Changes between Version 3 and Version 4 of ticket/0663_MLF


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Timestamp:
2010-04-22T18:48:52+02:00 (11 years ago)
Author:
gm
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  • ticket/0663_MLF

    v3 v4  
    1313=== Description === 
    1414 
    15 Introduce the modified Leap-Frog Robert-Asselin filter to fit the one presented in Leclair and Madec Ocean Modelling (2009). 
     15Introduce the modified Leap-Frog -- Robert-Asselin filter (LF-RA) to fit the one presented in Leclair and Madec Ocean Modelling (2009). 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. Furthermore, the scheme allows to use a much smaller Asselin filter parameter (10^-4 instead of 10^-1) decreasing the damping effect of the LF-RA scheme, so that it is actually a quasi second order scheme. 
    1616 
    17 This update is part of the LOCEAN.8 task (Update of the ocean physics) due by June 25th 
     17The LF-RA flow chart of ''step'' is the one introduce in the trunk after the v3.1, to restore the key_vvl option. Its main characteristic is that the ''ssh'' is time-stepped first, followed immediately by the computation of the ''now'' vertical velocity (see figure below). [[BR]] 
     18 
     19[[Image(NEW_LF_RA_FlowCart.jpg, 50%)]][[BR]] 
     20Step routine flow chart, form Leclair and Madec (OM 2009).[[BR]] 
     21 
     22 
     23 
     24This update is part of the LOCEAN.8 task (Update of the ocean physics) due by June 25th 2010. 
    1825 
    1926The main remaining modifications concern :  
    2027 
    21 (1) forcing terms as a mean between time step n-1/2 and n+1/2 (sbcmod) 
     28(1) forcing terms as a mean between time step n-1/2 and n+1/2 (sbcmod.F90, sbc.F90, trasbc.F90, dynzdf....F90) 
     29  
     30[[Image(NEW_LF_RA.jpg, 50%)]][[BR]] 
     31Illustration of forcing integration methods. Old formulation (top) and new formulation (bottom). 
    2232 
    23 (2) Asselin filter: remove the forcing from the filter (dynnxt, tranxt, traqsr) 
     33   At each time a forcing term appears in the 3 time evolution equations solved in NEMO/OPA (ssh, dynamics and tracer). The mean of kt+1/2 and kt+1/2 has to be used in order to remove the largest source of divergence of two consecutive time step. Therefore, the surface module (including the sea-ice!) must now provide both kt+1/2 and kt-1/2 forcing fields, and all the module involving a forcing term have to be modified. 
     34   Note that TKE equation is solved with a two level time stepping scheme : no need of a before field for its forcing term (i.e. taum) 
     35 
     36   sbc_oce.F90 : add "before" forcing fields (denoted by a '_b' sufix) for all ocean forcing components (utau_b, vtau_b, qns_b, qsr_b, emp_b, emps_b) 
     37   sbcmod.F90 : add the swap of surface fields and the read/write in the restart file. 
     38      Beginning of sbs routine, introduce the swap, but only for kt >= nit000+1. If no restart, the '_b' fields is known only at the end of the sbc calculation. Therefore we have choosen to regroup the setting of '_b' fields at nit000 at the end of sbc routine. CAUTION : here the swap must be done at each kt, what ever the nn_fsbc value is. Otherwise the ocean forcing will be wrong. 
     39 
     40(2) Asselin filter: remove the forcing from the filter (dynnxt (?), tranxt, traqsr) 
    2441 
    2542(3) tranxt.F90 : semi-implicit hydrostatic pressure gradient : change the coefficients to take into account the Asselin filter parameter 
    2643 
    27    time mean T and S for the semi implicit hpg computation is now    rbcp*ta+(2-rbcp)*tn+rbcp*tb  
    28    with rbcp=(1+atfp)*(1+atfp*atfp) / 4, and atfp is the Asselin filter parameter. It used to be (ta+2*tn + tb) / 4. 
     44   time mean T and S for the semi implicit hpg computation was (ta+2*tn + tb) / 4 . It is now replaced by 
    2945 
    30    The effect of the change is to further extend the stability limit of the time stepping scheme when ln_dynhpg_imp = T 
     46   rbcp*ta+(2-rbcp)*tn+rbcp*tb  
     47 
     48   with rbcp=(1+atfp)*(1+atfp*atfp) / 4, and atfp is the Asselin filter parameter. 
     49 
     50   The change extend further the stability limit of the time stepping scheme when ln_dynhpg_imp = T 
    3151 
    3252 
    33 === Current missing points === 
     53=== Open questions === 
    3454 
    35 (1) and (2) 
     55(2) Do we need to remove the stress component from the filter in dynnxt?  
    3656 
    37 (3) open question in vvl case : thickness weighted tracer in the time mean, do we need to compute and use a time mean thickness in the hpg computation? 
     57(3) in vvl case : thickness weighted tracer in the time mean, do we need to compute and use a time mean thickness in the hpg computation? 
    3858 
    3959----