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Chemistry and diff_6th_opt

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Short version of my question:
The WRF-Chem user guide recommends to set diff_6th_opt=0, but I think diff_6th_opt=2 would improve the wind fields in my simulation. So I would like to know why the user guide recommends this, and whether it's safe to turn it on in my case.

And here is the long version:
I'm using WRF-Chem (version 4.1) to simulate transport of passive trace gases (chem_opt=17, and I define the tracers in the registry myself). The WRF-Chem user guide ( recommends the following advection options in namelist.input. Quote from the user guide:

chem_adv_opt = 2
diff_6th​_opt = 0

The above options should always be used when running chemistry simulations. The WRF advection scheme has the tendency to overshoot and produce locally unrealistically low values (referred to at times as “digging holes”) if those options are not turned on. This “digging” is stronger with stronger gradients like those found where there are high emission rates.

It's not clear to me why the recommendation is to turn diff_6th_opt off. I have the impression that, on the contrary, turning it on can be important for making wind fields (and thus tracer transport) more realistic. See this report:, where they show how diff_6th_opt corrects an oscillating up- and downdraft pattern. I see something similar in my simulations. Furthermore, it's recommended to set diff_6th_opt=2 when using sf_urban_physics, which I do, "in order to avoid oscillations that may arise at the border of the city where a strong change in roughness is present" (

For these reasons I would like to set diff_6th_opt=2, while sticking to the other recommendations from the user guide quoted above. But why does the user guide recommend diff_6th_opt=0? Does diff_6th_opt=2 negatively affect tracer transport? In my case, mass conservation and realistic dispersion of the tracer are important.