# Numerical Noise in distant regions by changing runoff **URL:** \ **Category:** Versions\ **Created:** [3 January 2025 10:10 UTC](https://nemo-ocean.discourse.group/t/numerical-noise-in-distant-regions-by-changing-runoff/814 "2025-01-03T10:10:29Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![saad](https://yyz2.discourse-cdn.com/free1/user_avatar/nemo-ocean.discourse.group/saad/32/239_2.png) [@saad](https://nemo-ocean.discourse.group/u/saad)\ **Post date:** [3 January 2025 10:10 UTC](https://nemo-ocean.discourse.group/t/numerical-noise-in-distant-regions-by-changing-runoff/814/1 "2025-01-03T10:10:29Z")
Hi everyone, I’m conducting a regional parametric study where I perturbed a river and observed unexpected noise in a distant region, just after few hours (very low) but starts to grow as days pass and becomes significantly high, like after few months i saw a change in Atlantic after perturbating a river in Eastern Mediterranean, i was able to see a change of around 1 psu in atlantic thats not physically advected. The change is in tracers, velocities, and sea surface height (SSH). The change may originated near the bathymetry since i saw change in velocities and tracers near the base like in the deep ocean in Atlantic **Here are the key details of my setup** : I am using a 1/16° grid with z-coordinates and partial steps (no AGRIF zoom) with Split-Explicit formulation. I am using open boundary only in the Atlantic, with flow relaxation for tracers and baroclinic variables, and Flather conditions for barotropic variables and SSH. For time stepping, my Baroclinic step = 200 seconds; barotropic step = ~2 seconds using nn\_baro (also tested CFL-dependent barotropic stepping). I am using MUSCL for tracers advection. I am using vector form for momentum advection however i also tested flux form. My river is just at the top cell with runoff and salinity only. I tried changing turbulence schemes, the use of GLS/TKE closure models reduced the error signficantly in comparison with Richardson number-dependent mixing however smaller error persist that grows over months. I am using MPI, and in NEMO 5 i used halo cells = 2. I ran experiments using both **NEMO 3.6** and **NEMO 5 beta** , testing both linear and non-linear SSH formulations. I also tried different configurations, including: • Reducing the timestep (affected but didn’t eliminate the noise) • Splitting the river into multiple cells instead of a single input. • Using the Runge-Kutta scheme in NEMO 5. • Running the simulation on a smaller domain with finer resolution (similar kindof noise observed). I would greatly appreciate any insights or suggestions on how to address this issue. Please let me know if i could provide any more information that could be helpful. ---
**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\ **Post date:** [3 January 2025 11:09 UTC](https://nemo-ocean.discourse.group/t/numerical-noise-in-distant-regions-by-changing-runoff/814/2 "2025-01-03T11:09:13Z")
Hello Saad, Welcome to Nemo. Could you be a bit more precise regarding the perturbation you add to the river runoff ? Cheers, Robinson ---
**Author:** ![saad](https://yyz2.discourse-cdn.com/free1/user_avatar/nemo-ocean.discourse.group/saad/32/239_2.png) [@saad](https://nemo-ocean.discourse.group/u/saad)\ **Post date:** [3 January 2025 11:36 UTC](https://nemo-ocean.discourse.group/t/numerical-noise-in-distant-regions-by-changing-runoff/814/3 "2025-01-03T11:36:57Z")
Dear Robison, thank you for reading. I am running two identical cases and i just change the monthly runoff of the river (two different runoffs with different seasonal cycle). Apart from the flow change around the grid cell of the river, i see change in far away regions. ---
**Author:** ![robinson](https://avatars.discourse-cdn.com/v4/letter/r/848f3c/32.png) [@robinson](https://nemo-ocean.discourse.group/u/robinson)\ **Post date:** [3 January 2025 12:30 UTC](https://nemo-ocean.discourse.group/t/numerical-noise-in-distant-regions-by-changing-runoff/814/4 "2025-01-03T12:30:04Z")
And are you sure the distant noise appears only if you change the runoff ? Could you please send me the runoff files before & after (you can gzip them) to robinson.hordoir@hi.no ---
**Author:** ![saad](https://yyz2.discourse-cdn.com/free1/user_avatar/nemo-ocean.discourse.group/saad/32/239_2.png) [@saad](https://nemo-ocean.discourse.group/u/saad)\ **Post date:** [3 January 2025 13:46 UTC](https://nemo-ocean.discourse.group/t/numerical-noise-in-distant-regions-by-changing-runoff/814/5 "2025-01-03T13:46:51Z")
Yes, thats the only thing that i change. I will share the files with you. Thank you!