Question about the extra model top layer for radiation

[zshen]

Hi,

I am looking at the radiation calculation in CAM, and notice this in the CAM3 documentation:

The CAM 3.0 model atmosphere is divided into K + 1 layers in the vertical; an extra top layer (with index 0, above the K layers specified by CAM 3.0) is added. This extra layer prevents excessive heating in the top layer when the top pressure is not very low; also, as the model does not specify absorber properties above its top layer, the optical properties of the top layer must be used for the extra layer.

I wonder if this is still true in CAM5 or CAM6, and if there is any more detailed documentation on how the properties of this layer are chosen? Thanks!

 

[brianpm]

It is the case that there is an "extra layer" added in the radiation calculation. That's why, for example, FSNT and FSNTOA are not identical in the output files. I'm not aware of any documentation of how that extra layer is treated. I think there's an assumption about the composition and temperature, but I'm not super sure. If you look at rrtmg_state.F90 it seems like it is using the top interface temperature and just putting it as the temperature of the upper layer and similarly for the gas concentrations. Maybe someone else can chime in if this is wrong.

 

[zshen]

Thanks for the information! I will try to look at the code.