What's the difference between VAR and VAR_SSO

[Qianqian Huang]

Hi,
I noticed that there are VAR and VAR_SSO in geo_em*.nc file. May I ask, what's the difference between those two?
The description of VAR is "stdev of subgrid-scale orographic height", and the description of VAR_SSO is "Variance of Subgrid Scale Orography". So except that one is stdev and the other is variance, is there any other difference? I have calculated that VAR is not equal to sqrt(VAR_SSO).

Thanks in advance.
Qianqian

 

[Ming Chen]

These two variables have the same physical meaning. The only difference is that VARSSO is based on higher resolution terrain than VAR.

 

[Qianqian Huang]

Thank you for your reply.

I want to run some simulation with 1km and 333m resolution, using topo_wind. The default data path for VAR_SSO is varsso_10m written in GEOGRID.TBL. That is almost 20km resolution, a very low resolution for mesoscale simulation. I wonder why the default value setup like this? Is there any profound meaning in doing this? I noticed that the default value for other variable is high resolution data, for example HGT_M is 30sec.

So when I perform simulation with 1km and 333m resolution, should I use VARSSO_1sec ?

Thanks.
Qianqian

 

[Ming Chen]

Hi Qianqian,

VAR_SSO is used when topo_wind is activated to adjust surface winds. This scheme is more effective over areas with large variation of terrain. However, if your terain resolution is much higher than the resolution of VAR_SSO, I suppose you don't need to turn on topo_wind.

Another option is that you create your own high-resolution VAR_SSO dataset and use it WPS/WRF.