ADCIRC2D+

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ADCIRC2D+, is a mode which allows for the inclusion of baroclinic (density-driven) effects into the traditionally barotropic (depth-averaged) system. This is achieved by coupling ADCIRC with temperature and salinity fields from a coarser-resolution Ocean General Circulation Model (OGCM), enabling improved representation of long-term sea level dynamics. Description of results using this capability are available in (Pringle et al. 2019) [1] and (Blakely et al. 2024) [2]

Version

Some of the capability of ADCIRC2D+ is available in v55; however improvements and modifications were made that are present only in v56 and above.

Governing Equations

ADCIRC2D+ solves a modified form of the shallow water equations using the Generalized Wave Continuity Equation (GWCE). The momentum equation includes baroclinic pressure gradient and internal wave drag terms:

\[\frac{\partial \mathbf{U}}{\partial t} + \mathbf{U} \cdot \nabla \mathbf{U} + f \mathbf{k} \times \mathbf{U} = - \nabla\left( \frac{p_s}{\rho_0} + g(\eta - \eta_{EQ} - \eta_{SAL}) \right) - \frac{\text{BPG}}{H} + \frac{\boldsymbol{\tau}_s - \boldsymbol{\tau}_b}{\rho_0 H} - \gamma_D \mathbf{C} \mathbf{U}\]

Where:

  • BPG is the depth-integrated baroclinic pressure gradient

  • γD is a scaling parameter for internal wave drag

  • C is the internal wave drag tensor

The baroclinic pressure gradient is computed as:

\[\text{BPG} = \frac{g}{\rho_0} \left[ \int_{-h}^{0} \nabla \left( \int_{0}^{z} (\rho - \rho_0) \, dz' \right) dz + \eta \nabla[\eta(\rho_s - \rho_0)] \right]\]

The internal tide drag tensor is scaled using a dissipation ratio \(\gamma_D\) defined by:

\[\gamma_D = \frac{\text{Diss}_{\text{tidal}}}{\text{Diss}_{\text{total}}} = \frac{\mathbf{U}_{\text{tidal}} \cdot \mathbf{C} \cdot \mathbf{U}_{\text{tidal}}}{\mathbf{U} \cdot \mathbf{C} \cdot \mathbf{U}}\]

Tidal velocity is estimated using a lagged 25-hour filter and removing the resultant mean signal from the total velocity. This ensures that dissipation from internal wave drag occurs predominantly at tidal frequencies, preserving tidal fidelity in the coupled model.

densityControl Namelist

Activating ADCIRC2D+ is accomplished through the use of the densityControl namelist.This namelist has the following options and defaults (denoted by (D))

  • densityRunType (string)

    'none' (D): By default ADCIRC2D+ is not active. If densityRunType='none' the rest of the namelist values do not matter. 'prognostic' 'diagnostic'

  • densityFileName (string): The name of the file from which pre-computed baroclinic pressure gradients and stratification information are read.

  • densityTimeIterator (integer): The stride used when reading in densityFileName. For example, if densityFileName contains data at hourly timesteps and densityTimeIterator=2 then data will be read in every two hours.

  • densityForcingType (string)

    'SigmaT' 'Salinity' 'Temperature' 'SalinityTemperature' 'Baroclinicgradients' 'BaroclinicgradientsDispersion' 'Buoyancyfrequencies' 'BCForcingOnADCIRCGrid' 'BuoyancyFrequenciesOnGrid'

Within the ADCIRC code, the various settings of these namelist parameters is internally translated to an IDEN value which determines the baroclinic terms in the shallow water equations to include.

References