Changes between Version 2 and Version 3 of 2020WP/ENHANCE-01_davestorkey_fix_3D_momentum_trends


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Timestamp:
2020-01-21T15:08:25+01:00 (8 months ago)
Author:
davestorkey
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  • 2020WP/ENHANCE-01_davestorkey_fix_3D_momentum_trends

    v2 v3  
    2222=== Description 
    2323 
    24 {{{#!box width=25em help 
    25 Describe the goal of development and the methodology, \\ 
    26 add reference documents or publications if relevant. 
    27 }}} 
     24There are various issues with the 3D momentum trends diagnostics (ln_dyn_trd=T) when using the split-explicit solution of the momentum equation (dynspg_ts - default from NEMO 4.0 onwards): 
    2825 
    29 ''...'' 
     26 1. The "SPG" trend is not the trend due to the surface pressure gradient. It is the change in the velocities due to the solution of the barotropic part of the equations in dyn_spg_ts.  
     27 
     28 2. More generally, for trends which are allowed to vary over the barotropic subcycling (PVO, BFR), there is a small inaccuracy because currently this variation is not taken into account.  
     29 
     30 3. At various points in the code the barotropic part of the velocities is subtracted or added back in (eg. at the beginning of dyn_spg_ts, or at the beginning of dyn_zdf for the implicit bottom friction option). This subtraction/addition of the barotropic component is sometimes incorrectly included in the trend associated with that module.  
     31 
     32These problems mean that it is currently not possible to close the momentum budget, as already noted in tickets #1584 and #1984. The 3D momentum trends are used to calculate various other diagnostics such as 2D vorticity trends (ln_vor_trd=T) or kinetic energy trends (ln_KE_trd=T) so the problems also affect these derived diagnostics.  
     33 
    3034 
    3135=== Implementation 
    3236 
    33 {{{#!box width=35em help 
    34 Describe flow chart of the changes in the code. \\ 
    35 List the Fortran modules and subroutines to be created/edited/deleted. \\ 
    36 Detailed list of new variables to be defined (including namelists), \\ 
    37 give for each the chosen name and description wrt coding rules. 
    38 }}} 
     37 1. Momentum trends diagnostics should correspond to terms in the primitive equations (rather than NEMO modules).  
    3938 
    40 ''...'' 
     39 2. We should have a small number of 3D trends which are sufficient to close the budget: 
     40  a) Each 3D trend should include the barotropic and baroclinic part. (We can output the barotropic part separately if we want). Where the barotropic part of the trend varies over the subcycling this part can be calculated accurately using the time-averaging weights already available in dynspg_ts.F90. 
     41  b) The horizontal pressure gradient trend (HPG) should include the surface pressure gradient (SPG). The SPG trend can be output as an extra diagnostics not required to close the budget. 
     42  c) The vertical diffusion trend (ZDF) already includes the contribution of the wind stress, top stress (in ice shelf cavities) and implicit bottom stress, but it should also include the contribution from the explicit bottom stress terms. The explicit bottom stress can still be output as an extra diagnostic not required to close the budget.  
     43 
     44 3. We should make sure that the subtraction or addition of the barotropic component of velocities or velocity trends is not included in the momentum trend diagnostics. This particularly applies to the ZDF trend. (This is similar to a fix proposed but not implemented by Christoph in ticket #1584).  
     45 
    4146 
    4247=== Documentation updates