Runtime configurable removal of frozen runoff associated with snowcapping#8384
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Copilot-generated draft code to find snowcapped runoff within a user specified latitude band and remove it from the frozen runoff stream and put it in the river runoff stream, with the goal of raising the temperature to river runoff temperatures, thus preventing snowcapped runoff from leading to sea ice formation with that latitude band. Yet to be implemented is energy conservation -- currently energy violation is simply tracked and report in the mpas-o log file. Add configurable conversion of frozen runoff to liquid runoff within a symmetric equatorial latitude band: config_convert_ice_to_river_runoff_latband config_convert_ice_to_river_runoff_latband_width_degrees Apply conversion in MCT ocean import while conserving local freshwater mass (rofl += rofi, rofi = 0 in selected cells) Add diagnostics for converted flux and associated energy deficit: convertedIceRunoffFlux / avgConvertedIceRunoffFlux convertedIceRunoffEnergyDeficitFlux / avgConvertedIceRunoffEnergyDeficitFlux Add runtime monitoring log output with global converted mass flux, instantaneous estimated energy deficit power, and cumulative deficit energy Update MPAS-O namelist registry/definition/defaults for new controls
version first routes removed snowcapped runoff through MOSART rather than directly to the ocean, thereby allowing mass changes to show up appropriately in coupler budget tables. Runs of <1 day, 1 day, and 1 month appear to converm overall mass conservation. Energy conservation remains to be tested / confirmed. - add ELM SnowCapping heat sink to cool snowpack by capped-ice latent energy (uniform vertical distribution) - move rofi->rofl latitude-band conversion upstream into MOSART export - add MOSART namelist controls for latitude-band conversion - remove obsolete MPAS-O conversion options, diagnostics, and driver plumbing - keep conversion upstream so coupler-side budget accounting remains consistent
CoPilot generated code towards fix to snowcapping-related sea ice production in low-lat. coastal regions. Additions and edits on top of previous two related commits. Align snowcapping physics with the MOSART lat-band routing intent by making ELM local latent-heat extraction conditional on the same latitude-band controls. When the switch is enabled, ELM removes latent heat only within the configured band; outside the band, behavior remains the default downstream treatment. Add ELM namelist controls and defaults: convert_ice_to_river_runoff_latband convert_ice_to_river_runoff_latband_width_degrees Wire controls through elm_varctl/control and apply latitude masking in SnowCapping using topounit latitude. Fix a coupled-build regression in MPAS-Ocean by removing a duplicate declaration of avgRemovedIceRunoffHeatFlux in mpas_ocn_time_average_coupled. No behavior change when the new lat-band switch is disabled.
CoPilot generated code to add optional SnowCapping diagnostics in ELM to quantify latent-energy removal and cooling where snowcapped ice conversion is active in the configured latitude band. Add new elm_inparm controls: snowcap_latband_diag_write snowcap_latband_diag_nstep Wire controls through varctl, control namelist handling, definitions, defaults, and MPI broadcast. Extend SnowHydrology SnowCapping with per-step diagnostics for: capped ice mass latent energy removed area-weighted mean cooling max cooling affected column count Add OpenMP-safe local-to-step accumulation and MPI global reductions. Add cumulative diagnostic tracking and master-task iulog reporting. Add SnowCappingDiagReset and SnowCappingDiagLog APIs. Hook reset and log calls in elm_driver so diagnostics are tied to the land-model timestep and logged with diagnostics cadence.
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ELM: use gridcell latitude for snowcap cooling lat-band gate Switch SnowHydrology lat-band gating for local latent-cooling extraction from topounit latitude to gridcell latitude. This aligns the ELM gate with the lat basis used by runoff routing conversion and avoids missing in-band cooling due to topounit-lat source differences. Additional changes by SP (from Az) to support running E3SM on bebop.
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CoPilot-authored code changes to do the following: Extract latent-heat extraction and diagnostic accumulation from SnowCapping into a new subroutine SnowCapLatentCoolingAndDiag, called once per time step from elm_driver after HydrologyDrainage. Previously, latent cooling was applied inside SnowCapping, which is only called when use_firn_percolation_and_compaction=.true. This meant that in standard (non-firn) configurations, snowcapped ice was silently converted to liquid runoff downstream without any compensating thermal adjustment to the snowpack. The new design accumulates all sources of qflx_snwcp_ice first (CanopyHydrology, SoilHydrology excess-ice, and SnowCapping for firn configs), then applies the latent cooling in a single pass over all columns, gated by the existing latitude-band flag and using gridcell latitude (consistent with the runoff routing conversion gate).
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stephenprice
changed the title
Copilot code changes made during the process of writing the PR description and double checking that copilot's description of all the code changes is consistent with the requirements. Fix SnowCapLatentCoolingAndDiag so that no local snowpack cooling is applied when convert_ice_to_river_runoff_latband is .false. (the default). Previously, apply_cooling was initialized to .true. and the latitude-band check was skipped entirely when the feature was disabled, causing latent heat to be extracted from the snowpack unconditionally — even though the snowcapped ice was still being sent to the ocean as ice runoff, where the ocean would also account for the phase-change energy. Change apply_cooling default from .true. to .false. so that cooling is only activated when the lat-band feature is enabled and the column falls within the configured band.
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Additional feature design discussion and initial evaluation via ad hoc analysis (e.g., visualization using ncview and paraview, coupler budget examination) can be found here: https://e3sm.atlassian.net/wiki/spaces/pd/pages/6082429113/V4+snowcapping+fixes A set of copilot derived python scripts for analyzing a run with these fixes in place (for a given latitude band) relative to a control run where these features are inactive can be found here: https://github.com/stephenprice/analysis-scripts/tree/main/snowcapping The config.py file in the directory contains additional user defined variables for the analysis (e.g., simulations paths) and other options for doing a comparative analysis between simulations in order to look at outputs like sea ice concentration, ice runoff flux, river runoff flux, snowcapping flux, and snowpack temperatures. |
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Overview
By default, frozen runoff from snowcapping is sent to coastal ocean cells via nearest neighbor mapping, where the necessary latent heat of fusion (to convert ice to liquid water) is extracted from the ocean water in those cells. This can lead to excessive cooling of the coastal ocean and, sometimes, non-physical sea ice formation (e.g., in low-lat. regions of the planet). The code changes in this PR add a runtime configurable option (via user namelist settings) to instead send that runoff to the rof stream where it enters the ocean at the same temperature as all other runoff. The latent heat is instead extracted from the snowpack where (and proportional to) the mass that was removed from the snowpack. Within each component section below is additional information on how to configure the necessary changes at run time.
** Note that below here, this description remains in draft form and is still being updated **
Changes by component
ELM (land model) —
SnowHydrologyMod.F90,elm_varctl.F90,controlMod.F90,elm_driver.F90, namelist XML filesNew subroutine SnowCapLatentCoolingAndDiag: when the latitude-band snowcapping feature is enabled (convert_ice_to_river_runoff_latband = .true.), snowcapped ice within the configured band is rerouted through MOSART as liquid runoff rather than being sent to the ocean as ice. To conserve energy, this subroutine extracts the corresponding latent heat of fusion from the local snowpack by cooling snow layer temperatures. The cooling is distributed uniformly (or optionally surface-weighted) across all active snow layers with meaningful heat capacity. Columns outside the latitude band and all columns when the feature is disabled are unaffected — their snowcapped ice continues to the ocean as ice runoff, where the ocean handles the phase-change energy as in the default code path.
New subroutines
SnowCappingDiagReset/SnowCappingDiagLog: accumulate and write per-timestep and cumulative snowcapping diagnostics to the ELM log, including total ice mass removed, latent energy extracted, area-weighted mean and maximum column cooling, and number of affected columns. Diagnostics useMPI_ALLREDUCEfor global aggregation. Writing frequency is configurable using the user_nl_elm options noted below.Called from
elm_driver.F90:SnowCappingDiagResetis called once before the clump loop each timestep;SnowCapLatentCoolingAndDiagis called after drainage within each clump;SnowCappingDiagLogis called afterwrite_diagnostic.Four new namelist parameters in
elm_inparm:convert_ice_to_river_runoff_latband(logical, default.false.) — enable lat-band gatingconvert_ice_to_river_runoff_latband_width_degrees(real, default20.0) — half-width of the latitude band in degreessnowcap_latband_diag_write(logical, default.false.) — enable diagnostic loggingsnowcap_latband_diag_nstep(integer, default1) — diagnostic write frequency in timesteps (e.g., for a land model timestep of 30 mins, setting equal to 48 will provide a single set of daily values for the diagnostics in the lnd.log file).MOSART (river routing) —
rof_comp_mct.F90,RtmMod.F90,RtmVar.F90, namelist XML filesrof_export_mct), whenconvert_ice_to_river_runoff_latbandis enabled, frozen runoff (rofi) is converted to liquid runoff (rofl) for grid cells within the configured latitude band. This ensures snowcapped ice mass is routed through the river network as liquid water rather than being passed to the ocean as ice runoff, which allows the mass to appear correctly in coupler budget tables.mosart_inparmmirror the ELM settings:convert_ice_to_river_runoff_latband(logical, default.false.)convert_ice_to_river_runoff_latband_width_degrees(real, default20.0)MPAS-Ocean —
ocn_comp_mct.FareaCellpointer declaration and loop variablei(supporting infrastructure for related ice-runoff diagnostics).Registry.xml.Key design decisions
convert_ice_to_river_runoff_latband = .true.and only to columns within the configured band. When the feature is disabled (the default), no local cooling occurs — snowcapped ice is sent to the ocean as ice runoff and the ocean handles the phase-change energy, exactly as in the baseline code.qflx_snwcp_ice) — including contributions from CanopyHydrology, SoilHydrology excess-ice, and SnowCapping — not just the portion gated by the firn percolation code path. This ensures energy conservation regardless of whetheruse_firn_percolation_and_compactionis active.rofi→roflconversion, ensuring the two are applied to the same geographic region.convert_ice_to_river_runoff_latband = .false.), preserving bit-for-bit results for existing configurations.!$omp atomicupdates and!$omp criticalfor the max-cooling tracker.