# Adding a Diagnostic¶

This is a five step process. There are three changes to make in the Fortran code. The indexing type is in marbl_interface_private_types.F90, and the rest of the code is in marbl_diagnostics_mod.F90. (If your diagnostic is part of the carbon isotope tracer module, that code belongs in marbl_ciso_diagnostics_mod.F90.) There are also two steps to make sure the diagnostic is known the GCM so it is included in the output.

For this example, we follow the in situ temperature, which uses the insitu_temp index.

## Step 1. Add to MARBL diagnostic indexing type¶

To reduce the number of string comparisons inside routines called every time-step, MARBL uses integer indices to track many different variables. These indices are packed into datatypes to group common indices together. So the indices for diagnostics variables are split into marbl_surface_flux_diagnostics_indexing_type and marbl_interior_tendency_diagnostics_indexing_type. insitu_temp is an interior forcing diagnostic.

type, public :: marbl_interior_tendency_diagnostics_indexing_type
! General 2D diags
integer(int_kind) :: zsatcalc
integer(int_kind) :: zsatarag
.
.
.
! General 3D diags
integer(int_kind) :: insitu_temp
.
.
.
end type marbl_interior_tendency_diagnostics_indexing_type

## Step 2. Add to diagnostic structure¶

Another common feature among MARBL datatypes is the idea of adding an element to a derived type to contain all the data. Most derived types, including as marbl_diagnostics_type, are “reallocating”: when a field is added, a new array of size N+1 is created, the existing array is copied into the first N elements and then deallocated, and the new entry becomes element N+1. In these situations, pointers are used instead of allocatable arrays so that marbl_instance%{surface,interior}_forcing_diags%diags can point to the new array.

lname = 'in situ temperature'
sname = 'insitu_temp'
units = 'degC'
vgrid = 'layer_avg'
truncate = .false.
call diags%add_diagnostic(lname, sname, units, vgrid, truncate,     &
ind%insitu_temp, marbl_status_log)
if (marbl_status_log%labort_marbl) then
call marbl_logging_add_diagnostics_error(marbl_status_log, sname, subname)
return
end if

## Step 3. Populate diagnostic type with data¶

The purpose of the marbl_diagnostics_type structure is to allow an easy way to pass diagnostics through the interface. This step copies data only available in MARBL into the datatype that is available to the GCM.

associate( &
kmt   => domain%kmt, &
diags => marbl_interior_tendency_diags%diags, &
ind   => marbl_interior_tendency_diag_ind &
)
diags(ind%insitu_temp)%field_3d(1:kmt, 1) = temperature(1:kmt)
end associate

Note

In situ temperature is copied to the diagnostic type in marbl_diagnostics_interior_tendency_compute(). This subroutine also calls many different store_diagnostics_* subroutines, but in a future release the store_diagnostics routines will be condensed into a smaller subset of routines. Regardless, find the routine that makes the most sense for your diagnostic variable. (Surface forcing fields are copied to the diagnostic type in marbl_diagnostics_surface_flux_compute().)

## Step 4. Update the Diagnostics YAML files¶

We use a YAML file to provide an easy-to-edit and human-readable text file containing a list of all diagnostics and the recommended frequency of output. Developers adding or removing diagnostics should make changes to defaults/diagnostics_latest.yaml.

insitu_temp :
longname : in situ temperature
units : degC
vertical_grid : layer_avg
frequency : medium
operator : average

Note that insitu_temp matches what we used for the short name in Step 2. Add to diagnostic structure. The frequency medium means “we recommend outputting this variable monthly”. Other acceptable frequencies are never, low (annual), and high (daily).

The operator means “average over this time period.” Other acceptable operators are instantaneous, minimum, and maximum. You can recommend multiple frequencies by adding a list to the YAML, as long as the operator key is a list of the same size:

CaCO3_form_zint :
longname : Total CaCO3 Formation Vertical Integral
units : mmol/m^3 cm/s
vertical_grid : none
frequency :
- medium
- high
operator :
- average
- average

## Step 5. Convert the YAML file to JSON¶

We prefer editing YAML files to editing JSON files because they are much easier to maintain (and allow user comments). Unfortunately, python does not include a YAML parser in the default distributions. Rather than require all users to install pyYAML, we require that of MARBL developers and then ask them to convert the YAML files to JSON. The MARBL_tools/yaml_to_json.py script is provided to do just that:

$cd MARBL_tools$ ./yaml_to_json.py

The rest of the python scripts provided in the MARBL_tools/ subdirectory rely on the JSON file rather than the YAML. MARBL_tools/MARBL_generate_diagnostics_file.py will turn the JSON file into a list for the GCM to parse:

# This file contains a list of all diagnostics MARBL can compute for a given configuration,
# as well as the recommended frequency and operator for outputting each diagnostic.
# The format of this file is:
#
# DIAGNOSTIC_NAME : frequency_operator
#
# And fields that should be output at multiple different frequencies will be comma-separated:
#
# DIAGNOSTIC_NAME : frequency1_operator1, frequency2_operator2, ..., frequencyN_operatorN
#
# Frequencies are never, low, medium, and high.
# Operators are instantaneous, average, minimum, and maximum.
.
.
.
CaCO3_form_zint : medium_average, high_average
.
.
.
insitu_temp : medium_average

It is then up to the GCM to convert this text file into a format it recognizes for output (e.g. POP will add to the tavg_contents file).