HYSPLIT Forum: hysplitbbs.arl.noaa.gov

Hi, everyone.
I'm looking through the code of concentration program.
I notice that there are two ways to generate the random number for particle dispersion:
1. If Krand=1, will use the pre-calculated random number array.
2. If Krand=2, will use the "parvar" subroutine inside the dispersion subroutine to generate the random number.
I find that if the user not specify the krand in the Namelist file, the program will use 2 when particle smaller than 5000, otherwise the first method.

I try both, finding that the result are very similar with a little different.

I just curious if both random number method's result are acceptable. Because I'm trying to parallelize the inner loop through the particles. And the first random number method (pre-calculated) is good for parallelization while the second one cannot preserve the same result as the original sequential program. So I try to only use the first random number method for my parallel version if that is acceptable.

Thank you.

The first method is acceptable too. However, it is less accurate than the second. Having precomputed random numbers is a good approximation but it is not a truly random distribution.
The code is already parallelized through the particles. Please check the conditional compilation statements in hymodelc.F

Thank you！
I Checked the source code and saw how it is using MPI to parallelize the particle loop.
And I used -DMPI flag to compile the "hycm_std" to run the same example as "hycs_std".
However I found the result PostScript produced by "hycm_std" was not the same as the one produced by "hycs_std".
I guessed that it may due to the parallel random number generation is not the same as the sequential random number generation.

If there is nothing wrong with what I said above, I wonder if this small difference between MPI version and sequential version acceptable?

Thank you.

Here are two results:
The result from sequential version: hycs_std. The result with the same input data from MPI version: hycm_std:

You are right, the differences you are seeing are due to the different random numbers generated by the parallelized code. However, I can see that you are not using enough particles so your concentration fields are noisy. I would suggest increasing the number of particles you release and you will see that the differences get smaller.

Thank you.
I tried to increase the numpar from 7500 to 30000. And the differences are much smaller:

The sequential version:
The MPI version:
So, I think that this differences now acceptable right?

Thank you.

Yes, they are acceptable.
If you increase your particle number even more you will reach a point in which you should not see any difference. No need to do that though.