2. CCPP-Compliant Physics Parameterizations

The rules for a scheme to be considered CCPP-compliant are summarized in this section. It should be noted that making a scheme CCPP-compliant is a necessary but not guaranteed step for the acceptance of the scheme in the pool of supported CCPP-Physics. Acceptance is dependent on scientific innovation, demonstrated value, and compliance with the rules described below. The criteria for acceptance of a scheme into the CCPP is under development.

It is recommended that parameterizations be comprised of the smallest units that will be used. For example, if a given set of deep and shallow convection schemes will always be called together and in a pre-established order, it is acceptable to group them within a single scheme. However, if one envisions that the deep and shallow convection schemes may someday operate independently, it is recommended to code two separate schemes to allow more flexibility.

Some schemes in the CCPP have been implemented using a driver as an entry point. In this context, a driver is defined as a wrapper that sits on top of the actual scheme and provides the CCPP entry points. In order to minimize the layers of code in the CCPP, the implementation of a driver is discouraged, that is, it is preferable that the CCPP be composed of atomic parameterizations. One example is the implementation of the MG microphysics, in which a simple entry point leads to two versions of the scheme, MG2 and MG3. A cleaner implementation would be to retire MG2 in favor of MG3, to put MG2 and MG3 as separate schemes, or to create a single scheme that can behave as MG2 and MG3 depending on namelist options.

The implementation of a driver is reasonable under the following circumstances:

  • To preserve schemes that are also distributed outside of the CCPP. For example, the Thompson microphysics scheme is distributed both with the Weather Research and Forecasting (WRF) model and with the CCPP. Having a driver with CCPP directives allows the Thompson scheme to remain intact so that it can be synchronized between the WRF model and the CCPP distributions. See more in mp_thompson.F90 in the ccpp-physics/physics directory.

  • To deal with optional arguments. A driver can check whether optional arguments have been provided by the host model to either write out a message and return an error code or call a subroutine with or without optional arguments. For example, see mp_thompson.F90, radsw_main.F90, or radlw_main.F90 in the ccpp-physics/physics directory.

  • To perform unit conversions or array transformations, such as flipping the vertical direction and rearranging the index order, for example, cu_gf_driver.F90 in the ccpp-physics/physics directory.

Schemes in the CCPP are classified into two categories: primary schemes and interstitial schemes. A primary scheme is one that updates the state variables and tracers or that produces tendencies for updating state variables and tracers based on the representation of major physical processes, such as radiation, convection, microphysics, etc. Exclusions are:

  • Schemes that compute tendencies exclusively for diagnostic purposes.

  • Schemes that adjust tendencies for different timesteps (e.g., create radiation tendencies based on a radiation scheme called at coarser intervals).

  • Schemes that update the model state based on tendencies generated in primary schemes.

Interstitial schemes are modularized pieces of code that perform data preparation, diagnostics, or other “glue” functions and allow primary schemes to work together as a suite. They can be categorized as “scheme-specific” or “suite-level”. Scheme-specific interstitial schemes augment a specific primary scheme (to provide additional functionality). Suite-level interstitial schemes provide additional functionality on top of a class of primary schemes, connect two or more schemes together, or provide code for conversions, initializing sums, or applying tendencies, for example. The rules and guidelines provided in the following sections apply both to primary and interstitial schemes.

2.1. General Rules

A CCPP-compliant scheme is in the form of Fortran modules. Listing 2.1 contains the template for a CCPP-compliant scheme (ccpp/framework/doc/DevelopersGuide/scheme_template.F90), which includes at least one of these five components: the _timestep_init, _init, _run, _finalize, and _timestep_finalize subroutines. Each .f or .F90 file that contains an entry point(s) for CCPP scheme(s) must be accompanied by a .meta file in the same directory as described in Section 2.2

    module scheme_template

      contains

!> \section arg_table_scheme_template_run Argument Table
!! \htmlinclude scheme_template_run.html
!!
      subroutine scheme_template_run (errmsg, errflg)

         implicit none

         !--- arguments
         ! add your arguments here
         character(len=*), intent(out)   :: errmsg
         integer,          intent(out)   :: errflg

         !--- local variables
         ! add your local variables here

         continue

         !--- initialize CCPP error handling variables
         errmsg = ''
         errflg = 0

         !--- initialize intent(out) variables
         ! initialize all intent(out) variables here

         !--- actual code
         ! add your code here

         ! in case of errors, set errflg to a value != 0,
         ! assign a meaningful message to errmsg and return

         return

      end subroutine scheme_template_run

    end module scheme_template

Listing 2.1: Fortran template for a CCPP-compliant scheme showing the _run subroutine. The structure for the other phases (_timestep_init, _init, _finalize, and _timestep_finalize is identical.

More details are found below:

  • Each scheme must be in its own module and must include at least one of the following subroutines (entry points): _timestep_init, _init, _run, _finalize, and _timestep_finalize. The module name and the subroutine names must be consistent with the scheme name. The _run subroutine contains the code to execute the scheme. If subroutines _timestep_init or _timestep_finalize are present, they will be executed at the beginning and at the end of the host model physics timestep, respectively. If present, the _init and _finalize subroutines associated with a scheme are run before and after the _run phase of the scheme. The _init and _finalize phases can be used even if the _run phase if absent. The _init and _finalize subroutines may be called more than once depending on the host model’s parallelization strategy, and as such must be idempotent (the answer must be the same when the subroutine is called multiple times).

  • Each .f or .F90 file with one or more CCPP entry point schemes must be accompanied by a a .meta file containing metadata about the arguments to the scheme(s). For more information, see Section 2.2.

  • All schemes must be preceded by the three lines below. These are markup comments used by Doxygen, the software employed to create the scientific documentation, to insert an external file containing metadata information (in this case, schemename_run.html) in the documentation. See more on this topic in Section 2.7.

!> \section arg_table_schemename_run Argument Table
!! \htmlinclude schemename_run.html
!!
  • All external information required by the scheme must be passed in via the argument list. Statements such as ‘use EXTERNAL_MODULE’ should not be used for passing in data and all physical constants should go through the argument list. See Section 2.5 for more information on how to use physical constants.

  • Note that standard names, variable names, module names, scheme names and subroutine names are all case insensitive.

  • Interstitial modules (scheme_pre and scheme_post) can be included if any part of the physics scheme must be executed before (_pre) or after (_post) the module scheme defined above.

2.2. Metadata Table Rules

Each CCPP-compliant physics scheme (.f or .F90 file) must have a corresponding metadata file (.meta) that contains information about CCPP entry point schemes and their dependencies. These files contain two types of metadata tables: ccpp-table-properties and ccpp-arg-table, both of which are mandatory. The contents of these tables are described in the sections below.

Metadata files (.meta) are in a relaxed config file format and contain metadata for one or more CCPP entry points.

2.2.1. ccpp-table-properties

The [ccpp-table-properties] section is required in every metadata file and has four valid entries:

  1. type: In the CCPP Physics, type can be scheme, module, or ddt and must match the type in the associated [ccpp-arg-table] section(s).

  2. name: This depends on the type. For types ddt and module (for variable/type/kind definitions), name must match the name of the single associated [ccpp-arg-table] section. For type scheme, the name must match the root names of the [ccpp-arg-table] sections for that scheme, without the suffixes _timestep_init,``_init``, _run, _finalize, or _timestep_finalize.

  3. dependencies: type/kind/variable definitions and physics schemes often depend on code in other files (e.g. “use machine” –> depends on machine.F). These dependencies must be listed in a comma-separated list. Relative path(s) to those file(s) must be specified here or using the relative_path entry described below. Dependency attributes are additive; multiple lines containing dependencies can be used.

  4. relative_path: If specified, the relative path is added to every file listed in the dependencies.

The information in this section table allows the CCPP to compile only the schemes and dependencies needed by the selected CCPP suite(s).

An example for type and variable definitions in GFS_typedefs.meta is shown in Listing 2.2.

Note

A single metadata file may require multiple instances of the [ccpp-table-properties] section.

########################################################################
[ccpp-table-properties]
  name = GFS_statein_type
  type = ddt
  dependencies =

[ccpp-arg-table]
  name = GFS_statein_type
  type = ddt
[phii]
  standard_name = geopotential_at_interface
...
########################################################################
[ccpp-table-properties]
  name = GFS_stateout_type
  type = ddt
  dependencies =

[ccpp-arg-table]
  name = GFS_stateout_type
  type = ddt
[gu0]
  standard_name = x_wind_updated_by_physics
...
########################################################################
[ccpp-table-properties]
  name = GFS_typedefs
  type = module
  relative_path = ../../ccpp/physics/physics
  dependencies = machine.F,physcons.F90,radlw_param.f,radsw_param.f
  dependencies = GFDL_parse_tracers.F90,rte-rrtmgp/rrtmgp/mo_gas_optics_rrtmgp.F90
  dependencies = rte-rrtmgp/rte/mo_optical_props.F90
  dependencies = rte-rrtmgp/extensions/cloud_optics/mo_cloud_optics.F90
  dependencies = rte-rrtmgp/rrtmgp/mo_gas_concentrations.F90
  dependencies = rte-rrtmgp/rte/mo_rte_config.F90
  dependencies = rte-rrtmgp/rte/mo_source_functions.F90

[ccpp-arg-table]
  name = GFS_typedefs
  type = module
[GFS_cldprop_type]
  standard_name = GFS_cldprop_type
  long_name = definition of type GFS_cldprop_type
  units = DDT
  dimensions = ()
  type = GFS_cldprop_type
...

Listing 2.2: Example of a CCPP-compliant metadata file showing the use of the [ccpp-table-properties] section and how it relates to [ccpp-arg-table].

An example metadata file for the CCPP scheme mp_thompson.meta is shown in Listing 2.3.

[ccpp-table-properties]
  name = mp_thompson
  type = scheme
  dependencies = machine.F,module_mp_radar.F90,module_mp_thompson.F90
  dependencies = module_mp_thompson_make_number_concentrations.F90

########################################################################
[ccpp-arg-table]
 name = mp_thompson_init
 type = scheme
...

########################################################################
[ccpp-arg-table]
  name = mp_thompson_run
  type = scheme
...

########################################################################
[ccpp-arg-table]
  name = mp_thompson_finalize
  type = scheme
...

Listing 2.3: Example metadata file for a CCPP-compliant physics scheme using a single [ccpp-table-properties] and how it defines dependencies for multiple [ccpp-arg-table] . In this example the timestep_init and timestep_finalize phases are not used.

2.2.2. ccpp-arg-table

For each CCPP compliant scheme, the ccpp-arg-table should start with this set of lines

[ccpp-arg-table]
 name = <name>
 type = <type>
  • ccpp-arg-table indicates the start of a new metadata section for a given scheme.

  • <name> is name of the corresponding subroutine/module.

  • <type> can be scheme, module, or DDT.

  • The metadata must describe all input and output arguments to the scheme using the following format:

[varname]
 standard_name = <standard_name>
 long_name = <long_name>
 units = <units>
 rank = <rank>
 dimensions = <dimensions>
 type = <type>
 kind = <kind>
 intent = <intent>
 optional = <optional>
  • The intent argument is only valid in scheme metadata tables, as it is not applicable to the other types.

  • The following attributes are optional: long_name, kind, and optional.

  • Lines can be combined using | as a separator, e.g.,

type = real | kind = kind_phys
  • [varname] is the local name of the variable in the subroutine.

  • The dimensions attribute should be empty parentheses for scalars or contain the standard_name for the start and end for each dimension of an array. ccpp_constant_one is the assumed start for any dimension which only has a single value. For example:

dimensions = ()
dimensions = (ccpp_constant_one:horizontal_loop_extent, vertical_level_dimension)
dimensions = (horizontal_dimension,vertical_dimension)
dimensions = (horizontal_dimension,vertical_dimension_of_ozone_forcing_data,number_of_coefficients_in_ozone_forcing_data)
  • The order of arguments in the entry point subroutines must match the order of entries in the metadata file.

  • Listing 2.4 contains the template for a CCPP-compliant scheme (ccpp/framework/doc/DevelopersGuide/scheme_template.meta),

[ccpp-table-properties]
  name = ozphys
  type = scheme
  dependencies = machine.F

[ccpp-arg-table]
  name = ozphys_run
  type = scheme
[errmsg]
  standard_name = ccpp_error_message
  long_name = error message for error handling in CCPP
  units = none
  dimensions = ()
  type = character
  kind = len=*
  intent = out
  optional = F
[errflg]
  standard_name = ccpp_error_flag
  long_name = error flag for error handling in CCPP
  units = flag
  dimensions = ()
  type = integer
  intent = out
  optional = F

Listing 2.4: Fortran template for a metadata file accompanying a CCPP-compliant scheme.

2.2.3. horizontal_dimension vs. horizontal_loop_extent

It is important to understand the difference between these metadata dimension names.

  • horizontal_dimension refers to all (horizontal) grid columns that an MPI process owns/is responsible for, and that are passed to the physics in the init, timestep_init, timestep_final, and final phases.

  • horizontal_loop_extent or, equivalent, ccpp_constant_one:horizontal_loop_extent stands for a subset of grid columns that are passed to the physics during the time integration, i.e. in the run phase.

  • Note that horizontal_loop_extent is identical to horizontal_dimension for host models that pass all columns to the physics during the time integration.

Since physics developers cannot know whether a host model is passing all columns to the physics during the time integration or just a subset of it, the following rules apply to all schemes:

  • Variables that depend on the horizontal decomposition must use horizontal_dimension in the metadata tables for the following phases: init, timestep_init, timestep_final, final.

  • Variables that depend on the horizontal decomposition must use horizontal_loop_extent or ccpp_constant_one:horizontal_loop_extent in the run phase.

2.3. Input/output Variable (argument) Rules

  • Variables available for CCPP physics schemes are identified by their unique standard_name. While an effort is made to comply with existing standard_name definitions of the Climate and Forecast (CF) conventions (http://cfconventions.org), additional names are used in the CCPP (see below for further information).

  • A list of available standard names and an example of naming conventions can be found in ccpp/framework/doc/DevelopersGuide/CCPP_VARIABLES_${HOST}.pdf, where ${HOST} is the name of the host model. Running the CCPP prebuild script (described in Chapter 8) will generate a LaTeX source file that can be compiled to produce a PDF file with all variables defined by the host model and requested by the physics schemes.

  • A standard_name cannot be assigned to more than one local variable (local_name). The local_name of a variable can be chosen freely and does not have to match the local_name in the host model.

  • All variable information (standard_name, units, dimensions) must match the specifications on the host model side, but sub-slices can be used/added in the host model. For example, when using the UFS Atmosphere as the host model, tendencies are split in GFS_typedefs.meta so they can be used in the necessary physics scheme:

[dt3dt(:,:,1)]
  standard_name = cumulative_change_in_temperature_due_to_longwave_radiation
  long_name = cumulative change in temperature due to longwave radiation
  units = K
  dimensions = (horizontal_dimension,vertical_dimension)
  type = real
  kind = kind_phys
[dt3dt(:,:,2)]
  standard_name = cumulative_change_in_temperature_due_to_shortwave_radiation
  long_name = cumulative change in temperature due to shortwave radiation
  units = K
  dimensions = (horizontal_dimension,vertical_dimension)
  type = real
  kind = kind_phys
[dt3dt(:,:,3)]
  standard_name = cumulative_change_in_temperature_due_to_PBL
  long_name = cumulative change in temperature due to PBL
  units = K
  dimensions = (horizontal_dimension,vertical_dimension)
  type = real
  kind = kind_phys
  • The two mandatory variables that any scheme-related subroutine must accept as intent(out) arguments are errmsg and errflg (see also coding rules in Section 2.4).

  • At present, only two types of variable definitions are supported by the CCPP-framework:

    • Standard Intrinsic Fortran variables are preferred (character, integer, logical, real). For character variables, the length should be specified as * in order to allow the host model to specify the corresponding variable with a length of its own choice. All others can have a kind attribute of a kind type defined by the host model.

    • Derived data types (DDTs). While the use of DDTs is discouraged, some use cases may justify their application (e.g. DDTs for chemistry that contain tracer arrays or information on whether tracers are advected). It should be understood that use of DDTs within schemes forces their use in host models and potentially limits a scheme’s portability. Where possible, DDTs should be broken into components that could be usable for another scheme of the same type.

  • It is preferable to have separate variables for physically-distinct quantities. For example, an array containing various cloud properties should be split into its individual physically-distinct components to facilitate generality. An exception to this rule is if there is a need to perform the same operation on an array of otherwise physically-distinct variables. For example, tracers that undergo vertical diffusion can be combined into one array where necessary. This tactic should be avoided wherever possible, and is not acceptable merely as a convenience.

  • If a scheme is to make use of CCPP’s subcycling capability, the current loop counter and the loop extent can be obtained from CCPP as intent(in) variables (see a mandatory list of variables that are provided by the CCPP-Framework and/or the host model for this and other purposes).

  • It is preferable to use assumed-size array declarations for input/output variables for CCPP schemes, i.e. instead of

    real(kind=kind_phys), dimension(is:ie,ks:ke), intent(inout) :: foo
    

    one should use

    real(kind=kind_phys), dimension(:,:), intent(inout) :: foo
    

    This allows the compiler to perform bounds checking and detect errors that otherwise may go unnoticed.

    Warning

    Fortran assumes that the lower bound of assumed-size arrays is 1. If foo has lower bounds is and ks that are different from 1, then these must be specified explicitly:

    real(kind=kind_phys), dimension(is:,ks:), intent(inout) :: foo
    

2.4. Coding Rules

  • Code must comply to modern Fortran standards (Fortran 90/95/2003).

  • Labeled end statements should be used for modules, subroutines and functions, for example, module scheme_template end module scheme_template.

  • Implicit variable declarations are not allowed. The implicit none statement is mandatory and is preferable at the module-level so that it applies to all the subroutines in the module.

  • All intent(out) variables must be set inside the subroutine, including the mandatory variables errflg and errmsg.

  • Decomposition-dependent host model data inside the module cannot be permanent, i.e. variables that contain domain-dependent data cannot be kept using the save attribute.

  • goto statements are not alowed.

  • common blocks are not allowed.

  • Errors are handled by the host model using the two mandatory arguments errmsg and errflg. In the event of an error, a meaningful error message should be assigned to errmsg and set errflg to a value other than 0, for example:

write (errmsg, ‘(*(a))) ‘Logic error in scheme xyz: …’
errflg = 1
return
  • Schemes are not allowed to abort/stop the program.

  • Schemes are not allowed to perform I/O operations except for reading lookup tables or other information needed to initialize the scheme, including stdout and stderr. Diagnostic messages are tolerated, but should be minimal.

  • Line lengths of no more than 120 characters are suggested for better readability.

Additional coding rules are listed under the Coding Standards section of the NOAA NGGPS Overarching System team document on Code, Data, and Documentation Management for NOAA Environmental Modeling System (NEMS) Modeling Applications and Suites (available at https://docs.google.com/document/u/1/d/1bjnyJpJ7T3XeW3zCnhRLTL5a3m4_3XIAUeThUPWD9Tg/edit#heading=h.97v79689onyd).

2.5. Using Constants

There are two principles that must be followed when using physical constants within CCPP-compliant physics schemes:

  1. All schemes should use a single, consistent set of constants.

  2. The host model must control (define and use) that single set, to provide consistency between a host model and the physics.

As long as a host application provides metadata describing its physical constants so that the CCPP framework can pass them to the physics schemes, these two principles are realized, and the CCPP physics schemes are model-agnostic. Since CCPP-compliant hosts provide metadata about the available physical constants, they can be passed into schemes like any other data.

For simple schemes that consist of one or two files and only a few “helper” subroutines, passing in physical constants via the argument list and propagating those constants down to any subroutines that need them is the most direct approach. The following example shows how the constant karman can be passed into a physics scheme:

subroutine my_physics_run(im,km,ux,vx,tx,karman)
  ...
real(kind=kind_phys),intent(in)   ::  karman

Where the following has been added to the my_physics.meta file:

[karman]
  standard_name = von_karman_constant
  long_name = von karman constant
  units = none
  dimensions = ()
  type = real
  intent = in
  optional = F

This allows the vonKarman constant to be defined by the host model and be passed in through the CCPP scheme subroutine interface.

For pre-existing complex schemes that contain many software layers and/or many “helper” subroutines that require physical constants, another method is accepted to ensure that the two principles are met while eliminating the need to modify many subroutine interfaces. This method passes the physical constants once through the argument list for the top-level _init subroutine for the scheme. This top-level _init subroutine also imports scheme-specific constants from a user-defined module. For example, constants can be set in a module as:

module my_scheme_common
  use machine,     only : kind_phys
  implicit none
  real(kind=kind_phys)   ::  pi, omega1, omega2
end module my_scheme_common

Within the _init subroutine body, the constants in the my_scheme_common module can be set with the ones that are passed in via the argument list, including any derived ones. For example:

module my_scheme
  use machine, only: kind_phys
  implicit none
  private
  public my_scheme_init, my_scheme_run, my_scheme_finalize
  logical :: is_initialized = .false.
contains
  subroutine my_scheme_init (a, b, con_pi, con_omega)
    use my_scheme_common, only: pi, omega1, omega2
      ...
    pi = con_pi
    omega1 = con_omega
    omega2 = 2.*omega1
      ...
    is_initialized = .true.
  end subroutine my_scheme_init

  subroutine my_scheme_run (a, b)
    use my_scheme_common, only: pi, omega1, omega2
      ...
  end subroutine my_scheme_run

  subroutine my_scheme_finalize
      ...
    is_initialized = .false.
    pi = -999.
    omega1 = -999.
    omega2 = -999.
      ...
  end subroutine my_scheme_finalize
end module my_scheme

After this point, physical constants can be imported from my_scheme_common wherever they are needed. Although there may be some duplication in memory, constants within the scheme will be guaranteed to be consistent with the rest of physics and will only be set/derived once during the initialization phase. Of course, this will require that any constants in my_scheme_common that are coming from the host model cannot use the Fortran parameter keyword. To guard against inadvertently using constants in my_scheme_common without setting them from the host, they should be initially set to some invalid value. To ensure that this only happens once, the is_initialized flag should be set to true after setting/deriving constants. To clean up during the finalize phase of the scheme, the is_initialized flag can be set back to false and the constants can be set back to an invalid value.

In summary, there are two ways to pass constants to a physics scheme. The first is to directly pass constants via the subroutine interface and continue passing them down to all subroutines as needed. The second is to have a user-specified scheme constants module within the scheme and to sync it once with the physical constants from the host model at initialization time. The approach to use is somewhat up to the developer. It is not recommended to use the physcons module, since it is specific to FV3.

2.6. Parallel Programming Rules

Most often shared memory (OpenMP: Open Multi-Processing) and MPI (Message Passing Interface) communication are done outside the physics in which case the physics looping and arrays already take into account the sizes of the threaded tasks through their input indices and array dimensions. The following rules should be observed when including OpenMP or MPI communication in a physics scheme:

  • Shared-memory (OpenMP) parallelization inside a scheme is allowed with the restriction that the number of OpenMP threads to use is obtained from the host model as an intent(in) argument in the argument list (Listing 6.2).

  • MPI communication is allowed in the _timestep_init, _init, _finalize, and _timestep_finalize phases for the purpose of computing, reading or writing scheme-specific data that is independent of the host model’s data decomposition.

  • If MPI is used, it is restricted to global communications: barrier, broadcast, gather, scatter, reduction.

  • An example of a valid use of MPI is the initial read of a lookup table of aerosol properties by one or more MPI processes, and its subsequent broadcast to all processes. Several restrictions apply:

    • The implementation of reading and writing of data must be scalable to perform efficiently from a few to millions of tasks.

    • The MPI communicator must be provided by the host model as an intent(in) argument in the argument list (see list of mandatory variables).

    • Point-to-point communication is not allowed.

    • The use of MPI_COMM_WORLD is not allowed.

  • Calls to MPI and OpenMP functions, and the import of the MPI and OpenMP libraries, must be guarded by C preprocessor directives as illustrated in the following listing. OpenMP pragmas can be inserted without C preprocessor guards, since they are ignored by the compiler if the OpenMP compiler flag is omitted.

#ifdef MPI
  use mpi
#endif
#ifdef OPENMP
  use omp_lib
#endif
...
#ifdef MPI
  call MPI_BARRIER(mpicomm, ierr)
#endif

#ifdef OPENMP
  me = OMP_GET_THREAD_NUM()
#else
  me = 0
#endif

2.7. Scientific Documentation Rules

Technically, scientific documentation is not needed for a parameterization to work with the CCPP. However, scientific and technical documents are important for code maintenance and for fostering understanding among stakeholders. As such, it is required of physics schemes in order to be included in the CCPP. This section describes the process used for documenting parameterizations in the CCPP. Doxygen was chosen as a tool for generating human-readable output due to its built-in functionality with Fortran, its high level of configurability, and its ability to parse inline comments within the source code. Keeping documentation with the source itself increases the likelihood that the documentation will be updated along with the underlying code. Additionally, inline documentation is amenable to version control.

The purpose of this section is to provide an understanding of how to properly document a physics scheme using doxygen inline comments in the Fortran code and metadata information contained in the .meta files. It covers what kind of information should be in the documentation, how to mark up the inline comments so that doxygen will parse them correctly, where to put various comments within the code, how to include information from the .meta files, and how to configure and run doxygen to generate HTML output. For an example of the HTML rendering of the CCPP Scientific Documentation, see https://dtcenter.org/GMTB/v5.0.0/sci_doc. Part of this documentation, namely metadata about subroutine arguments, has functional significance as part of the CCPP infrastructure. The metadata must be in a particular format to be parsed by Python scripts that “automatically” generate a software cap for a given physics scheme. Although the procedure outlined herein is not unique, following it will provide a level of continuity with previous documented schemes.

Reviewing the documentation for CCPP parameterizations is a good way of getting started in writing documentation for a new scheme.

2.7.1. Doxygen Comments and Commands

All doxygen commands start with a backslash (“\”) or an at-sign (“@”). The doxygen inline comment blocks begin with “!>”, and subsequent lines begin with “!!”, which means that regular Fortran comments using “!” are not parsed by doxygen.

In the first line of each Fortran file, a brief one-sentence overview of the file purpose is present using the doxygen command “\\file”:

! !>  \file cires_ugwp.F90
!! This file contains the Unified Gravity Wave Physics (UGWP) scheme by Valery Yudin (University of Colorado, CIRES)
A parameter definition begins with “!<”, where the sign ‘<’ just tells

Doxygen that documentation follows. Example:

integer, parameter, public :: NF_VGAS = 10   !< number of gas species
integer, parameter         :: IMXCO2  = 24   !< input CO2 data longitude points
integer, parameter         :: JMXCO2  = 12   !< input CO2 data latitude points
integer, parameter         :: MINYEAR = 1957 !< earlist year 2D CO2 data available

2.7.2. Doxygen Documentation Style

To document a physics suite, a broad array of information should be included in order to serve both software engineering and scientific purposes. The documentation style could be divided into four categories:

  • Doxygen Files

  • Doxygen Pages (overview page and scheme pages)

  • Doxygen Modules

  • Bibliography

2.7.2.1. Doxygen files

Doxygen provides the “\\file” tag as a way to provide documentation on the Fortran source code file level. That is, in the generated documentation, one may navigate by source code filenames (if desired) rather than through a “functional” navigation. The most important documentation organization is through the “module” concept mentioned below, because the division of a scheme into multiple source files is often functionally irrelevant. Nevertheless, using a “\\file” tag provides an alternate path to navigate the documentation and it should be included in every source file. Therefore, it is prudent to include a small documentation block to describe what code is in each file using the “\\file” tag, e.g.:

!>\file cu_gf_driver.F90
!! This file is scale-aware Grell-Freitas cumulus scheme driver.

The brief description for each file is displayed next to the source filename on the doxygen-generated “File List” page:

_images/DoxygenFileList.png

2.7.2.2. Doxygen Overview Page

Pages in Doxygen can be used for documentation that is not directly attached to a source code entity such as file or module. They are external text files that generate pages with a high-level scientific overview and typically contain a longer description of a project or suite. You can refer to any source code entity from within the page.

The DTC maintains a main page, created by the Doxygen command “\\mainpage”, containing an overall description and background of the CCPP. Physics developers do not have to edit the file with the mainpage, which has a user-visible title, but not label:

/**
\mainpage Introduction
...
*/

All other pages listed under the main page are created using the Doxygen tag “\\page” described in the next section. In any Doxygen page, you can refer to any entity of source code by using Doxygen tag “\\ref” or “@ref”. Example in suite_FV3_GFS_v15p2.xml.txt:

The GFS v15p2 physics suite uses the parameterizations in the following order, as defined in

\c FV3_GFS_v15p2 :
 - \ref fast_sat_adj
 - \ref GFS_RRTMG
 - \ref GFS_SFCLYR
 - \ref GFS_NSST
 - \ref GFS_NOAH
 - \ref GFS_SFCSICE
 - \ref GFS_HEDMF
 - \ref cires_ugwp
 - \ref GFS_RAYLEIGH
 - \ref GFS_OZPHYS
 - \ref GFS_H2OPHYS
 - \ref GFS_SAMFdeep
 - \ref GFS_SAMFshal
 - \ref GFDL_cloud
 - \ref GFS_CALPRECIPTYPE
 - \ref STOCHY_PHYS

The HTML result is here. You can see that the “-” signs before “@ref” generate a list with bullets. Doxygen command “\\c” displays its argument using a typewriter font.

2.7.2.3. Physics Scheme Pages

Each major scheme in CCPP should have its own scheme page containing an overview of the parameterization. These pages are not tied to the Fortran code directly; instead, they are created with a separate text file that starts with the command “\\page”. Scheme pages are stored in the ccpp-physics/physics/docs/pdftxt directory. Each page has a label (e.g., “GFS_SAMFdeep” in the following example) and a user-visible title (“GFS Scale-Aware Simplified Arakawa-Schubert (sa-SAS) Deep Convection Scheme” in the following example). It is noted that labels must be unique across the entire doxygen project so that the “\\ref” command can be used to create an unambiguous link to the structuring element. It therefore makes sense to choose label names that refer to their context.

/**
\page GFS_SAMFdeep GFS Scale-Aware Simplified Arakawa-Schubert (sa-SAS) Deep Convection Scheme
\section des_deep Description
 The scale-aware mass-flux (SAMF) deep convection scheme is an
 updated version of the previous Simplified Arakawa-Schubert (SAS) scheme
 with scale and aerosol awareness and parameterizes the effect of deep
 convection on the environment (represented by the model state variables)
 in the following way …

\section intra_deep  Intraphysics Communication
\ref arg_table_samfdeepcnv_run

\section gen_al_deep General Algorithm
\ref general_samfdeep

*/

The physics scheme page will often describe the following:

  1. Description section (“\\section”), which usually includes:

    • Scientific origin and scheme history (“\\cite”)

    • Key features and differentiating points

    • A picture is worth a thousand words (“\\image”)

      To insert images into doxygen documentation, you’ll need to have your images ready in a graphical format, such as Portable Network Graphic (png), depending on which type of doxygen output you are planning to generate. For example, for LaTeX output, the images must be provided in Encapsulated PostScript (.eps), while for HTML output the images can be provided in the png format. Images are stored in ccpp-physics/physics/docs/img directory. Example of including an image for HTML output:

\image  html  gfdl_cloud_mp_diagram.png "Figure 1: GFDL MP at a glance (Courtesy of S.J. Lin at GFDL)" width=10cm
  1. Intraphysics Communication Section (“\\section”)

    The argument table for CCPP entry point subroutine {scheme}_run will be in this section. It is created by inserting a reference link (“\\ref”) to the table in the Fortran code for the scheme.

  2. General Algorithm Section (“\\section”)

    The general description of the algorithn will be in this section. It is created by inserting a reference link (“\\ref”) in the Fortran code for the scheme.

The symbols “/\*\*” and “*/” need to be the first and last entries of the page. See an example of GFS Scale-Aware Simplified Arakawa-Schubert (sa-SAS) Deep Convection Scheme page in the previous page.

Note that separate pages can also be created to document something that is not a scheme. For example, a page could be created to describe a suite, or how a set of schemes work together. Doxygen automatically generates an index of all pages that is visible at the top-level of the documentation, thus allowing the user to quickly find, and navigate between, the available pages.

2.7.2.4. Doxygen Modules

The CCPP documentation is based on doxygen modules (note this is not the same as Fortran modules). Each doxygen module pertains to a particular parameterization and is used to aggregate all code related to that scheme, even when it is in separate files. Since doxygen cannot know which files or subroutines belong to each physics scheme, each relevant subroutine must be tagged with the module name. This allows doxygen to understand your modularized design and generate the documentation accordingly. Here is a list of module list defined in CCPP.

A module is defined using:

!>\defgroup group_name group_title

Where group_name is the identifier and the group_title is what the group is referred to in the output. In the example below, we’re defining a parent module “GFS radsw Main”:

!> \defgroup module_radsw_main GFS radsw Main
!! This module includes NCEP's modifications of the RRTMG-SW radiation
!! code from AER.
!! ...
!!\author   Eli J. Mlawer, emlawer@aer.com
!!\author   Jennifer S. Delamere, jdelamer@aer.com
!!\author   Michael J. Iacono, miacono@aer.com
!!\author   Shepard A. Clough
!!\version NCEP SW v5.1  Nov 2012 -RRTMG-SW v3.8
!!

One or more contact persons should be listed with author. If you make significant modifications or additions to a file, consider adding an author and a version line for yourself. The above example generates the Author, Version sections on the page. All email addresses are converted to mailto hypertext links automatically:

Author

Eli J. Mlawer, emlawer@aer.com

Jennifer S. Delamere, jdelamer@aer.com

Michael J. Iacono, miacono@aer.com

Shepard A. Clough

Version

NCEP SW v5.1 Nov 2012 -RRTMG-SW v3.8

In order to include other pieces of code in the same module, the following tag must be used at the beginning of a comment block:

\ingroup group_name

For example:

!>\ingroup module_radsw_main
!> The subroutine computes the optical depth in band 16:  2600-3250
!! cm-1 (low - h2o,ch4; high - ch4)
!-----------------------------------
      subroutine taumol16
!...................................

In the same comment block where a group is defined for a physics scheme, there should be some additional documentation. First, using the “\\brief” command, a brief one or two sentence description of the scheme should be included. After a blank doxygen comment line, begin the scheme origin and history using “\\version”, “\\author” and “\\date”.

Each subroutine that is a CCPP entry point to a parameterization, should be further documented with a documentation block immediately preceding its definition in the source. The documentation block should include at least the following components:

  • A brief one- or two-sentence description with the "\\brief" tag

  • A more detailed one or two paragraph description of the function of the subroutine

  • A comment indicating that metadata information about the subroutine arguments follows (in this example, the subroutine is called SUBROUTINE_NAME. Note that this line is also functional documentation used during the CCPP prebuild step.

!! \section arg_table_SUBROUTINE_NAME Argument Table
  • A second comment indicating that a table of metadata to describe the subroutine arguments will be included from a separate file in HTML format (in this case, file SUBROUTINE_NAME.html). Please refer to the section below for information on how to generate the HTML files with metadata information from the .meta files.

    The argument table should be immediately followed by a blank doxygen line “!!”.

!! \htmlinclude SUBROUTINE_NAME.html
!!
  • A section called “General Algorithm” with a bullet or numbered list of the tasks completed in the subroutine algorithm

  • At the end of initial subroutine documentation block, a “Detailed algorithm” section is started and the entirety of the code is encompassed with the “!> @{” and “!> @}” delimiters. This way, any comments explaining detailed aspects of the code are automatically included in the “Detailed Algorithm” section.

For subroutines that are not a CCPP entry point to a scheme, no inclusion of metadata information is required. But it is suggested that following “\\ingroup” and “\\brief”, use “\\param” to define each argument with local name, a short description and unit, i.e.,

!>  \ingroup HEDMF
!!  \brief This subroutine is used for calculating the mass flux and updraft properties.
!!  ...
!!
!!  \param[in] im      integer, number of used points
!!  \param[in] ix      integer, horizontal dimension
!!  \param[in] km      integer, vertical layer dimension
!!  \param[in] ntrac   integer, number of tracers
!!  \param[in] delt    real, physics time step
!!  ...
!!  \section general_mfpbl mfpbl General Algorithm
!!  -# Determine an updraft parcel's entrainment rate, buoyancy, and vertical velocity.
!!  -# Recalculate the PBL height ...
!!  -# Calculate the mass flux profile and updraft properties.
!!  \section detailed_mfpbl mfpbl Detailed Algorithm
!>  @{
       subroutine mfpbl(im,ix,km,ntrac,delt,cnvflg,                       &
       &   zl,zm,thvx,q1,t1,u1,v1,hpbl,kpbl,                              &
       &   sflx,ustar,wstar,xmf,tcko,qcko,ucko,vcko)

       end subroutine mfpbl
!>  @}

2.7.2.5. Bibliography

Doxygen can handle in-line paper citations and link to an automatically created bibliography page. The bibliographic data for any papers that are cited need to be put in BibTeX format and saved in a .bib file. The bib file for CCPP is included in the repository, and the doxygen configuration option cite_bib_files points to the included file.

Citations are invoked with the following tag:

\cite bibtex_key_to_paper

2.7.2.6. Equations

See link for information about including equations. For the best rendering, the following option should be set in the Doxygen configuration file:

USE_MATHJAX            = YES
MATHJAX_RELPATH        =  https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2

There are many great online resources to use the LaTeX math typesetting used in doxygen.

2.7.3. Doxygen Configuration

2.7.3.1. Configuration File

The CCPP is distributed with a doxygen configuration file ./ccpp/physics/physics/docs/ccpp_doxyfile, such that you don’t need to create an additional one.

If starting from scratch, you can generate a default configuration file using the command:

doxygen -g <config_file>

Then you can edit the default configuration file to serve your needs. The default file includes plenty of comments to explain all the options. Some of the important things you need to pay attention to are:

  • The name of your project:

PROJECT_NAME = ‘your project name’
  • The input files (relative to the directory where you run doxygen):

INPUT =

The following lines should be listed here: the doxygen mainpage text file, the scheme pages, and the source codes to be contained in the output. The order in which schemes are listed determines the order in the HTML result.

  • The directory where to put the documentation (if you leave it empty, then the documentation will be created in the directory where you run doxygen):

OUTPUT_DIRECTORY = doc
  • The type of documentation you want to generate (HTML, LaTeX and/or something else):

GENERATE_HTML = YES

If HTML is chosen, the following tells doxygen where to put the documentation relative to the OUTPUT_DIRECTORY:

HTML_OUTPUT = html

and

HTML_FILE_EXTENSION = .html

determines the extension of the files.

  • Other important settings for a Fortran code project are:

OPTIMIZE_FOR_FORTRAN        =    YES
EXTENSION_MAPPING           = .f=FortranFree        \
                              .F90=FortranFree      \
                              .f90=FortranFree
LAYOUT_FILE                 = ccpp_dox_layout.xml
CITE_BIB_FILES              = library.bib
FILE_PATTERN                = *.f     \
                              *.F90   \
                              *.f90   \
                              *.txt
GENERATE_TREEVIEW           = yes

Doxygen files for layout (ccpp_dox_layout.xml), a HTML style (ccpp_dox_extra_style.css), and bibliography (library.bib) are provided with the CCPP. Additionally, a configuration file is supplied, with the following variables modified from the default:

2.7.3.2. Diagrams

On its own, Doxygen is capable of creating simple text-based class diagrams. With the help of the additional software GraphViz, Doxygen can generate additional graphics-based diagrams, optionally in Unified Modeling Language (UML) style. To enable GraphViz support, the configure file parameter “HAVE_DOT” must be set to “YES”.

You can use doxygen to create call graphs of all the physics schemes in CCPP. In order to create the call graphs you will need to set the following options in your doxygen config file:

HAVE_DOT             = YES
EXTRACT_ALL          = YES
EXTRACT_PRIVATE      = YES
EXTRACT_STATIC       = YES
CALL_GRAPH           = YES

Note that will need the DOT (graph description language) utility to be installed when starting doxygen. Doxygen will call it to generate the graphs. On most distributions the DOT utility can be found in the GraphViz package. Here is the call graph for subroutine mpdrv in GFDL cloud microphysics generated by doxygen:

_images/DoxygenCallGraph.png

2.7.4. Including metadata information

As described above, a table of metadata information should be included in the documentation for every CCPP entrypoint scheme. Before doxygen is run, the table for each scheme must be manually created in separate files in HTML format, with one file per scheme. The HTML files are included in the Fortran files using the doygen markup below.

!! \htmlinclude SUBROUTINE_NAME.html
!!

The tables should be created using a Python script distrbuted with the CCPP Framework, ccpp/framework/scripts/metadata2html.py. The syntax for running this script from the directory above where the CCPP is installed is:

./ccpp/framework/scripts/metadata2html.py -m ccpp/physics/physics/file.meta -o ccpp/physics/physics/docs

where -m is used to specify a file with metadata information and -o is used to specify the directory for output. Note that a single input file (.meta) may have more than one CCPP entrypoint scheme, and therefore can be used to generate more than one HTML file.

Note that the .meta files are supplied with the CCPP Physics, and that there is a .meta file for each Fortran file that contains one or more CCPP entrypoint schemes. The .meta files are located in the same directory as the scheme Fortran files (ccpp/physics/physics).

To generate a complete Scientific Documentation, documentation, script ./ccpp/framework/scripts/metadata2html.py must be run separately for each .meta file available in ccpp/physics/physics. Alternatively, a batch mode exists that converts all metadata files associated with schemes and variable definitions in the CCPP prebuild config:

./ccpp/framework/scripts/metadata2html.py -c ccpp/config/ccpp_prebuild_config.py

Note that the options -c and -m are mutually exclusive, but that one of them is required. Option -m also requires to specify -o, while option -c will ignore -o. For more information, use

./ccpp/framework/scripts/metadata2html.py --help

2.7.5. Using Doxygen

In order to generate the doxygen-based documentation, one needs to follow five steps:

  1. Have the doxygen executable installed on your computer. For the NOAA machine Hera and the NCAR machine Cheyenne, the doxygen executable resides in /usr/bin, which should be in your $PATH. If you need to install doxygen in another location, add the following line into the .cshrc file in your home directory:

    alias doxygen /path/to/doxygen

    Source your .cshrc file.

  2. Document your code, including doxygen main page, scheme pages and inline comments within source code as described above.

  3. Run metadata2html.py to create files in HTML format containing metadata information for each CCPP entrypoint scheme.

  4. Prepare a Bibliography file in BibTex format for papers referred to in the physics suites.

  5. Create or edit a doxygen configuration file to control what doxygen pages, source files and bibliography file get parsed, how the source files get parsed, and to customize the output.

  6. Run doxygen from directory ccpp/physics/physics/docs using the command line to specify the doxygen configuration file as an argument:

$doxygen $PATH_TO_CONFIG_FILE/<config_file>

Running this command may generate warnings or errors that need to be fixed in order to produce proper output. The location and type of output (HTML, LaTeX, etc.) are specified in the configuration file. The generated HTML documentation can be viewed by pointing an HTML browser to the index.html file in the ./docs/doc/html/ directory.

For precise instructions on creating the scientific documentation, contact the CCPP Forum at https://dtcenter.org/forum/ccpp-user-support.