Build CROCO tidal forcing

1.7. Build CROCO tidal forcing#

CROCO is able to propagate the different tidal constituents from its lateral boundaries by forcing tidal signal both in elevation and velocity.

Note

To get a clean signal you need to provide harmonic components from both tide elevation and tide velocity. In case you don’t have velocity harmonics (undef UV_TIDES in cppdefs.h) a set of reduced equations is available to compute velocity from SSH (OBC_REDUCED_PHYSICS)

1.7.1. Description of make_tides#

make_tides creates a forcing file containing the amplitude and phase of each of the desired tidal components for tide elevation and currents. These values are interpolated over the entire CROCO grid, but the simulation only uses the values at boundaries for forcing.

In most cases, the simulations are only forced by the main tidal components. To take disturbance waves into account, a nodal correction is applied to these main waves. This nodal correction is of low amplitude and only slightly variable (considered constant over a year) but can have a significant impact on the tidal solution in coastal areas.

CROCO’s common practice is to set the nodal correction value at the simulation start date and keep it constant thereafter. In this case, the correction is directly included in the forcing file. However, CROCO also has the ability to calculate these corrections at each time step in the simulation with the cppkey TIDES_MAS. In this case, the nodal correction is not included in the forcing file. make_tides manages these different cases.

Note

The TIDES_MAS key applies nodal correction only on tidal elevation. In the current state, it is therefore necessary to keep the nodal correction on currents in the forcing file.

1.7.2. Fill the Reader#

The associated reader, Readers/tides_reader.py, needs to be filled. Two kind of input data exist, either providing Real/Imaginary part for each wave (like TPXO) or providing Amplitude/Phase (like FES).

As for the initial conditions, the reader contains information on the spatial coordinates at the rho,u,v points, and on the different variables. For each variable, the first letter indicates the variable, H for elevation, (U,V) for respectively eastward and northward currents; and the indices r,i,a,p indicate whether the data are real/imaginary or amplitude/phase type.

For TPXO, which gives elevation and transport data in real/imaginary format, the reader writes:

if input == 'tpxo':
      dico={ 'lonr':'lon_z','lonu':'lon_u','lonv':'lon_v',\
             'latr':'lat_z','latu':'lat_u','latv':'lat_v',\
             'H_r':'hRe',\
             'H_i':'hIm',\
             'U_r':'uRe',\
             'U_i':'uIm',\
             'V_r':'vRe',\
             'V_i':'vIm'\
             'topor':'h','topou':'hu','topov':'hv'\
            }

As TPXO does not provide meridional and zonal current transport, an extra line is added to read the TPXO topography to be able to compute the transport.

1.7.3. Using make_tides#

Let’s build a forcing file from TPXO7. This dataset is available in the DATASETS_CROCO . The reader for this file is:

elif input == 'tpxo7':
    dico={ 'lonr':'lon_r','lonu':'lon_u','lonv':'lon_r',\
           'latr':'lat_r','latu':'lat_r','latv':'lat_v',\
           'H_r':'ssh_r',\
           'H_i':'ssh_i',\
           'U_r':'u_r',\
           'U_i':'u_i',\
           'V_r':'v_r',\
           'V_i':'v_i',\
           'topor':'h','topou':'h','topov':'h'\
          }

Before running make_tides.py, USER CHANGES section needs to be filled. There are several parts in it:

  • Dates:

    # Dates
# Origin year
Yorig = 2000 # 1900 if TIDES_MAS defined in cppdef.h
# Initial date
Yini, Mini, Dini = 2013, 1, 1

  • Input data informations

    # Input data information and formating
# Note: if you are using a tpxo dataset please be sure to have somewhere in
#       inputdata 'tpxo'. This will allow the code to use the OTIS (TPXO is obtained with it)
#       convention a-b*i for complex.
#       Also, if you have already preprocess TPXO, make sure that you have correct units and
#       u,v are in m/s and not m**2/s
inputdata = 'tpxo7_croco' # Input data dictionnary as defined in the Readers/tides_reader.py
input_dir = '../../DATASETS_CROCOTOOLS/TPXO7/'
input_file = 'TPXO7.nc' # Leave empty if you have multiple files
input_type = 'Re_Im' # Format of the input data 'Amp_phase'or 'Re_Im'
multi_files  = False # Set to True if several input files
if multi_files:
    waves_separated = True # Set to True if input files waves are separated
    elev_file = 'h_<tides>_tpxo9_atlas_30_v5.nc' # elevation file names. if wave_separated put <tides> where wave name is found
    u_file = 'u_<tides>_tpxo9_atlas_30_v5.nc' # eastward currents file names. if wave_separated put <tides> where wave name is found
    v_file = 'u_<tides>_tpxo9_atlas_30_v5.nc' # northward currents file names. if wave_separated put <tides> where wave name is found

Here we select the reader for the tpxo7 data. We also select input file location and data format (Re/Im or Amp/Pha). Elevation and current data may not be in the same file. The multi_files option is then useful for specifying each of them. If Eastward and Northward components are in the same file put the same name in u_file and v_file. It is also possible to have waves that are in different files using the <tides> key, which will be replaced by the wave list specified below.

Note

TPXO follows complex convention a-b*i. You therefore need to pay attention to the sign when calculating the phase. For the scripts to correctly take this convention into account, you need to ensure that inputdata contains the tpxo characters.

  • CROCO grid informations

    # CROCO grid informations
croco_dir = '../../CROCO_FILES/'
croco_grd = 'croco_grd.nc'

Informations about your CROCO grid. Indicate the path (croco_dir) and the input grid to use (croco_grd).

  • Tide file and settings:

    # Tide file informations
croco_filename = 'croco_frc.nc'
tides = ['M2','S2','N2','K2','K1','O1','P1','Q1','Mf','Mm']

cur = True # Set to True if you to compute currents
pot = False # Set to True if you to compute potiential tides

# Nodal correction
Correction_ssh = True
Correction_uv = True

Contains informations about the output file name(croco_filename) and which tidal components it will contain.

You can also choose whether you want to calculate currents (cur) and/or the generating potential (pot).

If you want to apply nodal correction on elevation and/or current.

Note

As previously said, CROCO can compute nodal correction for elevation. In this case you must set Correction_ssh = False, choose (Yorig = 1900 and define TIDES_MAS in cppdefs.h)

To use make_tides.py do:

python make_tides.py