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# Difference between revisions of "IM"

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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.

## Shortcut IM Values

The most common combination of options used for simulation can be specified through the shortcut values

## 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 formulation of certain terms or integration methods in the GWCE or momentum equations. The available options for each digit are specified below:

Value Digit 1: 2DDI/3D, Lateral Stress in GWCE[1] 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 Non conservative Integration by parts, velocity-based Non conservative Corrected Consistent (semi-implicit), barotropic
2 2DDI, 2-part flux-based Conservative form 1 Integration by parts, flux-based Conservative form 1 Original Lumped (explicit), barotropic
3 2DDI, 2-part velocity-based Conservative form 2 Integration by parts, velocity-based symmetrical Conservative form 2 - Lumped (explicit), baroclinic (not yet implemented in ADCIRC release version)
4 2DDI, 2-part flux-based symmetrical - Integration by parts, flux-based symmetrical - - -
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 uses default options (same as IM = 0), but simulates in explicit mass-lumping mode. This is a useful alternative to the (default) semi-implicit consistent GWCE mass matrix mode, which requires a matrix solve increasing computational time and memory compared to the explicit mass-lumping mode, which as about twice as fast and scales to fewer grid nodes per computational core.[2]

## References

1. K.M. Dresback, R.L. Kolar, R.A. Luettich, Jr., 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, 2005: pp. 399–418. doi:10.1061/40876(209)23.
2. 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