HOWTO open boundaries

GETM is not limited to have the open boundaries at the edges of the topo.nc but defines boundaries as western, northern, eastern and southern (not the real geographic orientation, but orientation on a printout) boundary sections which can be locatede inside the domain. The boundary sections are defined in the 'bdyinfo.dat' file.

a) The principle ordering of the boundaries is  (notice order of sequence!) western, northern, eastern and southern boundaries.

b) The sections within one direction are ordered after the start index.

c) The sections must not overlap. 

d) If a point belongs to both, a 'vertical' and a 'horizontal' section, it belongs by definition to the 'vertical' section.

e) The first index in the section specification is always <= the last index for all sections and sides 

The bdyinfo file has at least four lines (one for each direction) with the number of boundary sections per direction (remember sequence: western, northern, eastern, southern); followed by one line per section giving a constant i-index for western/eastern direction (respectively a const. j index for northern and southern direction) followed by an including first and last j-index for the y axis  for western and eastern direction (respectively a first and last i-index for the x axis for northern and southern direction).
For the GETM devel release (!), there are two additional digits in each line, related to the type of boundary condition. The first additional integer specifies the 2D boundary type as follows:
1: zero gradient in elevation
2: Sommerfeldt radiation scheme (also known as Chapman) gradient in elevation
3: Clamped in elevation (used in the stable release)
4: Flather condition in elevation
The second integer should just be 0 for now.
The example of bdyinfo.dat (below) does not include the two digits for the type of boundary since this is still a possibly unstable option of the developers code (!).

Example:

 The figure shows the landmask of a setup's topo.nc (50x30 cells), where blue indicates water cells and brown indicates land. The white cells are unspecified in the model setup and MUST be masked out in 'mask.adjust' ; the transitions between water and unspecified cells are open boundaries as well as water cells at the border of the domain. In the figure, the black lines indicate the cell borders. The cells with i-index=1are located between the i=0 and i=1 lines. The water cell in the upper-right corner (j=30,i=38) is defined to be opened to the east in the bdyinfo.dat example (see below).

The bdyinfo.dat is:

1
6 9 12
2
30 11 20
30 26 37
2
50 5 11
38 21 30
0

 

 

 

 

 

commented:

1 - one western boundary section
6 9 12 - section located at i=6 and reaches from j=9 up to j=12
2  - two northern boundary sections
30 11 20 - section located at j=30 and reaches from i=11 up to i=20
30 26 37
2 - two eastern boundary sections
50 5 11
38 21 30
0 - no southern boundary sections