What is the role of the tropopause_climo_file in CAM4?

[gus@ldeo_columbia_edu]

I am intrigued by this new tropopause_climo_file.

It was absent in CAM3 and versions prior to CAM4.
It contains a climatology of the tropopause pressure, correct?
It also seems to be mandatory, and the code will fail if it is not specified, right?

The source file tropopause.F90 suggests that the file is only
a diagnostic feature, to support one of the 7 algorithms used
to locate the tropopause, and therefore allow the model to
output the tropopause pressure, temeprature, and geopotential height fields.

Questions:

1) Is this the only role of the tropopause_climo_file and its data?

2) Couldn't the tropopause location be determined based on the
model variables alone, with no extra help from a climatology?

3) Does this climatology file/data affect in any way the values of the model variables?
(and perhaps condition the resulting climate)

4) In other words: What if I change this file/data, say,
to a zonally simmetric version, for instance?
Will the model output still be the same as when
I use the original tropopause_climo_file that comes in the CAM4 distribution?

5) Is this tropopause_climo_file somehow another boundary condition data set
(besides the sst/sea ice)?

Thank you,
Gus Correa

 

[bardeenc]

Hi Gus,

1) Yes, the tropopause_climo_file is a new required file that is used by the diagnostics and contains the climatological average of the tropopause pressure. However, the diagnosed tropopause is also used by the chemistry code and the volcanic aerosol code to locate the tropopause, so the file isn't just for diagnostic purposes.

2) There are times when the diagnostic routines based upon the model state are unable to identify the tropopause. The doesn't happen often (< 1% of the time), but when it does happen the climatological tropopause to used. Backing the tropopause diagnostic with the climatology means that the chemistry and volcanic aerosol code will always have a tropopause location.

3) The definition of the climatological tropopause can change the model state, but it should be a very minor impact since most of the time the tropopause is diagnosed and the climatology is not used.

4) If you change the tropopause climatology file, I would expect you to get a numerically different result, but I wouldn't expect those differences to be significant.

5) Since this file is only used when the state based tropopause diagnostic fails, it is not a boundary condition in the sense of sst and sea ice.

I hope that helps,

Chuck Bardeen

 

[gus@ldeo_columbia_edu]

Dear Chuck Bardeen
Thank you very much for your thorough and very helpful answers.
Gus Correa