3.6. Merging

Merging combines one or more mapped source fields into a single destination field in a component’s export bundle. It is declared per destination field by addmrg calls in esmFldsExchange_<host>_mod.F90 and executed by the prep phases; the machinery lives in med_merge_mod.F90. Where Mapping fills the off-diagonal FBImp(n,k) entries, merging reads down a destination column and writes FBExp.

3.6.1. Automatic merging

The prep phases call med_merge_auto, which is driven by the ``addmrg`` declarations rather than by hand-written per-field logic. For each destination field in fldListTo(comp) it looks up the source contributions that were declared for that field — each contribution’s source component (mrg_from), source field (mrg_fld), merge type (mrg_type) and, where needed, fraction name (mrg_fracname) — and applies them in turn to build the output field. The mapped source fields are taken from the FBImp bundles already on the destination grid; the weights are taken from FBfrac on the destination grid.

3.6.2. The merge operations

Every merge reduces to one of a small set of per-gridcell operations. Writing dst for the destination field, src for the mapped source field, and f for the fraction weight (mrg_fracname in FBfrac of the destination):

mrg_type	operation	meaning
copy	dst = src	overwrite with the source
copy_with_weights	dst = src * f	copy, fraction-weighted
sum	dst = dst + src	cumulative sum (unweighted)
sum_with_weights	dst = dst + src*f	cumulative sum, fraction-weighted
merge	dst = dst + src*f	fraction-weighted accumulate

merge and sum_with_weights perform the same arithmetic (dst = dst + src*f); the different names document intent. The *_with_weights and merge types read their weight f from the destination’s fraction bundle; copy and sum use no weight.

3.6.3. Multi-source (surface) merges

A field that the mediator forms from several surfaces — for example the surface temperature the atmosphere sees over a cell that is part land, part ocean and part sea ice — is declared as one ``addmrg_to`` call per source, each with mrg_type = 'merge' and that surface’s fraction. Because merge accumulates src*f, the three calls together compute the fraction-weighted sum

Sx_t = f_lnd * Sl_t  +  f_ice * Si_t  +  f_ocn * So_t

Since the land, ice and ocean fractions sum to one over each atmosphere cell, this is a proper fraction-weighted average — the gridcell-average surface temperature. The first contribution behaves like an accumulate onto a zero-initialized output, and each subsequent merge call adds its surface’s weighted contribution.

3.6.4. Merging and conservation

The fraction-weighted merge here is one half of the conservation strategy developed in Core concepts: the mediator maps fields with a fraction-weighted, normalized map (Mapping) and then merges them with fraction weights. Applied consistently, the two keep coupled fluxes conserved and coupled states as meaningful gridcell-average values across mismatched grids.