ESMF Unmapped Points Value Handling During Regrid

[danrosen25]

Add a feature to fill unmapped points with a custom fill value (or NaN) during ESMF regrid. This is similar to a user filling a destination field with a custom fill value before the regrid step and utilizing zeroRegion=select.

Feature enhances a user's ability to identify and handle unmapped points, which is important in regional coupling cases.

https://earthsystemmodeling.org/docs/nightly/develop/ESMF_refdoc/node5.html#SECTION050366900000000000000

[anntsay]

Core team discussion: may be able to achieve this by going through, zeroRegion=select
The benefit of this feature request may making this operation simple, more easily understood, minimize mistakes (without many steps).
Gerhard: maybe implement this in the connector. -> so it will make it easy for users.
It may be a convenience used in the mediator.
Gerhard: performance impact: may increase since we are filling everything and select. you would go through grid twice.
Dan: the benefit for putting this deeper, then there are more use cases that can take advantage of this.
Bob: may want to go with another option or simple routine, allow user to set value. and if user want to do something more complicated, then go through the original.
Dan: maybe leverage the existing option for unmapped.
Bob: add a new optional argument (Fill_value) to allow user to fill custom value (include non-physical values).. or if not set, they will default to zero.
Gerhard: field in fieldbundle have different types.
Dan: may also want to consider global defined variable such as ESMF_IEEE_NAN .. so they can sub the field value correctly. ESMF_IEEE_NAN = ieee_value(ESMF_IEEE_NAN, ieee_quiet_nan) -> will create a ticket for this. ESMF_NAN_I4=0
ESMF_NAN_I8=0
ESMF_NAN_R4=
ieee_value(ESMF_NAN_R4, ieee_quiet_nan)
ESMF_NAN_R8=ieee_value(ESMF_NAN_R8, ieee_quiet_nan)
Dan: or maybe use ESMF_FILLVAL_I4
Dan: call ESMF_FillValueSet(...) allow user definition of fill value.
Gerhard: may want to now consider how to bubble up to NUOPC connector and how it will be used at that layer.

Action:

1. Bob to consider the points above.. will reconvene for implementation decision. current idea is use field attribute at ESMF level.. and if don't set, it will take the default. the default could be defined by user.