion includes a Smagorinski term, so its calculation isn't straightforward.  All the horizontal diffusion and viscosity terms have parameters in blkdat.input, and many of them are negative which indicates that they are are spatially varying and are the same as for the Reanalysis, see http://data.hycom.org/datasets/GLBb0.08/expt_53.X/relax/

The vertical diffusion is from KPP, which in HYCOM is applied over the full water column and includes background diffusion and potentially other terms as well as the KPP boundary layer near the surface which depends on the surface forcing.  The vertical visc and diffusion can optionally be written into the archive file, but this has not been done for GOFS.

Alan.

[Rodrigo Duran]

Alan,

Thanks very much. I have downloaded some daily mean files. I have some follow-up questions if I may:

1) Could you point to software that can write the ab outputs as netcdf files? If there were Matlab routines that would be great, otherwise we will work with whatever is available (Fortran, etc). 

2) Are there similar average files for the GOFS 3.1: 41-layer HYCOM + NCODA Global 1/12° Reanalysis ? I would like to analyze July--September 1994 for example. 

3) If the data in the average files has not been interpolated, I think I might need specific routines to be able to differentiate, for example, to include chain-rule terms from the vertical coordinate. If so is there code available? Or perhaps there are no additional computations? I do not fully understand the way HyCOM solves momentum equations. Where can I read about this? 

Thanks again.

Rodrigo.

[Alan Wallcraft]

1) Use HYCOM-tools, an example script is at https://github.com/HYCOM/HYCOM-examples/blob/master/GLBt0.72/archive/ncdf2d_archm.csh

2) They are not currently on-line.  If you can extract the diagnostics you want from  the existing archm files we can likely restore selected 53.X archm files for you.

3) A layered ocean model starts with a stack of shallow water layers with no diapycnal mixing.  However, these are made more complicated by the split-explcit time step (separate baroclinic and barotripic terms), the use of dp' in  place of dp for most terms, and by vertical mixing (which HYCOM refers to as KPP).  Also HYCOM uses relative vorticity in the momentum equations.  There is no perfect source for the equations, but see https://www.hycom.org/attachments/063_hycom_users_manual.pdf and some of the short notes by George Halliwell may also be of interest at https://www.hycom.org/hycom/documentation. The best single reference is Bleck, R., 2002: An oceanic general circulation model framed in hybrid isopycnic-cartesian coordinates. Ocean Modelling, 4, 55-88.

You will need the "metric" terms from regional.grid at http://data.hycom.org/datasets/GLBb0.08/expt_93.0/topo/

Alan.