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High Model Temperature Bias

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twglotfe@...
High Model Temperature Bias

Hello,

 

I'm currently running a version of CESM 1.2.2 using the fully coupled B_2000_STRATMAM7_CN componet set configureation. My research group has also added several new aerosol and cloud interaction treatments to the model but we have noticed that our global mean temperature bias compared to NCDC observation is around -2.9 deg C. This is bias is greater than what is typically accepted for global climate models and I was wondering if there are any parameters or namelist options that can be tuned or adjusted to bring the model into better agreement with the observations?

 

Thanks,

Tim Glotfelty

jedwards

What grid resolution are you using?   Have you compared against an unmodified version of the model to insure that you did not introduce the bias?  

CESM Software Engineer

twglotfe@...

The grid rsolution I use is f09_g16 so 0.9x1.25 degree for the land and atmosphere and gx1v6 for the ice and ocean components. I did a run using the default out of the box B_2000_STRATMAM3_CN componet set. I couldn't use the B_2000_STRATMAM7_CN component set because there was some namelist error in the default version. The default temperature bias is still a little high -2.3 deg C which indicates my additinal aerosol and cloud changes add about -0.6 deg C to the mean bias.

 

Tim Glotfelty

santos

I've moved this since I think that the people who frequent the CAM forums are more likely to have an answer.

Sean Patrick Santos

CESM Software Engineering Group

hannay

You may need to retune your modified version of the model. 

hannay

From user:


1) Firstly I was wondering what radiation variables from CAM5 are used to compute the radiative balance. Is is something like FSNTOA+FSNIRTOA = RESTOM or is it calculated using other variables? 

2) Is the method any different because I will be tuning a current year time period (2001-2010) instead of pre-industrial (1850)?

3) I also did some checking on the LWCF and SWCF from my simulations compared against CERES data. It appears that on global average the SWCF from my simulations is around 5.0 W m-2 greater (more negative) than CERES and the LWCF is about 2.0 W m-2 smaller. Based on that is there any parameters to tune that make the SWCF less negative and increase the LWCF? 

4) Lastly, is there an acceptable range of values with which to adjust the parameters that you suggested to be physically realistic?

hannay

1) Firstly I was wondering what radiation variables from CAM5 are used to compute the radiative balance. Is is something like FSNTOA+FSNIRTOA = RESTOM or is it calculated using other variables? 

In CAM5: RESTOM = FSNT - FLNT 


2) Is the method any different because I will be tuning a current year time period (2001-2010) instead of pre-industrial (1850)? 

The difference would be that you are not in radiative equilibrium for current time
1850 => RESTOM = 0 W/m2
2000 => RESTOM will be positive (in CERES_EBAF it is close to 0.8 W/m2) 


3) I also did some checking on the LWCF and SWCF from my simulations compared against CERES data. It appears that on global average the SWCF from my simulations is around 5.0 W m-2 greater (more negative) than CERES and the LWCF is about 2.0 W m-2 smaller. Based on that is there any parameters to tune that make the SWCF less negative and increase the LWCF? 

increasing rhminl will allow you to decrease SWCF (less negative) without affecting too much LWCF, but 5 W/m2 is a lot so it depends where your rhminl is now

Decreasing Dcs might help too

FOR SWCF: are you comparing to CERES or CERES-EBAF ?  

 

4) Lastly, is there an acceptable range of values with which to adjust the parameters that you suggested to be physically realistic?

rhminl is not an observable. This is the threshold from low-level cloud and it is used in the model as a tuning parameter. Personally, I  have pushed it up to 0.93.

Dcs = [90-500] is probably a reasonable range (these values are coming from Andrew Gettelman). Dcs is the effective diameter at which we autoconvert to precipitation for ice crystals. So it could be refined based on observations. 

 

twglotfe@...

Thanks for the information it was quite helpful. I compared my SWCF against CERES-EBAF. By adjusting rhminl I was able to get an RESTOM of 0.7 W/m2 so it is comparable to CERES-EBAF. This adjustment also decresed the bias in SWCF to around -1.0 W/m2 and brought the temperature bias back down to around -2.0 degrees C annually. 

 

Thanks for the assistance,

Tim Glotfelty

 

hannay

Hi Tim,

Thanks for your feedback. It is nice to hear that it works for you. 

Cecile

 

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