Unrealistically high relative humidity (RELHUM) in the upper atmosphere in WACCM4 simulations

[deepashree_dutta@student_unsw_edu_au]

Hi all,

I'm running PI simulations using WACCM with CAM4 (cesm1.2.2).
I have also run some methane sensitivity experiments.

I was looking at the seasonal mean T and relative humidity (RELHUM variable) in the upper atmosphere (zonal mean, lat vs pr plots) for the PI simulation.
In the model levels between 10-16 (lev[10:16] = [0.000885, 0.001458, 0.002405, 0.003978, 0.006557, 0.010814]) zonal mean RELHUM in JJA and DJF is more than 1000% which does not make sense. (T at these levels < -100C).
(I checked the daily outputs at different level thinking the problem might be because of averaging such wildly varying states at these levels. But I still see unrealistically high RELHUM values at these levels on global maps)

I don't understand what is causing this issue.
Does the model microphysics not condense any water vapour at these very low densities?
Am I missing something here?

If you have any ideas please let me know,
Thanks a lot,
Deepashree

 

[mmills]

It sounds like you are looking at the summer mesopause. This is the coldest part of the atmosphere, and is the region where polar mesospheric clouds form. WACCM is not designed to deal with PMC ice microphysics. I don't think that the cloud microphysics operates at all at such altitudes.

 

[deepashree_dutta@student_unsw_edu_au]

Hi Mike,

Thank you so much for your response!
You are right, in the PI simulation these weird RH occurs near polar regions in summer, and I see formation of mesospheric clouds.
I found this interesting because in methane sensitivity experiments these regions shift towards tropics. If possible I would like to investigate this further.

I saw 1/2 articles that use WACCM/CARMA model to study mesospheric clouds (https://acd-ext.gsfc.nasa.gov/People/Jackman/Bardeen_2016.pdf). Does CARMA take care of the ice microphysics at such altitudes?

I have never used CARMA before and don't know how to use it. I couldn't find any information on how to enable CARMA with CAM4 and WACCM (in CESM 1.2.2). So, I tried to create a preindustrial case with CARMA microphysics using :
-user_compset 1850_CAM4%WCSF_CLM40%SP_CICE%PRES_DOCN%DOM_RTM_SGLC_SWAV
While it created the case successfully, I had this error message in ./cesm_setup

"Running preview_namelist script
infile is /g/data/y99/dd7103/CESM/case/CARMA_sept15_test/Buildconf/cplconf/cesm_namelist
CAM build-namelist - ERROR: No default value found for ncdata
user defined attributes:
key=ic_md val=00010101
Died at /home/561/dd7103/CESM/cesm1_2_2/models/atm/cam/bld/build-namelist line 3384.
ERROR: cam.buildnml.csh failed
ERROR: /g/data/y99/dd7103/CESM/case/CARMA_sept15_test/preview_namelists failed: 25344"

Do you think CARMA (with CAM4 and WACCM) would provide more realistic results in mesosphere?
If it is so, can you please tell me how to set up a preindustrial case with CARMA (or point to a user guide)?

Thanks again,
Deepashree

 

[mmills]

The biggest issue is that WACCM is tuned for realistic stratospheric temperatures, which leads to unrealistic temperatures at the mesopause, which is crucial for PMC formation. There is a PMC tuned version of WACCM4, which is described in:

Merkel, A. W., Marsh, D. R., Gettelman, A., & Jensen, E. J. (2009). On the relationship of polar mesospheric cloud ice water content, particle radius and mesospheric temperature and its use in multi-dimensional models. Atmospheric Chemistry and Physics, 9, 8889.

You might want to contact Aimee Merkel or Dan Marsh for assistance.

 

[deepashree_dutta@student_unsw_edu_au]

The biggest issue is that WACCM is tuned for realistic stratospheric temperatures, which leads to unrealistic temperatures at the mesopause, which is crucial for PMC formation. There is a PMC tuned version of WACCM4, which is described in:

Merkel, A. W., Marsh, D. R., Gettelman, A., & Jensen, E. J. (2009). On the relationship of polar mesospheric cloud ice water content, particle radius and mesospheric temperature and its use in multi-dimensional models. Atmospheric Chemistry and Physics, 9, 8889.

You might want to contact Aimee Merkel or Dan Marsh for assistance.

Thank you so much Mike!