---
# System prepended metadata

title: 'ICON4Py Granule Restrictions:'
tags: [architecture, icon4py-production-2025, icon4py]

---

# ICON4Py Granule Restrictions:

## Restrictions
The Python ported components do _not_ support all options that are available in Fortran. This was a deliberate decision by EXCLAIM when the granules were meant to be components of a Python standalone model and no back-integration of these components into the original Fortran code was foreseen.

Some restriction concern major architectural choices as for example nested grids, which if implemented in the same way as ICON might compromise the goal of reaching a more modern and flexible architecture in ICON4py

Some others concern only simple namelist parameters that were left out.

### Features
- No Nesting


### Namelist options
## Dycore Granule


| namelist parameter                  | ICON4Py                                            | reason                                                        |
| ----------------------------------- | -------------------------------------------------- | ------------------------------------------------------------- |
| non_hydrostatic_config:lextra_diffu | the parameter is absent and treated as always TRUE | there seems to be no experiment in ICON that sets it to FALSE |
| non_hydrostatic_config:itime_scheme | only itime_scheme=4 (default value in icon) is supported                   | It is the most efficient option and it is used by MeteoSwiss. itime_scheme=6 may be supported in the future with only a few extra existing stencils https://hackmd.io/FSMJYZEqQAC40hMnT6p-kg |
| non_hydrostatic_config:rayleigh_type | only rayleigh_type=2 (default value in icon) is supported                   | Klemp damping (rayleigh_type=1) damps both horizontal and vertical velocity and needs prescribed values of both. Not used in aquaplanet |
| non_hydrostatic_config:igradp_method | only igradp_method=3 (default value in icon) is supported                   |  |
| non_hydrostatic_config:iadv_rhotheta | only iadv_rhotheta=2 (default value in icon) is supported                   |  |




## Diffusion Granule
| namelist parameter | ICON4Py               | reason                  |
| ------------------ | --------------------- | ----------------------- |
| diffusion_type     | only diffusion_type=5 | used by MCH and EXCLAIM |
|   ...                 |                       |                         |