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How to specify relaxation zone in the WRF high resolution simulations?

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I am running the WRF model at 3 km resolution over complex terrain using high resolution forcing data which has 9 km spatial resolution. I perform one step nesting procedure, i.e. nesting directly the ECMWF 9 km forcing data to the WRF run at 3 km.
Considering that I have complex terrain over the simulation domain and only one-step nesting, do you suggest me to significantly increase the number of points in relaxation zone? say, relax_zone=20 or higher?

Do number of points in relaxation zone refer to the WRF parent model (with 3 km resolution) or to the forcing data (ECMWF with ~9km spatial resolution)?

Best regards,
The number of points in relaxation zone refers to the WRF parent domain.
It doesn't necessarily need to increase the number of points in relaxation zone for high resolution run. The model domain size and location are more important. The domain size must be large eight to ensure WRF dynamics and physics to fully evolve inside the model domain instead of being affected by large scale forcing. The lateral boundaries should be located away from complex terrain area with large topography gradient.

Ming Chen