I want to setup a mesoscale weather prediction model (WRF or something else) to work with NCEP/GFS meteorological input data with resolution of 0.25deg. My question is how to choose a proper size for the parent domain, considering the resolution of the input data. In my case GFS resolution is around 20km in mid latitudes, is it necessary for my parent domain to be around this value - for example 18km (or 21km) and then use nested domains to go down to 2km.
Or i can start directly with parent domain size 2km? What will be the effect of this big gap 20km to 2km? Do i need a longer spinup so i can resolve clouds for example, if i choose to start straight with 2km resolution? Am i going to miss some kinetic energy from the coarse meteorological input because of dissipation not represented properly? I suppose computation time is not much more if i use 18>6>2km compared to starting directly with 2km. From my experience with WRF, finer domains take most of the time to be computed -ar. 85% in my specific setup.