Orbital parameters in difference parts are inconsistent

[luoyangcheng96@pku_edu_cn]

Hi all,I am using CESM1.2.2 (F_1850_WACCM) to simulate planetary atmospheres. I want to change the orbital parameters, setting both eccentricity and obliquity to zero. Following the instructions I added four lines to user_nl_cpl before doing ./preview_namelists:  orb_mode = 'fixed_parameters' orb_eccen = 0. orb_mvelp = 0. orb_obliq = 0. After that I checked the output variable "SOLIN" and found the solar irradiance is well centered at the equator all the time. But then I checked the oxygen photolysis coefficient, and surprisingly I found a seasonal change. The maximum occurred in the northern hemisphere around June, and occurred in the southern hemisphere around December. This is something unexpected because I thought the different parts in the model (radiative transfer, photochemistry and so on) should have followed the exactly same orbital settings if I added the four lines to the coupler's namelist. Could anyone provide an explanation for this, or point out the mistakes in my methods? Thanks in advance. Yangcheng

 

[mmills]

I am not familiar with the process of modifying orbital parameters, but I did have a look in the CAM source code. It does look like the orb_obliq parameter should be picked up from the coupler and renamed "obliqr" in CAM. This is then used in the radiation modules. You might try printing the value of obliqr from inside atm/cam/src/physics/cam/radiation.F90 to see if it is getting there.You might also try posting this in a CAM forum if you don't get a better answer here.

 

[luoyangcheng96@pku_edu_cn]

Hi Mike,Thanks for your help. I printed out the "obliqr" in radiation.F90 and found indeed the modifications to the orbital settings in the namelist took effect - obliqr turned out to be zero. But I still don't understand what might have caused the seasonal change in photolysis coefficients. I thought perhaps it's the prescribed upper boundary conditions that have inherent seasonal cycles, so I modified the "tgcm_ubc_file" in the namelist, removing the seasonal cycle of CO, CO2, and CH4 distributions. However, this didn't make any differences. Do you have any further ideas about this?Thanks in advance.Yangcheng

 

[mmills]

I have sent your post out to others who know the photolysis code better than I do. Many people are not in the office this week.Photolysis rates are calculated in the chemistry code, which you can find under cam/src/chemistry/mozartSpecifically, they are calculated in the routine mo_photo.F90. The routine uses a look-up table, which is a function of solar zenith angle. I would think that SZA would respond appropriately to changes in obliquity, but I don't know. You might want to either look through the code there or put some print statements in various places to find out what is going on with SZA.