Nested Model Runs

A nested domain is used when you wish to have a finer-resolution domain used during a model run. The nested domain covers a portion of the parent domain, and is fully surrounded by the parent domain. It is driven along its lateral boundaries by the parent domain. Using a nest enables running at a higher-resolution without having uniformly high-resolution over a large domain (which can be VERY expensive), and prevents high resolution for a very small domain, with mismatched time and spatial lateral boundary conditions.

For these nested runs we will use the Hurricane Matthew case data (2016-10-06_00 to 2016-10-08_00).

The domain we are going to set up is shown below.

There are different ways to set up nested model runs:

Two-way Nesting

Two-way nesting is a method in which multiple domains at different grid resolutions are run simultaneously and communicate with each other. The coarser domain provides boundary values for the nest, and then the nest feeds its calculations back to the coarser domain. This option is turned on with the namelist.input variable feedback (=1).

a. Two-way nested run, with one input file
The preprocessing steps for this case will be similar to a single domain setup. The only difference takes place during the execution of wrf.exe, when multiple domains are initiated (at least 2).

b. Two-way nested run, with two input files
For this case the pre-processing programs need to be run to create extra input data for the nested domain(s), prior to running the wrf program. At the WRF model step, one has the choice to:

i. Use all the meteorological and static data for nested domains as input, or
ii. Use only the static data for nested domains as input.

One-way Nesting

One-way nesting is a method in which multiple domains at different grid resolutions are run simultaneously but the only communication is from the coarser domain, which provides boundary values for the nest. The nest does NOT feed its calculations back to the coarser domain. This method is run the same way as a 2-way nested run, but you will set feedback=0 in the namelist.input file.

c. One-way nesting using ndown
ndown is used to run one-way nested runs AFTER wrf has already been run for the mother domain.

Note: ndown is only recommended if you have already run a very long (several months/years) simulation for 1 domain, and then later decide that it would be more beneficial to have a nest at higher-resolution to observe, but is too expensive to run both domains simultaneously. If you have not already run the mother domain, then please use one of the 2-way methods, or the above-mentioned one-way method.