New URL for NEMO forge!   http://forge.nemo-ocean.eu

Since March 2022 along with NEMO 4.2 release, the code development moved to a self-hosted GitLab.
This present forge is now archived and remained online for history.
2009WP/2009Stream3/LeapFrogC – NEMO
wiki:2009WP/2009Stream3/LeapFrogC

Version 4 (modified by gm, 15 years ago) (diff)

--


Conservative Leap-Frog + Asselin filter

Last edited Timestamp?

Object:
Introduce in the trunk the modified Leap-Frog + Robert-Asselin filter time stepping scheme (thereafter LF-RA) described in (Leclair and Madec Ocean Modelling 2009, in press). The scheme allows a exact conservation of the heat and salt contents when using variable volume thickness (key_vvl). The non-conservation of the standard LF-RA due to the time diffusion of the forcing through the filter. In the modified scheme the exact conservation is achieved by removing the forcing from the filter. This requires 4 modifications regarding the forcing evaluation, its time discretisation, the filter, and the general flowchart :

  • Flow chart : general structure must be changed in a similar way to what have been done for vvl, see 2009WP/2009Stream3/VVL
  • Forcing evaluation : the momentum and tracers forcing are evaluated at kt+1/2, no-more at kt.
  • Forcing time discretisation : 0.5 * ( Q(k+1/2) + Q(kt-1/2) ), instead of simply Q(kt).
  • Filter : the RA filter no-more act on the forcing, i.e. -afp * rdt * ( Q(kt+1/2) - Q(kt-1/2) ) is removed from the filter

New versus old time discretisation of the forcing
Illustration of forcing integration methods. Old formulation (top) and new formulation (bottom).

Status:
ongoing work (NEMO-Paris).

Module involved:
fldread.F90
sbcmod.F90 ; trasbc.F90 , traqsr.F90 , dynzdf_exp.F90 , dynzdf_imp.F90
sshnxt.F90 , tranxt.F90 , dynnxt.F90

Pending issues:

  • The associated changes in TOP are still unclear. A priori, in PISCES p4zopt.F90 should be modified. Nevertheless, the TOP time stepping strategy seems to be a forward in time scheme. If yes, then the evaluation of the forcing at kt+1/2 is an improvement (if not to say a bug fix!). If not, action is required...
  • Check the consistency with the change planned in the emps field (switch to fsalt, a true salt flux).

0. Flow Chart

Flow chart of step with the new Leap-Frog + Robert-Asselin filter


1. Forcing evaluation

In order to evaluate the forcing at kt+1/2, all the input data in the surface module should be interpolated in time at kt+1/2.


2. Forcing time discretisation

Blah blah blah...


3. Modified filter

Blah blah blah...

Attachments (2)

Download all attachments as: .zip