topo_wind option and recommended grid size

With soon inclusion of the NEMS NMM-B model into the EMS package, WRF EMS changes its name and becomes UEMS.
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windyweek
Posts: 28
Joined: Thu Aug 21, 2014 8:46 am

topo_wind option and recommended grid size

Post by windyweek » Fri May 12, 2017 11:43 am

Hello all,

I would like to know what is the recommended grid size (if any) when using the topo_wind option.

This is an excerpt from UEMS run_physics.conf:
Topographic correction for surface winds to represent extra drag from sub-grid topography and enhanced flow at hill tops. Shown to reduce 10m wind biases, which is good, but designed for dx < 2km, which is a bummer.
However I'm not able to find any other (WRF official) documentation that says that the topo_wind would work as expected only for high resolution simulations.
I also see a lot of other users use it for any grid size (from dx=36000 to dx=500).

I have run my own experiments (with dx=6000) and my conclusion is that although it does a good job in correcting the wind speed, the latter seems to be underestimated in the plains (with a factor of 1.2-2).

I would highly appreciate your experience and opinion guys! Thanks much in advance.

- Ivan

P.S. This is regarding UEMS V15.89 (WRF 3.7.1) in 1-way nesting configuration (dx=6000, 2000).

meteoadriatic
Posts: 1512
Joined: Wed Aug 19, 2009 10:05 am

Re: topo_wind option and recommended grid size

Post by meteoadriatic » Fri May 12, 2017 11:57 am

Hello,

This is the most official source for topo_wind option, I hope you can find what you need inside. I know it is large and I didn't read it in details so I can't give more direct answer.
http://www2.mmm.ucar.edu/wrf/users/phys ... o_wind.pdf

Good luck!

windyweek
Posts: 28
Joined: Thu Aug 21, 2014 8:46 am

Re: topo_wind option and recommended grid size

Post by windyweek » Fri May 12, 2017 4:08 pm

Thanks Ivan!

I have read that study before but it was nice re-reading it again. However, it still does not shed light into the recommended grid size. The authors use a 2-km domain but don't discuss other grid sizes.

Here's one more question: as far as I understand the topo_wind option corrects only the U10/V10 fields and does not touch U-,V- at the model level. Let's say I run a nested simulation with two domains, the child domain has topo_wind turned ON but the parent has it OFF (e.g. topo_wind = 0,1).
Are U10/V10 in the child domain influenced by the U10/V10 in the parent?
Or in other words: Are U10/V10 part of the boundary conditions when we run a nested simulation?

Thanks!
- Ivan

meteoadriatic
Posts: 1512
Joined: Wed Aug 19, 2009 10:05 am

Re: topo_wind option and recommended grid size

Post by meteoadriatic » Fri May 12, 2017 5:00 pm

Good question. I don't know but if I'm very interested in finding out, I would make 2 simulations, one with topo_wind on on parent, other with off and then compare differences around boundaries in nests from those two runs. You can easy do that in GrADS, loading both ctl files into same session, then using something like

Code: Select all

d ugrd10m.1-ugrd10m.2
which will display difference between U values of those two files.

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