Lower SWDNB values in WRF4.3 compared to WRF4.2

[vaakos]

Hi all,

I noticed that I get considerably lower SWDNB (and thus T2) values with WRF4.3 than with WRF4.2, keeping all other settings and ICBC data identical.

See a difference map for SWDNB below:


This seems to be rather systematic than just some random noisy difference pattern that would be expected from a version change. Do you have any insight into the physical processes at play here?

Thank you and best regards,
Akos

 

[Ming Chen]

Hi Akos,
Would you please turn off adaptive time step (use_adaptive_time_step = .false.) and set swint_opt=0, then try again?
Remember to run the case with same number of processors.
Please keep me updated about the results. Thanks.

 

[vaakos]

Ming,

thank you for your quick response. I ran the experiment with use_adaptive_time_step = .false. and swint_opt=0 as you suggested.
However, I still see that SWDNB is considerably higher with the previous model version (WRF4.2).
See the SWDNB difference (WRF4.2 minus WRF4.3) below (WRF4.3 with swint_opt=1 left, WRF4.3 with swint_opt=0 right):



So swint_opt doesn't seem to play a big role here.

 

[Ming Chen]

Hi,
I run a test case using WRFv4.2 and WRFv4.3. Attached is the difference in SWDNB between the two versions.
I did see relatively large SWDNB over certain areas, which is attributed to the update of icloud=3 and Thompson microphysics in WRFv4.3.
Basically, the biggest microphsycis change is in how cloud ice converts to snow at a threshold size of 300 microns (up from 200
microns) and for rimed snow to convert to graupel (changed from 250 to 350 microns). A change to allowed
max size of ice means that the fall velocity constant for ice was changed to keep it aligned with snow at the
same cut-over size (of 300 microns). In addition, the cloud fraction scheme (icloud=3) can further improve
the clouds and radiation together with Thompson-MP due to the overall under-prediction of clouds. The
icloud3 option was adjusted so that cloud amounts of low, middle, high, and total cloud coverage, longwave
radiation outgoing at top-of-atmosphere, and shortwave radiation reaching the ground are more consistent with observations.

 

[vaakos]

Thank you for the clarification and the explanation, Ming.