Designing the GCOMS grid

Model domain

The GcOMS model will cover all the major shelf seas in the World's ocean. Shelf seas are defined as the shallow areas extending from the coastline to the shelf break, where the slope of the ocean floor is maximum. Continental shelves have a mean depth ranging from 100 to 1000 meters. Shelf breaks exhibit a slope around 3%. Oceanic plains start at the bottom of shelf breaks and are characterized by depths larger than 3000 meters.
To set the domain extent of GCOMS, we start from a bathymetry dataset covering the global ocean ( GEBCO). In this 2D field, we select the 800-meter isobath, which corresponds to the average depth of the shelf break.

Then this domain is extended to some distance toward the deeper ocean. This second operation ensures that the model domain includes the whole area of all the shelf seas, whatever are their depth, width and shape. A distance of 200 km is found to be necessary and sufficient for this purpose.
We extend the model domain to the deep ocean areas which are enclosed (such as the Gulf of Mexico, the Bafin Bay, some parts od the Indonesian Sea, the Japan Sea, etc.). Conversely, we remove the shallow areas around small and isolated islands (not connected to any continental shelf). For instance, we remove the Hawai and Kerguelen archipelagos. But we keep the islands within the Indonesian and Caribbean archipelagos, as well as New Zealand, Madagascar, etc.
Thus, the mean depth of the resulting global domain is ~ 1,290 m and its maximum depth is ~ 9,650 m.
The global domain represents approximately 16% of the global ocean surface area. As the model grid has a 1/10 degree resolution, the shelf sea model will contain approximately 1,100,000 points.

Domain decomposition

Thereafter, the global model must be decomposed into smaller suddomains, each of which will be simulated separately. The whole domain can not be integrated on a single computer, because of computational limitations. Nevertheless, during the global simulation, the subdomains will exchange information at their boundaries. Thus the final results would be identical to those obtained from a single domain computation. In order to reduce the amount of communications across the subdomain boundaries, their length should be as short as possible (e.g. lower than 100 points). Meanwhile. the surface area of each individual subdomain should be small enough to be suitable for a single computer (e.g. of the order of 200x200=40,000 points).
These technical constraints, along with some scientific considerations (dynamics, primary productivity, geometry, etc.), lead to decompose the global shelf sea domain into 52 subdomains.

Finally, each subdomain is attributed a ocean-land mask, a bathymetry field and a list of open boundary points.
The mean size of the subdomains is a rectangle area containing 326x183 ~ 56,200 points. The shelf seas cover an average of 19,000 points (33%) inside these rectangular domains. The open boundaries communicating with the deep ocean have an average length of 530 points. The deep ocean areas represent ~ 18,000 points on average (29%). Thus, the global model domain contains approximately 38% of land points. The mean depths of the subdomains range from 53 m (Baltic Sea) to 2,878 m (Peruvian shelf).

-- SylvainMichel - 13 Oct 2006