I'm doing extremely large area forecasts (think continental), of quite high resolution, 9km grid nesting 1:3 for a 3km inner grid, with the inner grid covering the whole continent.
How large should the outer grid be? I know the rule of thumb is the inner grid should be 1/3rd the size, but this is pretty excessive for grids of the size I'm working with.
I also understand I should avoid having the borders from irregular terrain, is this primarily for the inner grid, or the outer grid too? Is it worth compromising the size of the outer grid to preference flatland/ocean over terrain?