set ens members to one, this is about 3x faster for a single forecast

[JoachimKoenigslieb]

Running with this change I get:

((venv) ) (base) jjk@nwp-archive2:~/run-sunflow> time ./venv/bin/sunflow --run_mode files --dataset DWD --bbox CUSTOM --custom-bbox="-10.75,47.25,20,63.5" --time "2026-02-01T09:00"
Pysteps configuration file found at: /home/jjk/run-sunflow/venv/lib/python3.12/site-packages/pysteps/pystepsrc
...
2026-05-06 13:42:11.480 | INFO     | sunflow.main:cli:478 - Script end time = 2026-05-06 13:42:11 UTC

real    0m4.343s
user    0m3.673s
sys     0m0.762s

And without I get:

real    0m12.015s
user    0m57.413s
sys     0m10.522s

So wall-clock is about 3x faster and the user a lot lower because it does not do multi-processing stuff to get the ens members I presume.

[KristianHMoller]

Thanks @JoachimKoenigslieb !

This is great!

But perhaps, we should do it a bit more future proof and use this variable from config.py: ens_members=int(os.getenv("ENS_MEMBERS", "1")),
It defaults to one, so there should be no issue

[JoachimKoenigslieb]

Yes, that sounds like a great idea.

I've used the value from the config instead!

[KristianHMoller]

Thanks a lot @JoachimKoenigslieb !

I can confirm the speed up:

With update:

real	0m8.143s
user	0m10.613s
sys	0m1.547s

Without update :

real	0m15.351s
user	1m32.068s
sys	0m11.578s

If you fix the linting (with pre-commit install; pre-commit run --all-files it should be done automatically) and update the CHANGELOG.md, I will approve and then we will see who has authorization to merge.

[KristianHMoller]

With an entry in CHANGELOG.md, this is ready for approval and merge!

[KristianHMoller]

Suggested change

	- Pass down number of ensembles from configurations to `ProbabilisticAdvection`. This gives a roughly 3x speedup in the no ensembles case.
	- Pass down number of ensembles from configurations to `ProbabilisticAdvection`. This gives a roughly 3x speedup in the no ensembles case [!12](https://github.com/dmidk/sunflow/pull/12), @JoachimKoenigslieb