Barotropic model Blowing-up with high courant and dt = 1.

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asantana
Posts: 4
Joined: Fri Aug 16, 2019 5:30 pm
Location: SOCIB

Barotropic model Blowing-up with high courant and dt = 1.

#1 Unread post by asantana »

Hello,

This is my first post here.
So, I would like to say thank you for accepting me and for all ROMS developers for the tool and support.
I am modeling my first problem with ROMS.
It is a 2D model (barotropic only) with a resolution of 10m.

[Problem]
The model is blowing-up in step 9 with high barotropic velocities.

Code: Select all

      STEP   Day HH:MM:SS  KINETIC_ENRG   POTEN_ENRG    TOTAL_ENRG    NET_VOLUME
            C => (i,j)       Cu            Cv           C Max       Max Speed

      0 18707 00:00:00  0.000000E+00  0.000000E+00  0.000000E+00  5.811092E+07
             (000,000)  0.000000E+00  0.000000E+00  0.000000E+00  0.000000E+00
      DEF_HIS   - creating history file, Grid 01: /mnt/waverider/data/BRIFS_Tests/Outputs/ROMS/20190811_op_wrf3.6/roms_BRIFS_child_20190811_op_his_0001.nc
      WRT_HIS   - wrote history  fields (Index=1) into time record = 0000001
      1 18707 00:00:01  0.000000E+00  0.000000E+00  0.000000E+00  5.811092E+07
             (000,000)  0.000000E+00  0.000000E+00  0.000000E+00  0.000000E+00
      2 18707 00:00:02  1.084924E-14  0.000000E+00  1.084924E-14  5.811092E+07
             (001,001)  8.706092E-08  9.115097E-08  1.782119E-07  2.079068E-06
      3 18707 00:00:03  1.969264E-13  1.809879E-13  3.779143E-13  5.811092E+07
             (002,002)  3.749301E-07  3.928720E-07  7.678021E-07  4.664422E-06
      4 18707 00:00:04  5.893866E-13  7.733326E-13  1.362719E-12  5.811092E+07
             (004,004)  5.819446E-07  6.100008E-07  1.191945E-06  8.985014E-06
      5 18707 00:00:05  7.659377E-12  9.666165E-12  1.732554E-11  5.811092E+07
             (001,003)  1.448769E-06  2.626366E-06  4.075135E-06  4.763931E-05
      6 18707 00:00:06  4.201891E-10  4.859953E-10  9.061843E-10  5.811092E+07
             (002,004)  9.736066E-06  1.712538E-05  2.686145E-05  7.233986E-04
      7 18707 00:00:07  5.394890E-08  4.766217E-08  1.016111E-07  5.811092E+07
             (001,003)  5.420863E-04  4.928241E-05  5.913688E-04  1.775107E-02
      8 18707 00:00:08  5.679739E-05  3.656752E-05  9.336491E-05  5.811093E+07
             (001,002)  1.399511E-02  3.388924E-03  1.738403E-02  5.952867E-01
      9 18707 00:00:09  7.606368E-02  4.657086E-02  1.226345E-01  5.811088E+07
             (001,004)  5.894658E-01  6.879632E-02  6.582621E-01  2.162523E+01

Blowing-up: Saving latest model state into  RESTART file
Looking at the log files it is clear that I have a problem with the Courant numbers:

Code: Select all

 Minimum barotropic Courant Number =  1.19183864E+00
 Maximum barotropic Courant Number =  3.12685140E+00
 Maximum Coriolis   Courant Number =  9.37217488E-05
I could reduce my dt to control it ([1], [2], and [3]), but the problem is that I already have a dt = 1.

[Question]
How can I handle the Courant number in this case?
Or any ideas to identify where is my problem?

[More Information]
To identify the problem I did some tests.
First, to simplify my problem, I turn off the ATM_PRESS flag.
I have open boundaries in the South and West.
Also, I use Chapman implicit for free-surface and Flather for ubar and vbar.
I checked my boundary conditions and the velocities are okay:

Code: Select all

    GET_NGFLD   - free-surface western boundary condition,   t = 18707 00:02:00
                   (Rec=0000002, Index=1, File: roms_BRIFS_child_bry_20190811_op.nc)
                   (Tmin=      18707.0000 Tmax=      18709.0000)
                   (Min = -1.94486187E-04 Max = -1.64477533E-04)
    GET_NGFLD   - free-surface southern boundary condition,  t = 18707 00:02:00
                   (Rec=0000002, Index=1, File: roms_BRIFS_child_bry_20190811_op.nc)
                   (Tmin=      18707.0000 Tmax=      18709.0000)
                   (Min = -2.58851418E-04 Max = -1.64477533E-04)
    GET_NGFLD   - 2D u-momentum western boundary condition,  t = 18707 00:02:00
                   (Rec=0000002, Index=1, File: roms_BRIFS_child_bry_20190811_op.nc)
                   (Tmin=      18707.0000 Tmax=      18709.0000)
                   (Min = -8.54891041E-05 Max = -8.04100491E-05)
    GET_NGFLD   - 2D v-momentum western boundary condition,  t = 18707 00:02:00
                   (Rec=0000002, Index=1, File: roms_BRIFS_child_bry_20190811_op.nc)
                   (Tmin=      18707.0000 Tmax=      18709.0000)
                   (Min = -9.36471333E-05 Max = -9.36471333E-05)
    GET_NGFLD   - 2D u-momentum southern boundary condition, t = 18707 00:02:00
                   (Rec=0000002, Index=1, File: roms_BRIFS_child_bry_20190811_op.nc)
                   (Tmin=      18707.0000 Tmax=      18709.0000)
                   (Min = -8.54891041E-05 Max = -7.47929153E-05)
    GET_NGFLD   - 2D v-momentum southern boundary condition, t = 18707 00:02:00
                   (Rec=0000002, Index=1, File: roms_BRIFS_child_bry_20190811_op.nc)
                   (Tmin=      18707.0000 Tmax=      18709.0000)
                   (Min = -9.36471333E-05 Max = -8.32194564E-05)

This is my header with cppflags definition:

Code: Select all

# define UV_ADV
# define UV_QDRAG
# define UV_VIS2
# define UV_COR
# define RADIATION_2D

# define MASKING

# define VAR_RHO_2D
# define ASSUMED_SHAPE      
# define DOUBLE_PRECISION    
# define MPI                
# define NONLINEAR           
# define POWER_LAW           

# define ANA_INITIAL
# define ANA_DIAG
# define ANA_SMFLUX
# define ANA_STFLUX
# define ANA_SSFLUX

# define MIX_S_UV
# define MIX_GEO_TS
# define DEBUGGING
Some figures to ilustrate the problem:
Bathymetry
h_in_roms_BRIFS_child_20190811_op_rst.png
Zeta at step 9
zeta_in_roms_BRIFS_child_20190811_op_rst.png
V-bar at step 9
vbar_in_roms_BRIFS_child_20190811_op_rst.png
U-bar at step 9
ubar_in_roms_BRIFS_child_20190811_op_rst.png
Running the model with #ATM_PRESS, zeta at step 9.
zeta_in_roms_BRIFS_child_with_atm_press_20190811_op_rst.png
Thanks in advance,
Alex Santana
Ocean Modeller at SOCIB Modelling and Forecasting Facility

User avatar
kate
Posts: 4091
Joined: Wed Jul 02, 2003 5:29 pm
Location: CFOS/UAF, USA

Re: Barotropic model Blowing-up with high courant and dt = 1

#2 Unread post by kate »

Why not try dt=0.5 or shorter? Your horizontal grid spacing is quite a bit smaller than anything I've tried outside of test problems, so it makes sense that you'd need a short timestep.

asantana
Posts: 4
Joined: Fri Aug 16, 2019 5:30 pm
Location: SOCIB

Re: Barotropic model Blowing-up with high courant and dt = 1

#3 Unread post by asantana »

I did not try 0.5, because I just didn't know that it was possible.
I thought dt parameter was an integer. :oops:

I tested it with 0.5 and the max barotropic courant was higher than 1 (the model crashed - step 23).
But with dt <= 0.3 the Courant numbers are okay, so I adopted 0.25.
Now, I will let the model run during the weekend and evaluate the results on Monday.

Thanks for your answer kate!
Alex Santana
Ocean Modeller at SOCIB Modelling and Forecasting Facility

ymamoutos
Posts: 71
Joined: Fri Nov 19, 2010 2:33 pm
Location: University of Aegean

Re: Barotropic model Blowing-up with high courant and dt = 1

#4 Unread post by ymamoutos »

Greetings,

how many barotropic steps you set (ndtfast)?

if ndtdast=1 try more i.e 10 for beginning and
check the value of maximum barotropic Courant number.
In any case - if i recall correctly - for stability
reasons it cannot exceed 0.8

Giannis

User avatar
kate
Posts: 4091
Joined: Wed Jul 02, 2003 5:29 pm
Location: CFOS/UAF, USA

Re: Barotropic model Blowing-up with high courant and dt = 1

#5 Unread post by kate »

The OP said it was a barotropic model which requires NDTFAST = 1.

ymamoutos
Posts: 71
Joined: Fri Nov 19, 2010 2:33 pm
Location: University of Aegean

Re: Barotropic model Blowing-up with high courant and dt = 1

#6 Unread post by ymamoutos »

oops I missed that

I am apologizing. Please ignore my post

Giannis

asantana
Posts: 4
Joined: Fri Aug 16, 2019 5:30 pm
Location: SOCIB

Re: Barotropic model Blowing-up with high courant and dt = 1

#7 Unread post by asantana »

Updating, with dt equals 0.25 the model run without a problem until in an advanced where it blowed-up.
But this is not related to the original problem of this post.
Now, I need to adjust better the grid and bathymetry.

So, the original problem was solved!
Thanks!
Alex Santana
Ocean Modeller at SOCIB Modelling and Forecasting Facility

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