[Wrf-users] Are U10,V10 reliably correct?

[Bart Brashers]

I've been looking at WRF output from several different projects, with differing vertical grid structure, and noticed that often the wind speed at 10m (from U10,V10) seems wrong, generally too high.  I've written a little script that uses ncks (from NCO) to calculate and print the wind speed from the lowest 2 levels, along with the 10m speed, and tested it on about 5 different projects.

Some of the projects seem to have high 10m speeds for the first few hours, others have high 10m speeds throughout the simulation period.

Has anyone else noticed this, or am I doing the processing wrong?  U(i,j,k) was de-staggered by averaging with U(i+1,j,k), but otherwise it should be defined at the mid-point of the layer, right?

Any other suggestions?  Should I just not trust U10,V10 to be correct?  Lots of verification software extract U10,V10 to compare to OBS.

Some examples:

1. WRF for Hawaii:

# ./verify_wind_speed.csh /path/to/wrfout_d02_2008-07-04_12_00_00 68 35 10
Speed at 10.33100 meters =  6.84357
Speed at 10.00000 meters =  8.13718
Speed at 41.34533 meters =  7.97453

This work had the first few eta_levels = 1.0000,0.9975,0.9925,0.9875,0.9825, so the center of the 1st level is indeed about 10m, the center of the 2nd level is about 40m.
sf_sfclay_physics = sf_surface_physics = bl_pbl_physics = 2

Note that speed at 10m, calculated from U10,V10 in the wrfout* file, is much greater than the speed at 10.33m.

2. WRF for Alaska:

# ./verify_wind_speed.csh /path/to/wrfout_d02_2008-08-03_12:00:00 100 100 12
Speed at  9.99660 meters =  3.94742
Speed at 10.00000 meters =  4.80383
Speed at 39.98347 meters =  4.78945

Eta_levels = 1.0000,0.9975,0.9925,0.9875 too, but levels are lower because it's higher and colder.
sf_sfclay_physics = sf_surface_physics = bl_pbl_physics = 2

3. WRF for Colorado

# ./verify_wind_speed.csh /path/to/wrfout_d03_2008-09-10_12:00:00 100 100 12
Speed at  6.41493 meters =  4.63586
Speed at 10.00000 meters =  5.13724
Speed at 19.24236 meters =  5.38312

Eta_levels = 0.99925, 0.99775, 0.996, 0.994...
sf_sfclay_physics = sf_surface_physics = bl_pbl_physics = 7

Here it seems OK.  But early in the run (during the 1st 12 hours of spin-up) it's not:

# ./verify_wind_speed.csh /home/proj3/WestJumpAQS/wrfout_d03_2008-09-10_12:00:00 100 100 1
Speed at  6.25275 meters =  2.34345
Speed at 10.00000 meters =  2.72022
Speed at 18.75957 meters =  2.29107

Thanks,

Bart


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