[Wrf-users] Unexpected "hot-spot" like differences with WRF runs using Intel compiler

[Ligia Bernardet]

Yongtao,

Forecasts will be different when the same code is run on different computational platforms. Even different compilers, or different compiler flags in the same platform will yield different results. 

Chris Harrop and I did a study to quantify the differences in forecast when several models are run in various platforms (http://www.mmm.ucar.edu/wrf/users/workshops/WS2011/Extended%20Abstracts%202011/P52_Harrop_ExtendedAbstract_11.pdf).

It is very difficult to know when the differences one encounters are "normal", just caused by changing platform, or when they are the result of a bug. Different compilers will treat un-initialized variables in different ways: setting them to zero or to a random number. But since you used ifort v10.1 in both platforms, this should not be the issue you are facing.

Regards,
Ligia



On Jun 6, 2012, at 4:28 PM, Hu, Yongtao wrote:

> Hello All,
> 
> We have recently found out an "issue" with WRF modeling using Intel compilers (particularly ifort v10.1) on both Intel and AMD chips. I am wondering if anyone else finds similar "issues": We are conducting two WRF (V3 versions, particularly V3.3 and V3.1.1) runs with everything the same but with changes in landuse (e.g. change the forest to urban, or agriculture to forest in a limited area within the the domain). We found expected result-changes in/near the landuse-changed-area, but found unexpected result-changes in the areas that is far away. These unexpected differences are large, e.g. can be 4K in temperature. Spatially and time-wise, these hot spots "noise" look like come and go. But they apparently can not be seen as numerical noise level differences. 
> 
> We have tested the identical runs (means run twice the same run on the same machine), no difference was found. However, "semi-identical" runs (means run twice the same run but one time on a Intel chip machine and another time on a AMD chip machine) produce different results, with differences as high as 3-4K in temperature. Not at the noise level either.
> 
> By the way, we run WRF in parallel using MPICH2 and we use "-O3" optimization in the WRF compilation. We have followed the instructions from the Intel compiler's website regarding how to compile WRF using ifort. 
> 
> Thanks in advance, 
> 
> Yongtao
> 
> 
> -- 
> Yongtao Hu, Ph.D., Research Scientist II
> School of Civil and Environmental Engineering
> Georgia Institute of Technology
> 311 Ferst Drive, ES&T building
> Atlanta, GA 30332-0340
> tel: 404-385-4558
> fax: 404-894-8266
> e-mail:yh29 at mail.gatech.edu
> _______________________________________________
> Wrf-users mailing list
> Wrf-users at ucar.edu
> http://mailman.ucar.edu/mailman/listinfo/wrf-users