## Activity 3: More about nesting

Note: you should attempt this assignment only after you completed Assignment 2.

You may have noticed that in order to create nests, we also need to define two paramenters in the namelist called:

• i_parent_start
• j_parent_start

These two parameters refer to the position in your domain where you would like your nests to be. They refer to the grid point location of your nest relative to its parent. Just be careful when working with them, because you want one nest to be inside the other - without any overlapping - if they receive information from each other! The figure below exemplifies what it is meant by i_parent_start and j_parent_start:

Figure 1 - Illustration from the WRF Users' Guide, page 3-20, on the parameters used when creating a nest.

Activity 1
In the experiment you created in Activity 2, try to change the values for i_parent_start and j_parent_start. Can you create a setup where the nests are more centred over southern Norway?
Remember to: a) plot your result using plotgrids.ncl; b) run geogrid.exe and c) visualize the geo_em.d0X.nc files.

WARNING

You may have noticed that you should also define the number of grid points you want for a nest. This is done via the paramenters e_we and e_sn of your namelist.wps. In the WRF Users' Manual, page 3-20, it says:

to ensure that the upper-right corner of the nest's grid is coincident with an unstaggered grid point in the parent domain, both e_we and e_sn must be one greater than some integer multiple of the nesting ratio

This means that if your ratio is 3:1, then, your e_we and e_sn for your nest should be:

$(3*X)+1$

So, for example, if I want my nest to have 90 grid points by 90 grid points, then my e_we and e_sn values should be:

$(3*30) + 1 = 91$

So, we then have:

e_sn = 91
e_we = 91

Activity 2
If you have a ratio of 3:1, what should your e_we and e_sn values be if you want your nests to have:

a) 30 by 30 grid points?
b) 150 by 150 grid points?

Now, if you have a ratio of 4:1, what should these values be for:

a) 30 by 30 grid points?
b) 100 by 100 grid points?

You are welcome to post any questions on our Facebook group.