IM is an important parameter in the fort.15 file that defines numerical model formulation and dimension. Among other things,
IM specifies whether ADCIRC is solved in two-dimensional depth-integrated (2DDI) or in three-dimensions (3D), solution of the governing equations is semi-implicit or explicit in time, and whether the model formulation is barotropic or baroclinic. Popular values for 2D barotropic ADCIRC include
IM=111112; users should be aware that the
A00, B00, C00 coefficients must be specified differently in these two cases.
Default IM Values
Default simulation option combinations can be specified through single or double digit values, some of which are shortcuts to the six-digit codes described in the next heading.
IM values are specified in the table below and in the following section on 6-digit values:
|IM Value||Six-digit Equivalent||Description|
|1||611111||Barotropic 3D velocity-based momentum|
|2||-||Barotropic 3D stress-based momentum|
|10||-||Barotropic 2DDI with passive scalar transport|
|11||-||Barotropic 3D velocity-based momentum with passive scalar transport|
|21||611113||Baroclinic 3D velocity-based momentum|
|30||-||Baroclinic 2DDI with passive scalar transport|
|31||-||Baroclinic 3D velocity-based momentum with passive scalar transport|
Note that all default
IM values employ the semi-implicit consistent GWCE mass matrix solver. It has less numerical error and tends to be more stable than the explicit mass-lumping approach at the expense of computational time and memory.
Six-digit IM Codes
For fine-grained control of various options six-digit codes for
IM can be specified. Each digit represents a specific option regarding the dimension and the formulation of certain terms or integration methods in the GWCE or momentum equations.
The available options for each digit are specified below, with the first digit being the left-most. The internal flags that are set are listed to help users dig through the code.
|Value||Digit 1: 2DDI/3D, Lateral Stress in GWCE||Digit 2: Advection in GWCE||Digit 3: Lateral Stress in Momentum||Digit 4: Advection in Momentum||Digit 5: Area Integration in Momentum||Digit 6: GWCE Mass Matrix, Barotropic/Baroclinic|
|1 (default)||2DDI, Kolar-Gray flux-based
||Integration by parts, velocity-based
||Consistent (implicit for linear part of gravity wave term), barotropic|
|2||2DDI, 2-part flux-based
||Conservative form 1
||Integration by parts, flux-based
||Conservative form 1
||Lumped (explicit), barotropic|
|3||2DDI, 2-part velocity-based
||Conservative form 2
||Integration by parts, velocity-based symmetrical
||Conservative form 2
||-||Consistent (implicit for full gravity wave term), barotropic|
|4||2DDI, 2-part flux-based symmetrical
||-||Integration by parts, flux-based symmetrical
||-||-||A value of 4-6 does the same as 1-3 (same order) but in baroclinic mode|
|5||2DDI, 2-part velocity-based symmetrical
||-||2 Part, velocity-based (not implemented)
|6||3D, Kolar-Gray flux-based
||-||2 Part, flux-based (not implemented)
A common code combination is
IM=111112, which is identical to the default
111111 (same as
IM=0), but simulates in explicit mass-lumping mode. Note that
A00, B00, C00 must be set to
0.0 1.0 0.0 when in this mode. Lumped explicit mode is a useful alternative to the (default) semi-implicit consistent GWCE mass matrix mode, because the latter requires a matrix solve that increases computational time and memory. By comparison, the explicit mass-lumping mode is about twice as fast and scales to fewer grid nodes per computational core. Moreover, for model setups that are sufficiently resolved in space and time, differences in the solution between approaches should be small. Though, many users have reported somewhat lower stability in lumped explicit mode.
The most recent version (55+) also has an option that improves the (default) semi-implicit consistent GWCE mass matrix mode to compute the complete (total depth) gravity wave term (free surface gradient) implicitly; toggled by setting IMDigit-6 to 3. The default version (IMDigit-6=1), only computes the initial still water depth component of the free surface gradient implicitly, which might make it more susceptible to CFL violations in shallow depths and can encounter Matrix diagonality issues overland where the initial still water depth is negative.
- K.M. Dresback, R.L. Kolar, R.A. Luettich, Jr. (2005). On the Form of the Momentum Equation and Lateral Stress Closure Law in Shallow Water Modeling, in: Estuar. Coast. Model., American Society of Civil Engineers, Reston, VA, 399–418. doi:10.1061/40876(209)23
- K.M. Dresback, R.L. Kolar, J.C. Dietrich (2005). On the Form of the Momentum Equation for Shallow Water Models Based on the Generalized Wave Continuity Equation: Conservative vs. Non-Conservative. Advances in Water Resources, 28(4), 345-358. doi:10.1016/j.advwatres.2004.11.011
- S. Tanaka, S. Bunya, J.J. Westerink, C. Dawson, R.A. Luettich, Scalability of an Unstructured Grid Continuous Galerkin Based Hurricane Storm Surge Model, J. Sci. Comput. 46 (2011) 329–358. doi:10.1007/s10915-010-9402-1