First layer height and wind speed at 10m

[sluque]

Hello there.

I ran two WRF simulations with the only difference between them being the first layer height. The first simulation has an 50m first layer (in average) and the second one a 20m first layer (in average). Both simulations are run with BouLac PBL scheme, SLUCM and Revised MM5 for surface layer scheme.

I was expecting the 20m first layer height simulation to present values more similar to observations as the vertical resolution near the surface was increased.

Temperature and water vapor mixing ratio at 2m are very similar between both configurations. The same is true for wind direction at 10m, but not for wind speed at 10m. Wind speed at 10m is systematically higher with the 20m first layer.

I understand that wind speed and direction at 10m are computed with Monin-Obukhov similarity theory, but Im having a hard time understanding how the length of the first layer height is impacting it.

Is there some material I could read about this or someone with a deeper understanding in the topic?

Thank you
Sol

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[kwerner]

Hi Sol,
I had to forward your issue to our physics expert in the lab because I wasn't sure of the specifics. This is their response:

The value at 10 meters comes from the lowest level value and accounts for the height difference to 10 meters. This implies that the lowest level wind speeds are significantly different, even accounting for their height above ground - not sure why. YSU has a method for making this less sensitive, but I don't know about the BouLac method.

 

[sluque]

Hi there.

Ok, thank you for your answer! Ill keep looking into it.