global_ec

IceBin was originally designed to define elevation classes for a local ice sheet, eg. Greenland or Antarctica. global_ec uses IceBin to define elevation classes for global ice cover.

global_ec works with four grids, all of them latitude/longitude type grids:

  1. A: Atmosphere grid — defined by the GCM
  2. I: Ice grid — high-resolution
  3. E: Elevation grid — defined by interactions between A and I.

To define global elevation classes, global_ec requires the following input:

  1. <mask filename>: A global map of ice cover and elevation (on the ice grid), with each grid cell either being entirely covered by ice or entirely ice-free. It can be produced with the etopo1_ice_ command.

global_ec produces a single file containing the six regridding matrices between A, I and E.

Command Line

The global_ec command takes the following arguments:

$ global_ec <atm. grid name> <ice grid name> <display ice grid name>
            <mask filename> <focean filename>
            -o <output filename>

For example:

$ global_ec g1qx1 g1mx1m ghxh etopo1_ice_g1m.nc topoo.nc -o global_ec.nc

Note

The <focean filename> argument is only used if mismatched regridding is turned on. Otherwise, any filler is fine: foo, _, etc.

Grid Names

The grid names are drawn from a list of pre-defined grids that global_ec is able to use (others may be added in the future). They are:

  • g1mx1m: 1-minute (eg: ETOPO1)
  • g2mx2m: 2-minute (eg: ETOPO2)
  • g10mx10m: 10-minute
  • ghxh: \(\frac{1}{2} \times \frac{1}{2}\) degree
  • g1x1: 1 degee
  • g1qx1: \(1 \frac{1}{4} \times 1\) degree (eg: ModelE ocean)
  • g2hx2: \(2 \frac{1}{2} \times 2\) degree (eg: ModelE atmosphere)
  • g5x4 : \(5 \times 4\) degree (eg: Model3)

Elevations

Elevation classes are defined at a discrete set of elevations; by default, they go from \(-100 m\) to \(5100 m\), every \(200 m\). This is specified on the command line with:

Note

It is important that the highest elevation class is higher than the highest ice in your input file.

Radius of Earth

By default, the radius of the Earth is set to be the same as in ModelE (\(6.371 \times 10^6 m\)). This can be changed with the -R flag.

NetCDF Variable Names

By default, global_ec looks for the variables FGICE1m and ZICETOP1m from the mask file. global_ec can be directed to look for these quantities under different variable names, using the command line arguments --elev and --mask.

Mismatched Regridding

Due to properties of the ModelE ocean, the ice extent “seen” by ModelE might not be the same as is provided by the high-resolution ice grid. This is because ModelE must “round” ocean grid cells to be all ocean or all non-ocean; and also because ice extent might chnage over time, whereas the ModelE ocean cannot.

The --mismatched flag enables mismatched regridding for global_ec. If it is turned on, the behavior of global_ec changes:

  1. Instead of providing the atmosphere grid on the command line, the user must provide the ocean grid, which will be 1/2 the resolution of the desired atmosphere grid. All core calculations are done on the ocean grid, and then regridded to the atmosphere grid at the last step.
  2. The <focean filename> parameter is required: A map of which ocean grid cells are used by the ModelE ocean (ocean grid), as produced by the make_topoo_ command.

Logistical Issues

The computations involved in global_ec can require large amount of RAM, depending on the size of the ice grid and the number of ice-covered grid cells. For example, the ETOPO1 grid contains 233 million grid cells, of which 26 million are covered in ice. global_ec is not able to run with this much ice, on a typical computer with 16Gb RAM.

Therefore, global_ec works by splitting up the task into smaller sub-matrices covering portions of the globe; and then combining the matrices at the end. The domain decomposition process is independent of ice location or extent, and will work for exoplanets too.

When global_ec is run by the user, it examines the input files to plan its subtasks. It then writes the plan into a Makefile called <output>.nc.mk. For example, the following command line actually produces the file global_ec.nc.mk:

$ global_ec g1qx1 g1mx1m ghxh etopo1_ice_g1m.nc topoo.nc -o global_ec.nc

The computation is then completed with the command:

$ make -f global_ec.nc.mk

If you are using make to automate your workflow, the following placed in your makefile will be sufficient:

# The six regridding matrices (compressed) for global ice
global_ec.nc.mk : topoo.nc etopo1_ice_g1m.nc
    global_ec g1qx1 g1mx1m ghxh etopo1_ice_g1m.nc topoo.nc -o global_ec.nc

global_ec.nc : global_ec.nc.mk
     make -f global_ec.nc.mk