Meteorological Forcing

Cocoa supports atmospheric forcing from wind fields and surface pressure data for storm surge and other weather-driven simulations. Forcing data is read from external files, interpolated onto the computational mesh, and applied at each timestep.

Overview

Meteorological forcing in Cocoa includes two components:

• Wind stress: Surface drag from 10-meter wind velocity, computed via the Garratt (1977) bulk drag law

• Atmospheric pressure gradient: Barotropic forcing from spatially varying surface pressure

Both components are read together from the same source and share a common temporal interpolation framework. A run may also combine several sources – of the same or different file formats – into one composed forcing field; see Composing Multiple Sources. See Meteorological Forcing for the mathematical formulation.

To generate the wind and pressure field analytically from a storm track instead of (or in addition to) gridded data, see Parametric Vortex (GAHM).

Supported File Formats

Cocoa supports four meteorological file formats through the cocoa_meteo library:

Format	Config Value	Domains	Description
CF-compliant NetCDF	cf_netcdf	Single	Standard climate/forecast NetCDF with configurable variable names
OWI ASCII	owi_ascii	Multiple	ADCIRC-compatible paired pressure/wind ASCII files
OWI NetCDF	owi_netcdf	Multiple	NetCDF variant of OWI with support for moving (vortex-tracking) grids
GRIB2	grib	Multiple	Operational weather products (GFS, HRRR, RRFS, NAM, HWRF, HAFS) read directly via ecCodes; requires a build with -Dcocoa_ENABLE_GRIB=ON

The synonyms cf and netcdf (for cf_netcdf), owi (for owi_ascii), and grib2 (for grib) are also accepted.

Configuration

Meteorological forcing is configured in the forcing.meteorological section of the YAML configuration file.

Enabling Meteorological Forcing

To enable meteorological forcing, set enabled: true and specify the file format and path(s):

forcing:
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "path/to/meteo_data.nc"
  ramp:
    enabled: true
    duration: 1d

The ramp section controls the global spinup ramp applied to both tidal and meteorological forcing. To use a different ramp for meteorological forcing, add an optional ramp subsection under forcing.meteorological:

forcing:
  ramp:
    enabled: true
    duration: 5d                      # Applied to tidal forcing
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "path/to/meteo_data.nc"
    ramp:                           # Optional: overrides forcing.ramp for met
      enabled: true
      duration: 1d                     # Shorter met ramp

When the forcing.meteorological.ramp section is absent, the global ramp is used for both tidal and meteorological forcing.

CF NetCDF Format

forcing:
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "path/to/meteo_data.nc"

Optional variable name overrides:

By default, Cocoa expects the following NetCDF variable names. Override them under a nested variables map if your data uses different conventions:

Key	Default	Description
pressure	mslp	Mean sea level pressure variable name
wind_u	wind_u	Eastward 10-m wind component
wind_v	wind_v	Northward 10-m wind component
time	time	Time coordinate variable
lon	lon	Longitude coordinate variable
lat	lat	Latitude coordinate variable

Example with custom variable names:

forcing:
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "era5_data.nc"
    variables:
      pressure: "sp"
      wind_u: "u10"
      wind_v: "v10"
      time: "time"
      lon: "longitude"
      lat: "latitude"

OWI ASCII Format

OWI ASCII uses paired pressure and wind files per domain. This is the traditional ADCIRC meteorological forcing format.

Single domain:

forcing:
  meteorological:
    enabled: true
    format: owi_ascii
    filenames:
      - pressure: "path/to/fort.221"
        wind: "path/to/fort.222"

Multiple nested domains:

forcing:
  meteorological:
    enabled: true
    format: owi_ascii
    filenames:
      - pressure: "path/to/basin.pre"
        wind: "path/to/basin.wnd"
      - pressure: "path/to/region.pre"
        wind: "path/to/region.wnd"

Domain 0 (first entry) is the outer coarse grid. Subsequent entries are progressively finer inner grids. Where grids overlap, the innermost valid domain takes priority. Domains may also differ in time coverage; see Domain Time Windows.

OWI NetCDF Format

OWI NetCDF stores all domains in a single NetCDF file using group-based organization. It additionally supports moving (vortex-tracking) grids where the grid coordinates change at each time step.

forcing:
  meteorological:
    enabled: true
    format: owi_netcdf
    filename: "path/to/meteo_owi.nc"

GRIB2 Format

The GRIB reader ingests operational GRIB2 products directly – individual per-cycle snapshot files (each holding one valid time) or files carrying several times. At startup every file’s message headers are scanned and indexed by valid time, so filename order does not matter; incomplete or duplicated fields, grid changes within a domain, and misaligned time axes are all rejected with a descriptive error before the simulation starts.

GRIB support is compiled in with -Dcocoa_ENABLE_GRIB=ON, which requires an installed ECMWF ecCodes (point CMake at it with -Deccodes_DIR). ecCodes must be built with the JPEG2000 and CCSDS/AEC codecs that NCEP products use – spack install eccodes +memfs +aec jp2k=openjpeg provides a suitable build – and cocoa verifies those codec features are present at configure time (see Installation). GRIB2 fields are self-describing SI (Pa, m/s): pressure_scale is rejected for this format.

Single domain (GFS):

forcing:
  meteorological:
    enabled: true
    format: grib
    product: gfs
    filenames:
      - "gfs.t00z.pgrb2.0p25.f000"
      - "gfs.t00z.pgrb2.0p25.f003"
      - "gfs.t00z.pgrb2.0p25.f006"

The product preset selects which GRIB2 fields provide mean sea level pressure and 10-m winds. Available presets: gfs, hrrr, rrfs, nam, hwrf, hafs. GFS, NAM, HWRF, and HAFS use WMO-standard PRMSL; HRRR carries only the NCEP-local MSLMA reduction and RRFS only the NCEP-local MSLET (membrane) reduction, so those presets select the local parameters. NCEP files that pack the 10-m wind pair as a single multi-field GRIB message (e.g. NAM awphys) are handled transparently.

These products are 10-m winds from models that resolve land effects, so a mesh-based land wind reduction usually should not be applied on top: set wind_reduction: false at the meteorological level to leave the gridded field unreduced while, e.g., a composed parametric vortex is still reduced (see Per-source application).

Multiple GRIB resolutions (HRRR nested inside GFS), or GRIB mixed with another format:

Nesting a fine product inside a coarser one, or composing GRIB with a different format entirely, uses the general domains: list – each entry is a self-contained source with its own format (grib here, on every entry):

forcing:
  meteorological:
    enabled: true
    domains:
      - format: grib
        product: gfs          # domain 0: outer/coarse
        filenames: ["gfs_f000.grib2", "gfs_f001.grib2"]
      - format: grib
        product: hrrr         # domain 1: inner/fine, wins where it covers
        filenames: ["hrrr_f00.grib2", "hrrr_f01.grib2"]

See Composing Multiple Sources for the self-containment rule, coverage priority, and per-entry wind_reduction. Domains need not share a time step or phase – see Domain Time Windows.

A long forecast’s file list runs to hundreds of entries and is reusable across runs, so any filenames list (flat form or a domains entry) may instead be pulled from an external YAML file with an include node (see Configuration for the inclusion rules):

filenames:
  include: hafs_ida_files.yaml   # a YAML list of file paths

Regular latitude-longitude grids (GFS, HAFS parent domains) are handled natively, including global longitude wrapping and regional grids whose span crosses the antimeridian (routine for Pacific HAFS storms); projected grids (HRRR/RRFS/NAM Lambert conformal) are expanded to per-point coordinates and interpolated with the same inverse-bilinear machinery as irregular OWI grids. Bitmap-masked cells (missing data, e.g. outside a HAFS domain’s integration footprint) make the affected area fall through to the next-coarser domain.

Storm-following (moving) nests – the HWRF/HAFS storm output, where a fixed-shape regular grid translates with the storm each forecast hour – are detected automatically and handled by the same Lagrangian storm overlay used for OWI moving vortex grids (the grid origin is advected between snapshots and winds are blended magnitude-preserving, avoiding eye-wall vector cancellation). A moving nest may only change position: dimension, spacing, or projection changes are rejected, so a domain’s file list must come from a single forecast cycle. Moving nests must be regular lat-lon grids; a typical hurricane setup lists three domains, coarsest first (each entry still needs its own format: grib, like every domains: entry):

domains:
  - format: grib
    product: gfs
    filenames: [...]
  - format: grib
    product: hafs   # fixed parent domain
    filenames: [...]
  - format: grib
    product: hafs   # moving storm nest, highest priority
    filenames: [...]

For products the presets do not cover, the three fields can be selected explicitly by their numeric GRIB2 identity (shown here with the HRRR MSLMA values); level defaults to 0:

forcing:
  meteorological:
    enabled: true
    format: grib
    filenames: ["custom.grib2"]
    variables:
      pressure: {discipline: 0, category: 3, number: 198, surface_type: 101}
      wind_u:   {discipline: 0, category: 2, number: 2, surface_type: 103, level: 10}
      wind_v:   {discipline: 0, category: 2, number: 3, surface_type: 103, level: 10}

The same variables block may be given on a domains: entry (which still needs its own format: grib, like any other entry). surface_type is GRIB2 code table 4.5 (101 = mean sea level, 103 = height above ground).

Composing Multiple Sources

More than one meteorological source can drive a run at once – a basin-scale OWI background overlaid by a fine HAFS storm nest, or a GFS background with a GRIB HRRR inset – by listing them under forcing.meteorological.domains instead of the flat top-level keys used above. Each list entry is a fully self-contained source: it declares its own format and every key that format needs (files, product/variables for GRIB, wind_scale, pressure_scale, wind_reduction), so sources of different formats compose freely:

forcing:
  meteorological:
    enabled: true
    domains:
      - format: owi_ascii
        filenames:
          - pressure: "basin.pre"
            wind: "basin.wnd"
      - format: grib
        product: hafs
        filenames: ["hafs_storm_f00.grib2", "hafs_storm_f01.grib2"]

domains: cannot be combined with any top-level source key (format, filename/filenames, product, variables, wind_scale, pressure_scale, wind_reduction) – entries are self-contained, so there is nowhere else for that configuration to apply. In particular, format: grib at the top level combined with domains: is a startup error naming the offending key:

forcing.meteorological.domains cannot be combined with the top-level
'format' key; move the configuration into the domains entries

List order is coverage priority: entry 0 is the outermost/coarsest source, and later entries win at any node they cover (the same last-wins rule OWI’s own nested domains use, see OWI ASCII Format). A node not covered by any entry falls back to the ambient background (101300 Pa pressure, zero wind). The vortex block, if present, composes on top of whatever the domains: list produces exactly as it does over the flat form (Domains with a vortex below), unchanged – see Parametric Vortex (GAHM).

Per-entry wind_reduction

Each entry carries its own wind_reduction flag (default true, the same default as the flat form), so co-located sources can be corrected differently. A common pattern is a marine 10 m HAFS nest that still needs the land wind reduction, laid over an OWI background that is already a corrected surface wind and should not be reduced again:

forcing:
  meteorological:
    enabled: true
    domains:
      - format: owi_ascii        # basin-scale background, already a
        wind_reduction: false    # surface wind -- do not reduce again
        filenames:
          - pressure: "basin.pre"
            wind: "basin.wnd"
      - format: grib             # fine marine storm nest
        product: hafs
        wind_reduction: true     # reduce to a land wind where it covers
        filenames: ["hafs_storm_f00.grib2", "hafs_storm_f01.grib2"]

The flag is resolved per node from whichever entry actually covers it (the winner of the spatial combine), so a node under the HAFS nest is reduced even while a node left uncovered by it – and served by the OWI background – is not. See Per-source application for how the flag composes with the mesh’s roughness/canopy attributes, and Domain Time Windows for how sources with different time steps compose.

Domains with a vortex

A hurricane setup combining an OWI ASCII basin-scale background, a fine HAFS GRIB storm nest, and the GAHM parametric vortex – the full innermost-wins stack, gridded reader composition plus the vortex layered on top:

forcing:
  meteorological:
    enabled: true
    domains:
      - format: owi_ascii          # basin-scale background: already a
        wind_reduction: false      # surface wind, so leave it unreduced
        filenames:
          - pressure: "basin.pre"
            wind: "basin.wnd"
      - format: grib               # fine marine 10 m storm nest: reduce
        product: hafs               # it to a land wind where it covers
        wind_reduction: true
        filenames: ["hafs_storm_f00.grib2", "hafs_storm_f01.grib2"]
    vortex:
      model: gahm
      tracks:
        - "storm_bal.dat"
      wind_reduction: true          # the vortex is also a marine wind
  ramp:
    enabled: true
    duration: 1d

The HAFS nest and the OWI background can each be produced on their own native cadence (Domain Time Windows) – neither has to be resampled to match the other or the vortex’s per-snapshot track times. Both gridded sources reduce or not per their own physical nature (the OWI background is already surface-level; the HAFS nest and the vortex are both marine winds that need the mesh’s land correction), independent of composition order.

Activation Ramp

A domains: entry’s data window, or a vortex track’s own start/end, does not have to span the whole simulation – a fine HAFS storm nest may only start at hour 48, arriving against an ocean that has already spun up but is still storm-free. By default that arrival is a hard edge: the source comes in over a single data interval (Domain Time Windows), which for an intense, coarsely-sampled nest can still look like a step to the rest of the composed field.

Each domains: list entry – not the flat top-level source, which has no window of its own – and the vortex: block accept a ramp: <duration> key (e.g. 6h; default 0, off) that smooths this into an explicit cross-fade. The fade is a composition blend, not a scale on the source’s own values: at every node it interpolates between the composed field without that source and the composed field with it,

node = (1 - w) * underlying + w * source

with the weight w ramping from 0 to 1 as the window opens and back to 0 as it closes, symmetrically, using the same normalized tanh(2t/T) / tanh(2) shape as the top-level cold-start ramp block (Combining Meteorological and Tidal Forcing) – one curve shared by both uses so they cannot drift apart. Because the two sides of the blend already agree everywhere outside the source’s own spatial coverage, the fade is spatially seamless: it cannot introduce a seam at a nest boundary, and the composed field can never dip below the underlying data, unlike a plain scale on the source.

The fade is symmetric: the same duration applies at window-open and at window-close, each referenced to that edge’s own time. A window shorter than twice the ramp duration never reaches full weight – the fade-in and the fade-out are both still in progress when the other edge arrives – which is deliberate, not an error: a very short-lived nest fades smoothly in and back out rather than plateauing at full weight.

forcing:
  meteorological:
    enabled: true
    domains:
      - format: owi_ascii
        filenames:
          - pressure: "basin.pre"
            wind: "basin.wnd"
      - format: grib
        product: hafs
        ramp: 6h                 # cross-fade in/out over the nest's own window edges
        filenames: ["hafs_storm_f00.grib2", "hafs_storm_f01.grib2"]

See Parametric Vortex (GAHM) for the equivalent vortex.ramp key, which fades a storm in and out against its own track’s first and last snapshot times rather than a data window.

The full stack: a worked storm configuration

Everything above composes in a single run. This configuration layers four gridded sources of three formats, two parametric storms, and every kind of ramp the model has – each doing a different job on a different clock:

forcing:
  meteorological:
    enabled: true

    # Model cold-start spinup: ONE ramp for the whole composed field,
    # referenced to the start of the meteorological data. Protects the
    # quiescent ocean at initialization; unrelated to the per-entry
    # activation ramps below.
    ramp:
      enabled: true
      duration: 12h

    domains:
      # Layer 0: basin-scale OWI background spanning the whole run.
      # Already a corrected surface wind, so opt out of the land
      # reduction. Present from t=0, so it needs no activation ramp.
      - format: owi_ascii
        wind_reduction: false
        filenames:
          - pressure: "basin.pre"
            wind: "basin.wnd"

      # Layer 1: regional GFS on its own (coarser) cadence. Marine
      # 10 m wind: leave wind_reduction at its default (true). Its
      # files start a day into the run, so fade it in over 3 hours
      # instead of stepping in at its first snapshot.
      - format: grib
        product: gfs
        ramp: 3h
        filenames: ["gfs_f000.grib2", "gfs_f003.grib2", "gfs_f006.grib2"]

      # Layer 2: fixed HAFS parent domain around the storm basin.
      - format: grib
        product: hafs
        ramp: 6h
        filenames: ["hafs_parent_f00.grib2", "hafs_parent_f01.grib2"]

      # Layer 3: the moving HAFS storm nest, produced only around
      # landfall. The reader detects the translating grid from the
      # files; the 6 h ramp cross-fades its arrival and departure.
      - format: grib
        product: hafs
        ramp: 6h
        filenames: ["hafs_nest_f00.grib2", "hafs_nest_f01.grib2"]

    # Innermost: the parametric vortex, layered over whatever the
    # domains produce. Two storms, applied in list order (the later
    # track wins where they overlap). The single vortex ramp fades
    # EACH storm against its own track's first/last snapshot times,
    # so the two storms fade in and out independently.
    vortex:
      model: gahm
      tracks:
        - "storm_a.dat"
        - "storm_b.dat"
      ramp: 6h
      wind_reduction: true

Three kinds of ramp appear in that file, and they never share a clock:

Key	Clock	What it fades
meteorological.ramp	The model cold start (one reference time for the run)	The entire composed field – wind, stress, and the pressure anomaly about the ambient background – during spinup (Combining Meteorological and Tidal Forcing)
domains[i].ramp	Each of that entry’s data windows on the union time axis	That source’s arrival and departure, cross-faded against the field composed without it (Activation Ramp)
vortex.ramp	Each storm track’s own first and last snapshot times	Each storm’s overlay weight, independently per track (Parametric Vortex (GAHM))

Every gridded source above may also run on its own cadence – 15-minute OWI, 3-hourly GFS, hourly HAFS – with no resampling (Domain Time Windows), and the per-node land reduction follows whichever entry wins each node (Per-entry wind_reduction).

Domain Time Windows

Domains need not cover the same period or share a time step: each domain (an OWI ASCII file pair, an OWI NetCDF group, or a domains: list entry) carries its own time axis covering only the window it has data for. A typical layout pairs a basin-scale grid spanning the whole simulation with a fine storm grid produced only around landfall.

Cocoa builds the sorted union of every domain’s own times. At a union time that is exactly one of a domain’s own samples, that domain’s data is used verbatim (an exact-match short-circuit); at a union time strictly between two of a domain’s own samples, its pressure and wind are linearly interpolated from that domain’s own bracketing snapshots. A domain’s sample rate therefore never has to match another’s – an hourly HRRR nest composes with a 3-hourly GFS background with no configuration needed. A moving (storm-following) domain’s grid position is interpolated the same way, so a fine nest’s origin moves continuously between its own reported positions even while a coarser domain refills on a different cadence.

Outside its own window a domain is inactive: it contributes nothing, and its coverage falls through to the next outer domain or to the ambient background (background pressure, zero wind). By default the window edge is hard: the piecewise-linear interpolation between snapshots brings the domain in over a single data interval, which for a coarse cadence (a 3- or 6-hourly nest) can still look like a step to the rest of the composed field. Each domains: entry (and the vortex: block) can smooth this into an explicit, configurable-duration cross-fade with its own ramp key – see Activation Ramp – off by default, so the hard edge described above is unchanged unless configured.

Each domain’s data window is reported in the log at startup (debug level), and during the run each domain announces when the simulation first enters and leaves its window. A domain whose window never overlaps the simulated period produces a warning. The same announcements are made for each parametric vortex track (see Parametric Vortex (GAHM)).

Drag Law

The wind drag law used to convert 10-m wind velocity to surface stress is selected with forcing.meteorological.drag_law. The only accepted value is garratt (the Garratt 1977 bulk drag law), which is also the default:

forcing:
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "data.nc"
    drag_law: garratt   # default; only accepted value

Scale Factors

Cocoa applies configurable scale factors to convert raw file values to SI units (Pa for pressure, m/s for wind):

Parameter	Default	Description
pressure_scale	100.0	Multiplier for pressure values. Default converts hectopascals (hPa) to pascals (Pa).
wind_scale	1.0	Multiplier for wind velocity values. Default assumes m/s input.

Example for a dataset with pressure in Pa and wind in knots:

forcing:
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "data.nc"
    pressure_scale: 1.0        # Already in Pa
    wind_scale: 0.514444       # Convert knots to m/s

File Format Details

CF-Compliant NetCDF

The CF NetCDF reader expects a regular rectangular grid with the following structure:

Required dimensions and variables:

dimensions:
  time = UNLIMITED ;
  lat = <NY> ;
  lon = <NX> ;

variables:
  double time(time) ;
    time:units = "hours since 2026-01-01 00:00:00" ;
    time:calendar = "standard" ;
  double lat(lat) ;
  double lon(lon) ;
  float mslp(time, lat, lon) ;    // Surface pressure
  float wind_u(time, lat, lon) ;  // Eastward 10-m wind
  float wind_v(time, lat, lon) ;  // Northward 10-m wind

• The time variable must use CF-compliant units (e.g., "hours since ..." or "seconds since ...")

• Longitude and latitude are 1D coordinate arrays defining the grid

• Global grids (360° longitude span) are detected automatically; the reader handles wrap-around

OWI ASCII

OWI ASCII files use a fixed-width format with one pressure file and one wind file per domain.

File header (line 1):

Oceanweather WIN/PRE Format                            YYYYMMDDHH  YYYYMMDDHH

The header line contains a format identifier, the start time, and the end time.

Snapshot header (one per time step):

iLat= NNNiLong= NNNdx=D.DDDDdy=D.DDDDSWLat=LL.LLLLLSWLon=LLL.LLLLDT=YYYYMMDDHHNN

Field	Position	Description
iLat	cols 5-8	Number of latitude points (NY)
iLong	cols 15-18	Number of longitude points (NX)
dx	cols 22-27	Longitude grid spacing [degrees]
dy	cols 31-36	Latitude grid spacing [degrees]
SWLat	cols 43-50	Southwest corner latitude [degrees]
SWLon	cols 57-64	Southwest corner longitude [degrees]
DT	cols 68-79	Snapshot time (YYYYMMDDHHNN)

Data records:

Following each snapshot header, NX * NY floating-point values are written in free format. Pressure files contain one field per snapshot; wind files contain two fields (eastward U, then northward V).

OWI NetCDF

OWI NetCDF files use NetCDF groups to organize multi-domain data:

root group:
  dimensions:
    time = UNLIMITED ;
  variables:
    double time(time) ;

group: domain_0 {
  dimensions:
    node = <NX*NY> ;
  variables:
    double longitude(time, node) ;
    double latitude(time, node) ;
    float pressure(time, node) ;
    float wind_u(time, node) ;
    float wind_v(time, node) ;
}

group: domain_1 {
  ...
}

The per-domain coordinate arrays (longitude, latitude) vary with time, enabling vortex-tracking grids that follow a storm center. For stationary grids, the coordinates are the same at each time step.

Combining Meteorological and Tidal Forcing

Meteorological forcing is commonly used alongside tidal forcing for storm surge simulations. Both forcing types operate independently and can be configured together:

forcing:
  ramp:
    enabled: true
    duration: 2d
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "hurricane_met.nc"
  tide:
    potential:
      enabled: true
      type: astronomical
    boundary:
      enabled: true
      constituents:
        - name: M2
          frequency: 1.405189028e-04
          nodal_factor: 0.964
          equilibrium_arg: 23.06
          boundary_values:
            - { amplitude: 0.50, phase: 340.0 }

The ramp function is applied independently to tidal and meteorological components. If the simulation starts from a pre-spun-up tidal state, a shorter meteorological ramp avoids unnecessary delay:

forcing:
  ramp:
    enabled: true
    duration: 5d                           # Long tidal spinup
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: "hurricane_met.nc"
    ramp:
      enabled: true
      duration: 12h                          # Short met ramp

This forcing.ramp / forcing.meteorological.ramp pair is a cold-start spinup ramp: it fades the whole composed field in exactly once, from the start of the run. It is distinct from the per-source activation ramp (Activation Ramp), which fades one domains: entry or the vortex: block in and out mid-run, at its own data window’s edges, and can recur every time that source’s window opens or closes. The two compose independently – a source with its own ramp still passes through the cold-start ramp on top.

Wind Reduction

Cocoa supports two complementary wind reduction mechanisms that modify wind velocity over land before computing wind stress. These are particularly important for storm surge simulations where overland wind fields need to account for surface roughness and canopy sheltering.

Both reductions read spatially-varying data from the mesh file (configured in the physics section; see Wind Reduction) and are applied per meteorological source at composition time, gated by each source’s wind_reduction flag (see Per-source application). A source’s wind is corrected before it is blended into the combined field, so co-located sources can be treated differently – e.g. a parametric vortex reduced to a land surface wind while a background GFS field, already a surface wind, is left unreduced.

Directional Roughness Reduction

When surface_directional_roughness: mesh is specified, Cocoa applies a direction-dependent wind reduction based on upwind land surface roughness. This reproduces ADCIRC’s ApplyDirectionalWindReduction subroutine.

The algorithm:

1. Determines the wind direction at each node

2. Interpolates the land roughness length from the two nearest directional bins (12 bins at 30-degree spacing)

3. Computes the marine roughness from the current drag coefficient

4. Applies a reduction factor based on the roughness ratio

An overland flooding correction reduces the effective land roughness when the total water depth exceeds twice the minimum depth threshold (h0), accounting for the reduced influence of land surface features when they are submerged.

Configuration:

physics:
  surface_directional_roughness: mesh   # Read from mesh file

The mesh file must contain the surface_directional_effective_roughness_length variable with shape [num_nodes, 12]. See Mesh Preparation for details on the mesh variable format.

Canopy Coefficient

When surface_canopy_coefficient: mesh is specified, wind velocity components are multiplied by a per-node canopy sheltering factor. This represents wind attenuation under forest canopy.

Configuration:

physics:
  surface_canopy_coefficient: mesh   # Read from mesh file

The mesh file must contain the surface_canopy_coefficient variable with shape [num_nodes]. Values must be binary – exactly 0 (wind stress fully suppressed at that node) or 1 (no canopy effect). A fractional value fails loud at startup: the per-source model folds the canopy factor into the wind before the quadratic drag law, which is exact only for a 0/1 mask.

Per-source application

Each meteorological source carries a wind_reduction flag that gates both corrections (directional roughness and canopy) for that source. The flag defaults to true (opt-out): whatever wind field is present is reduced unless a source opts out. Set it to false for a source that should not be reduced.

forcing:
  meteorological:
    enabled: true
    format: cf_netcdf
    filename: gfs.nc
    wind_reduction: false      # GFS is already a surface wind -- do not reduce
    vortex:
      model: gahm
      tracks: [storm.dat]
      wind_reduction: true     # reduce the parametric vortex to a land wind

The gridded (reader) flag also governs the reader’s moving storm grids. The flags only have an effect when the mesh carries the corresponding nodal attributes; without them, no reduction is applied regardless of the flag.

With a domains: list (Composing Multiple Sources), each entry carries its own wind_reduction instead of one flag for the whole reader – see Per-entry wind_reduction for a worked example of a marine nest reduced over an already-corrected background.

Processing Order

The corrections are applied per source, as each source is composited into the combined field:

1. Directional roughness (if the source is flagged and the attribute is present): reduce the source’s wind based on upwind land roughness, using the marine drag coefficient computed from the source’s own (pre-reduction) wind speed.

2. Canopy coefficient (if flagged and present): multiply the source’s wind components by the binary canopy factor.

The corrected sources are then blended. Once the combined field is assembled, the remaining steps run once:

3. Ramp: scale the blended wind by the meteorological ramp factor.

4. Wind stress: compute kinematic stress from the ramped wind using the Garratt drag law.

Because the corrections are folded into each source’s wind before the drag law, a node covered by a single source produces the same stress as a post-blend reduction would; only feather/blend rings between a reduced and an unreduced source differ.

Diagnostics

Cocoa tracks peak wind speed and minimum atmospheric pressure at each node over the course of the simulation. These peak values are written to the output file alongside the standard elevation and velocity fields, which is useful for post-processing storm surge maxima.

Complete Example

A storm surge simulation with OWI ASCII forcing on a regional mesh:

mesh:
  filename: "gulf_mesh.nc"
  projection:
    type: "EquidistantCylindrical"
    center: [-90.0, 25.0]

simulation:
  start_time: 2026-08-01
  end_time: 2026-08-10
  time_step: 10s

initial_conditions:
  water_level: 0.0

physics:
  manning_n: mesh
  tau0: mesh
  cf_lower_limit: 0.001
  smagorinsky_coefficient: 0.2

numeric:
  solver: explicit
  gwce_coefficients: [0.0, 1.0, 0.0]

forcing:
  ramp:
    enabled: true
    duration: 2d
  meteorological:
    enabled: true
    format: owi_ascii
    filenames:
      - pressure: "basin_scale.pre"
        wind: "basin_scale.wnd"
      - pressure: "inner_region.pre"
        wind: "inner_region.wnd"
  tide:
    potential:
      enabled: true
      type: astronomical
    boundary:
      enabled: true
      constituents:
        - name: M2
          frequency: 1.405189028e-04
          nodal_factor: 0.964
          equilibrium_arg: 23.06
          boundary_values:
            - { amplitude: 0.50, phase: 340.0 }
            - { amplitude: 0.48, phase: 342.0 }

output:
  filename: "surge_output.nc"
  step_interval: 60

diagnostics:
  screen_interval: 3600