Currently employed at Para//ab, BCCS, where I mostly work on projects related to ocean modeling, but also with parallel computing and optimization of applications for supercomputing, e.g. climate models or other projects where Para//ab participate.

The equations we try to solve, also called "the model", is a system of partial differential equations that describes the dynamics of the ocean. This system is very complex. To get anywhere we do as most scientists; divide the problem into smaller subproblems that we know how to deal with efficiently. A "numerical model" is a result of such a process, it is a reformulation of the problem into a form suitable for solution by computers. One example is the Bergen Ocean Model that is developed and frequently used here in Bergen - look at this page or this page for graphical presentations of some of our results. Doing lots of simplifications and assumptions comes at a cost; we introduce error. Our primary concern is called numerical error, which we try to identify and remove/minimise. The underlying physical model has of course also weaknesses, and it is not always obvious if errors are numerical or due to the model itself. On our way we utilize tools from many fields of science and also try to pick up the essential ideas. Examples of such fields are e.g. oceanography, linear algebra, programming, visualisation and parallel computing.

Some of my recent activities are reflected in:

- Thiem, Ø., Avlesen, H., Alendal, G. and Berntsen, J. (2005): Internal waves and internal solitones shoaling and breaking along a continental slope. Report No. 14, BCCS Technical report series, University of Bergen, Norway.
- Øyvind Thiem, Helge Avlesen og Guttorm Alendal: Simulering av strømforhold i og rundt Vatlestraumen. August 2005. Online report
- Helge Avlesen. On the parallelization of a non hydrostatic, sigma co-ordinate ocean model. NOTUR advanced user support project 2004. PDF
- Avlesen, H. and Berntsen,J. (2004) A 60 day hindcast study of the Norwegian Seas with focus on Ormen Lange using 20km and 4 km resolution. Report No. 10, BCCS Technical report series, University of Bergen, Norway. Download (830kB)
- Avlesen, H. Berntsen,J. and Furnes,G. (2002) On the current conditions along the Ormen Lange pipeline path during an extreme, idealized storm passage. Report Nr. 238, Department of Informatics, University of Bergen, Norway. Download (830kB)
- Avlesen, H. and Berntsen,J. (2001) Flow over rough topography. A preliminary study with high resolution topography at Ormen Lange. Technical report No 209, The Nansen Environmental and Remote Sensing Center. Download (1307kB)
- Avlesen, H., Berntsen, J. and Espelid, T.O. (2001) A convergence
study of two ocean models applied to a density driven flow.
*International Journal for Numerical Methods in Fluids,***36:**639-657. - Avlesen, H. (2000) On the choice of numerical algorithms for ocean modeling. Ph.D. thesis, Department of Informatics, University of Bergen.
- Avlesen, H., Berntsen, J. (2000) A 2km resolution study of the Skagerak circulation, with a comparison of three internal pressure schemes Report Nr. 199, Department of Informatics, University of Bergen.
- Avlesen, H., Berntsen, J. and Espelid, T.O. (1998) A convergence study of two prognostic, sigma co-ordinate ocean models on a density driven flow in a quadratic basin. Report Nr. 157, Department of Informatics, University of Bergen. Download (290kB)
- Avlesen,H. (1998) A study of two new splitting methods for the gravity part of the shallow water equations. Report Nr. 145, Department of Informatics, University of Bergen. Download (164kB)

I finished my studies for the Dr degree modeling the
Skagerak area, some animations can be found
**here**.

In the present paper the domain of validity of implicit methods for solving the shallow water equations is investigated for a baroclinic benchmark, using both implicit and explicit versions of a sigma coordinate ocean model.

It is also demonstrated that the commonly used boundary conditions for the water elevation in ocean models, using semi-implicit time stepping methods, may not be in agreement with the thermal wind equations for geostrophic flow.

We find that the implicit method performs well with high spatial resolution, but on coarse grids the explicit method give much more accurate results. We believe that the enhanced resolution improve the results for the implicit method due to a better representation of the surface gravity waves, and that it also reduce the above mentioned problem with the boundary condition.