Experience Modifying CDNC in CAM6?

[AidanCraw]

Hi all,

I'm looking to run CAM6 (as part of a run on CESM2.1) and am looking to change cloud droplet number concentration geographically by latitude / longitude at specified atmospheric pressure levels (below 850 hPa). (I am trying to run a series of experiments to test ocean surface response to increased cloud cover.) Has anyone had any success manipulating CDNC in this build of CAM?

Thanks very much!

Running: cesm2.1.3-rc.01-0-g0596a97

 

[brianpm]

CAM6 predicts CDNC in almost all supported cases. The aquaplanet configurations default to a specified drop number concentration and crystal number concentration. To do that, we have some namelist flags that can be used:

micro_mg_nccons = .true.
micro_mg_nicons = .true.

That is only the very beginning of answering your question, though. These set constant values globally and uniformly in height. If you want to use specified drop number and control it in lat/lon and lev, you could modify micro_mg_cam.F90. Exactly how you do that will depend a lot on how you want to design your experiment.

There are other ways to change CDNC regionally. For example, you can specify aerosol emissions changes in a region. This is a less direct method, but wouldn't require code modifications.

 

[AidanCraw]

Hi Brian,

Thanks for the response! I've read the code for micro_mg_cam, and it looks to me like the best approach (assuming I want to directly modify CDNC and not go the emission-route) would be to modify ncic(pcols,pver) in micro_mg1_0.F90.

My only worries are that my ncic changes in micro_mg1_0.F90 will be overridden somewhere else and that I am not 100% confident of the units on ncic (although I assume they are the same as the constant CNDC variable ncnst at m^-3

 

[brianpm]

I'm not sure about the units. I'm fairly confident that ncic won't get changed outside of the microphysics (not 100% sure). You should make sure that you modify the correct MG code. The usual CAM6 configurations use MG2, which is in micro_mg2_0.F90.