fix: melt pond ice-lid sign + caller; tune pond parameters#912
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fix: melt pond ice-lid sign + caller; tune pond parameters#912JanStreffing wants to merge 2 commits into
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Three related bugs caused ipnd (melt pond ice lid thickness) to accumulate unbounded over the melt season (values >2 m in HR runs, >3 m in LR_run_test10), and the freezing branch in meltpond_area to be unreachable. - ice_thermo_cpl.F90: caller previously clamped meltt_rate/melts_rate to >=0 via max(0,-dhgrowth), making the freezing branch in meltpond_area dead code. Pass signed net thermodynamic tendency so the sign carries the freeze/melt distinction. - ice_meltponds.F90: melting branch had wrong sign (ipnd + meltt*dt grew the lid during melt instead of melting it). Also restructured to a clean 3-way conditional so the freezing branch only thickens lids where a pond exists, and a meltt=0 step is a no-op (previously the else branch accidentally grew ipnd when meltt<0 and apnd<=puny). - ice_meltponds.F90: clamp the melt-water input to >=0 in dvolpnd so freezing does not remove water from the pond (now that meltt can be negative). fpond (intent(out)) is declared but never consumed by the caller, so these sign changes have no downstream effect on the freshwater flux to the ocean.
…SM range Companion tuning to the sign fixes in the prior commit. With ipnd now behaving correctly, the existing parameters made the meltpond scheme too aggressive (LR Sep ice area ~3 Mkm^2 vs ~6 Mkm^2 in the no-pond reference, Sep volume ~5 vs ~17 x 10^3 km^3). - rfracmax 1.0 -> 0.5: less surface meltwater retained in ponds on thick ice, lowering pond depth and area. - albpnd 0.20 -> 0.30: open-pond albedo toward the CICE/CESM range reported in the literature. - albpnd_frz 0.36 -> 0.50: frozen-pond albedo also toward CICE range. Both albedo changes brighten ponded areas, partly compensating the additional summer SW absorption that the meltpond scheme introduces relative to a no-pond setup. Needs a multi-year LR validation run before merge.
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Peaks are still unrealistic. back to the drawing board. |
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Closing in favour of a cleaner approach. The sign fix here patched the immediate accumulation but left Superseded by #913, which replaces the home-grown ipnd accumulator with the Stefan ('hlid') branch of Icepack |
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May 22, 2026
refactor: meltpond ipnd via Icepack-style Stefan lid (supersedes #912)
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Fix two sign bugs in the melt-pond parameterization that let
ipndaccumulate unbounded (>3 m at LR, >2 m at HR), and re-tune pond parameters into the CICE/CESM range.ice_thermo_cpl.F90: caller clamped meltt/melts ≥ 0 viamax(0, -dhgrowth), making the freezing branch inmeltpond_areadead code. Pass signed net tendency.ice_meltponds.F90: melting branch had wrong sign (ipnd + meltt*dtgrew the lid during melt). Restructured to a 3-way conditional;meltt=0is now a no-op; freezing branch only thickens lid where a pond exists.ice_meltponds.F90: clamp melt-water input ≥ 0 indvolpndso freezing does not remove pond water.Tuning:
rfracmax1.0→0.5,albpnd0.20→0.30,albpnd_frz0.36→0.50.Direct bug fix impact:
Validation
20-yr AWI-ESM3 TCO95L91-core2 A/B (clim 1910–1919), no-fix vs fix+tune: