cu_physics, cugd_avedx settings for Grell-3D option (5)

[Sarath Guttikunda]

Instructions in the manual for G3 are the following

Chapter 5: WRF Model

Grell 3D is an improved version of the GD scheme that may also be used on high resolution (in addition to coarser resolutions) if subsidence spreading (option cugd_avedx) is turned on (5).

cugd_avedx	1​	(default) number of grid boxes over which subsidence is spread, for large grid distances
	3​	for small grid distances (DX < 5 km), G3 only

Since cugd_avedx is set as a single number, if set to 3 -- will it be automatically used for nests with dx < 5km and other nests use the default 1?

I have 3-nested domains @ 27, 9, and 3 km
cu_physics = 5,5,5
cugd_avedx = 3

Or

Should this be set to default
cugd_avedx = 1
and program automatically uses 3 for the 3rd nest with dx=3km?

 

[kwerner]

Hi Sarah,
This is a good question. Unfortunately because the parameter only allows a single entry, whichever option you choose will be used on all domains. I spoke to our physics specialist about this and they recommend that you try both to see which gives the best result. You can just do a quick, simple test for this if needed. They suspect option 3 may be the better choice.