Necessity of vertically resolved mask versus horizontal mask & set_bottom_index

[platipodium]

In SCHISM (unstructured grid), we typically use a vertically resolved mask, to

1. mask entire water column of dry elements
2. mask sub-ocean floor elements of any element, i.e. all below the index of the lowest water level along the vertical per element ibottom(1:ne)

Case (1) is covered by using a horizontal mask, the question is whether we need case (2) when we also have set_bottom_index(ibottom). So does FABM implicitly mask everything with index < ibottom ? If that was the case, we could switch over to using FABM_HORIZONTAL_MASK.

But even if not, perhaps it makes make sense to still use FABM_HORIZONTAL_MASK and ignore that means that sub-seafloor elements are calculated; might that be more efficient than implementing a vertically resolved mask?

[jornbr]

In principle, FABM has enough information to skip all points below bottom if you provide the horizontal mask and the index of bottom points. However, I expect that this is not fully implemented (certainly not yet tested) in the case where the vectorized dimension is depth and masks are being used - SCHISM is the only model doing this. Thus, some tweaks in the FABM codebase are likely needed to support this.

[platipodium]

Thanks for explaining this. I guess we'll just test both options and report back.

[platipodium]

I have two questions regarding this

1. Is it possible to make the mask setting a user option in CMake, such that cmake -D_FABM_HORIZONTAL_MASK_=OFF has an effect, overwriting the setting in fabm_driver.h?

2. It seems that within the host code, I don't have access to the _FABM_HORIZONTAL_MASK_ macro, is that correct? I wanted to do something like

#ifndef _FABM_HORIZONTAL_MASK_
call fs%model%set_mask(mask,mask_hz)
#else
call fs%model%set_mask(mask_hz)
#endif

[jornbr]

In the end, all that fabm_driver.h does is define a few preprocessor symbols. You can potentially set these too from the ouside when you compile FABM. For instance, if you have a cmake-based host such as SCHISM, you can use cmake's add_compile_definitions to set _FABM_HORIZONTAL_MASK_, potentiallly based on user-configurable cmake options. This would need to be done before incorporating FABM via add_subdirectory. For non-cmake-based hosts, you could achieve the same by including the preprocessor definitions in the FFLAGS environment variable before compiling FABM, If needed, you could also customize your fabm_driver.h to define or omit particular symbols, depending on other preprocessor symbols set by the host.

Regarding host access to the FABM preprocessor symbols: if you do an #include "fabm_driver.h" in the host file where you need them, you will have access to all symbols.