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# IM

**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 2DDI or 3D, solution of the governing equations is implicit or explicit in space, 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 in 2DDI simulations 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. Six-digit codes cannot be used to specify 3D simulations. Instead, use the shortcut values specified above.

The available options for each digit are specified below:

Digit 1 | Digit 2 | Digit 3 | Digit 4 | Digit 5 | Digit 6 |
---|---|---|---|---|---|

row 1, cell 1 | row 1, cell 2 | row 1, cell 3 | row 1, cell 4 | row 1, cell 5 | |

row 2, cell 1 | row 2, cell 2 | row 2, cell 3 | row 2, cell 4 | row 2, cell 5 |

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 (about twice as slow and reduces scalability^{[1]}) and memory compared the explicit mass-lumping mode.

- ↑ 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