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Nudging approach in SD-WACCM

Hi everyone,I have run the SD-WACCM with GEOS-5. I have 2 questions.1, The nudging approach is applied at every 6 hours (default), so the output at 0,6,12,18 o‘clock,is the model predictive field (before nudging)? or the analysis field (after nudging)?2, What’s the difference between SD and SC component. If I concentrate in winds/temperature, which I should use. thanks very much.Jeff
 

mmills

CSEG and Liaisons
Staff member
Jeff,

1. The nudging is applied every time step, which is 30 minutes in WACCM. Where did you got the idea that it is every 6 hours? The wind and temperature fields are nudged at every time step toward the analysis fields by 10% (default), or whatever fraction you set in the namelist variable met_max_rlx. The output is the model predictive field after nudging, which will be different from the analysis field unless you set the nudging to 100% (met_max_rlx=1.0)

2. The "standard" version of WACCM is neither SD nor SC; it calculates both dynamics and chemistry interactively. SD (specified dynamics) was developed to reproduce specific winds and temperatures, mostly for studying how well the model calculates chemistry in comparison to observations. SC (specified chemistry) was developed for users who are less interested in chemistry, and want to study how well the model calculates dynamics. The advantage of specifying chemistry using output from a previous WACCM run is that the model will run about 2 times faster. If you are more interested in studying dynamics and care less about chemistry, SC may be the configuration you would want. If you do care about interactive chemistry, you can use a standard free-running configuration of WACCM with full chemistry for double the cost of SC. SD is the most expensive version of WACCM because the number of vertical levels is increased from 66 to 88, and the model must calculate all of the dynamics and chemistry done in the standard version before nudging the dynamics.

I hope this helps.

Mike
 
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Hi Mike,

1. I have run the FSDW with GEOS5 met data for 5 model day (20050101-20050105). The time in the met_data (GEOS5.1_19x2_2005_JAN_c110728.nc) is 0,6,12,18 o‘clock everyday. My fraction is 0.1 and the output is 3-hourly. So I know the output at 3(9,15,21) o'clock are from interactive model. But I am confused that the output at 6(12,18,0) o'clock is before the nudging with 6(12,18,0) o'clock fieds in the met_data or after?

I don't know whether my question is clear.

2. the description of SD in the usr guide is : WACCM driven by GEOS5 meteorology with radiation feadbacks turned off. what's the meaning of the words ' radiation feadbacks turned off'.

For the SC, does it just calculate the dynamics without chemistry? Or it need the specified chemistry fields as input data like met_data in SD,and will calculate the effect of specified chemistry on dynamics? I did not find the decription of SC component in any papers, so I have those doubts.

Thanks for your help.

Best wishes.
Jeff
 
Last edited by a moderator:

mmills

CSEG and Liaisons
Staff member
The met_data may vary every 6 hours, or at whatever period is specified in the file. However, the nudging is applied at every timestep. This means that every 30 minutes, temperatures and winds are adjusted to be 0.9 times that calculated plus 0.1 times the current met data values. Model output at any time step will be nudged in this way. There is nothing different about these fields at 3:00 vs. 6:00 just because the met fields vary every 6 hours. At every time step, the output would be 90% from the interactive calculation, and 10% from the met data.

The description of the SD-WACCM compset you have copied is confusing, and will be updated in the future. I would not say it is "driven" by met data, but rather "nudged". The meaning of "radiation feedbacks turned off" is also unclear, and apparently has different meanings for SD-CAM and SD-WACCM. In SD-WACCM, the radiative heating from prescribed volcanic aerosols is turned off by default:

strat_aero_feedback = .false.

However, this is likely to change in future releases, as we have recently improved the prescribed volcanic aerosols.

The term "radiation feedbacks turned off" may have originated with SD-CAM, which uses 100% nudging. In that case, they have set the flag:

chem_rad_passive = .true.

meaning that chemistry does not impact radiation. This is not the case for SD-WACCM, however.
 
Last edited by a moderator:
Jeff,

1. The nudging is applied every time step, which is 30 minutes in WACCM. Where did you got the idea that it is every 6 hours? The wind and temperature fields are nudged at every time step toward the analysis fields by 10% (default), or whatever fraction you set in the namelist variable met_max_rlx. The output is the model predictive field after nudging, which will be different from the analysis field unless you set the nudging to 100% (met_max_rlx=1.0)

2. The "standard" version of WACCM is neither SD nor SC; it calculates both dynamics and chemistry interactively. SD (specified dynamics) was developed to reproduce specific winds and temperatures, mostly for studying how well the model calculates chemistry in comparison to observations. SC (specified chemistry) was developed for users who are less interested in chemistry, and want to study how well the model calculates dynamics. The advantage of specifying chemistry using output from a previous WACCM run is that the model will run about 2 times faster. If you are more interested in studying dynamics and care less about chemistry, SC may be the configuration you would want. If you do care about interactive chemistry, you can use a standard free-running configuration of WACCM with full chemistry for double the cost of SC. SD is the most expensive version of WACCM because the number of vertical levels is increased from 66 to 88, and the model must calculate all of the dynamics and chemistry done in the standard version before nudging the dynamics.

I hope this helps.

Mike
Hi Mike!


I was reading this post because I have a doubt about the nudging in CAM5, actually I am using FSTARTMAM7 compset but I modified the compset to put specif dynamics. Something like this :

"Component set: longname (shortname) (alias)

GEOS_CAM5%SMA7_CLM40%CN_CICE%PRES_DOCN%DOM_RTM_SGLC_SWAV (F_GEOS_STRATMAM7_CN) (FSTRATGEOSMAM7)

Component set Description:

CAM: CLM: RTM: CICE: DOCN: SGLC: SWAV: GEOS5 meteorology: cam5 physics: CAM trop_strat_mam7 chemistry: CAM winds and temperature driven by GEOS5 meteorology: clm4.0 physics: clm4.0 cn: prescribed cice: docn data mode:"


Now I am wondering if for this configuration the default nudging is also 10%? from where I can check this?

I was looking in my atm_in file but I didn't found a variable name "met_max_rlx"


And second question if I want to test different nudging %..... will work if I put met_max_rlx=0 in the way that will be "free running"? I mean without nudging?

I really appreciate any advice/help


Thanks in advance

Adriana
 
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