Can I get u, v wind components at 10m from U10 (10m wind speed), TAUX, and TAUY?

[jhyun91]

Hello, I'd like to calculate u, v wind components at 10m using 10m wind speed, and zonal and meridional wind stress components. This is because u, v wind components at 10m do not save in our runs.
Theoretically, zonal and meridional wind stress components can be explained as follows.
TAUX = rho_a * Cd * |U10| * U, TAUY = rho_a * Cd * |U10| * V,
where rho_a is the air density, Cd is the drag coefficient, U and V are zonal and meridional wind components at 10m and U10 is the wind speed at 10 m above the sea surface (=sqrt(U**2 + V**2)).
Does it make sense to calculate u, v wind components using the above equations?

 

[peverley]

Hello,

I consulted a scientist and this is what they said:

Though this is likely valid theoretically, this isn’t quite how the model computes the surface stress. A concise source on this would be the CAM5 documentation on surface flux exchanges: 5. Model Physics — camdoc documentation.

TAUX and TAUY, over ocean or land, are ultimately computed based on the Monin-Obukhov Similarity Theory (MOST). Here, the surface momentum exchanges aren’t directly computed from the 10m values of U and V, and U10 is not computed as the sqrt(u10**2 + v10**2). That is, by using MOST we aren’t directly computing everything at 10m but using an approximation to get those values. Here's a similar question from last year: calculation of zonal wind velocity at 10m

Ultimately in this case, the equations you listed aren’t the actual calculation of tau we use in the model; tau is based on bottom level density, an aerodynamic resistance, and the difference between UBOT (VBOT) and surface U (V) (but surface U and V are both zero, over land and over ocean). The wind velocity used in the resistance calculation is VBOT, which is the magnitude of the windspeed at the lowest model level as well, not at 10m. So I guess the short answer (after all that) is that no, I don’t think the proposed method would be suitable to derive 10m u and v components of the wind; MOST doesn’t inherently allow for that calculation.

Hope that helps

Courtney