Source Modification Example

Source Modification Example#

A common source code modification that you may want to do is to output a new variable that is not defined in CESM.

As an example, we will add a new variable to CAM: the atmopsheric temperature at 750hPa.

Adding a variable to CAM#

CAM has a history field that corresponds to the temperature at 500hPa and a number of other pressure levels, but not at 750hPa. Suppose you wanted to output the temperature at 750hPa. The following two calls are required to add an output variable:

call addfld(’T750’,...) (Add a field to the master field list)
call outfld(’T750’,...) (Collect values for this field and write to history file)

Each of these are now described in more detail:

addfld#

The sub-routine addfld adds a field to the master list with the following syntax:

addfld(fname,type,avgflag,units,long name)

where

  • fname = field name

  • type = the type of field. The entry for a single level field would be “horiz only” and the entry for a 3D field would be “(/ ’lev’ /)”.

  • avgflag = Averaging flag, A = average, I=instantaneous

  • units = the units of the field

  • long name = Field full name

Appropriate values of these parameters for the output of T750 are:

call addfld(’T750’,horiz only, ’A’, ’K’, ’Temperature at 750hPa pressure surface’)

outfld#

The subroutine outfld accumulates (or takes the minimum or maximum of, as appropriate) the field into the history buffer for the appropriate history tape with the following syntax

    outfld(fname, field, idim, c)

with

  • fname = Field name

  • field = array containing field values

  • idim = longitude dimension of field array

  • c = chunk (physics) or latitude (dynamics) index.

For example:

    call outfld(’T750’,t750, pcols, lchnk)

add_default#

Another useful subroutine is the subroutine add_default. The subroutine add_default Add a field to the list of default fields on history file with the following syntax

    subroutine add_default (fname, tindex, flag)

where

  • fname = Field name

  • tindex = history tape index

  • flag = Averaging flag: A = average (default). I = instantaneous

For example:

    call add_default ('T500', 1, ' ')
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