COAWST 3.8: SWAN crashes when NnodesWAV > 1 in WRF–SWAN–ROMS coupling

[Rocky_Wang]

Subject: COAWST 3.8: SWAN crashes when NnodesWAV > 1 in WRF–SWAN–ROMS coupling

Dear ROMS/COAWST community,

I am running a three-way coupled WRF–SWAN–ROMS application using COAWST 3.8. The model compiles successfully, but it crashes at runtime when the SWAN/WAV component uses more than one MPI task.

The key point is:

```fortran
NnodesOCN = 1
NnodesWAV = 1
NnodesATM = 1
```

This configuration runs.

Also:

```fortran
NnodesOCN = 2
NnodesWAV = 1
NnodesATM = 2
```

This configuration runs as well.

However:

```fortran
NnodesOCN = 2
NnodesWAV = 2
NnodesATM = 2
```

crashes immediately during initialization.

The `couple.log` shows that the WAV component is assigned to MPI tasks `00002 - 00003`:

```text
Ocean Model MPI nodes: 00000 - 00001
Waves Model MPI nodes: 00002 - 00003
Atmos Model MPI nodes: 00004 - 00005
```

The traceback shows that task 2, which is the first WAV/SWAN task, crashes in `waves_coupler.f`:

```text
forrtl: severe (174): SIGSEGV, segmentation fault occurred

coawstM
waves_coupler_mod
waves_coupler.f, line 636

waves_control_mod
waves_control.f, line 87

MAIN__
master.f90, line 363

task 2: Exited with exit code 174
```

Therefore, the problem seems to be triggered specifically by:

```text
NnodesWAV > 1
```

It does not appear to be caused by the ROMS or WRF parallel decomposition, because ROMS and WRF can run with two MPI tasks when SWAN remains serial.

## Model/environment information

```text
COAWST version: 3.8
WRF version: 4.5
SWAN library: libswan41.45.a
Compiler: Intel ifort 19.0.4
MPI: MPICH 4.0.2
NetCDF: 4.8.0
PnetCDF: 1.12.2
Coupler: MCT
Components: WRF + SWAN + ROMS
```

## Grid information

```text
ROMS grid: 480 x 430 x 20
SWAN grid: CGRID CURVILINEAR 481 431 ...
SWAN grid points: 482 x 432 = 208224
WRF domain: 547 x 485
```

## SCRIP weight file

The SCRIP weight file contains all six mappings:

```text
ocn1_to_wav1
wav1_to_ocn1
ocn1_to_atm1
atm1_to_ocn1
wav1_to_atm1
atm1_to_wav1
```

The OCN/WAV dimensions are consistent with the SWAN grid:

```text
ocn1_to_wav1: src_grid_size = 208224, dst_grid_size = 208224
wav1_to_ocn1: src_grid_size = 208224, dst_grid_size = 208224
atm1_to_wav1: src_grid_size = 264264, dst_grid_size = 208224
wav1_to_atm1: src_grid_size = 208224, dst_grid_size = 264264
```

The number of links is not zero.

## Active CPP options

The active options include:

```text
SWAN_COUPLING
WAVES_OCEAN
WRF_COUPLING
MCT_LIB
SPECTRUM_STOKES
WEC_VF
WDISS_WAVEMOD
UV_KIRBY
ATM2OCN_FLUXES
FRC_COUPLING
```

## Additional test

I also modified the character buffer lengths in `SWAN/src/waves_coupler.ftn` from:

```fortran
# ifdef SPECTRUM_STOKES
character (len=520) :: wostring
character (len=140) :: owstring
# else
character (len=120) :: wostring
character (len=120) :: owstring
# endif
```

to:

```fortran
# ifdef SPECTRUM_STOKES
character (len=5000) :: wostring
character (len=1000) :: owstring
# else
character (len=500) :: wostring
character (len=500) :: owstring
# endif
```

Then I recompiled SWAN and relinked COAWST, but the same crash still occurs when `NnodesWAV = 2`.

## Question

Does COAWST 3.8 support MPI-parallel SWAN coupling with `NnodesWAV > 1` in a three-way WRF–SWAN–ROMS configuration, especially when `SPECTRUM_STOKES`, `WEC_VF`, and `WAVES_OCEAN` are enabled?

If yes, could anyone suggest where to debug this `waves_coupler.f line 636` crash?

I suspect it may be related to one of the following:

```text
SWAN parallel grid decomposition
MCT GlobalSegMap initialization
AttrVect initialization
local/global WAV grid indexing
SPECTRUM_STOKES or WEC-related WAV fields in parallel mode
```

At present, the only stable workaround is:

```fortran
NnodesOCN = multiple tasks
NnodesWAV = 1
NnodesATM = multiple tasks
```

Any suggestions would be greatly appreciated.

Best regards,