U10m wind computed by CAM5
- Thread starter Weiyi
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Yes, it's interpolated from the lowest model level which is closer to around 50 m. I don't think there's a straightforward formula for interpolation. It is done using the Monin-Obhukov theory and using profiles that are determined by the model based on factors such as stability. That is, the shape of the profile that determined how the wind varies between the surface and the lowest model level will be a function of time and space as it depends on the climate state.
Hi Islas,
Thank you for your reply!
I looked into the Monin-Obukhov theory and found that it involves three key variables: the von Karman constant, \(z_0\) (the aerodynamic roughness length), and \(d\) (the zero-plane displacement, or the effective height of the ground). However, I didn't quite understand your point about "a function of time and space as it depends on the climate state." Could you elaborate on how this relates to the Monin-Obukhov theory? How do time and spatial factors influence the wind profile in this context?
Best,
Weiyi
Thank you for your reply!
I looked into the Monin-Obukhov theory and found that it involves three key variables: the von Karman constant, \(z_0\) (the aerodynamic roughness length), and \(d\) (the zero-plane displacement, or the effective height of the ground). However, I didn't quite understand your point about "a function of time and space as it depends on the climate state." Could you elaborate on how this relates to the Monin-Obukhov theory? How do time and spatial factors influence the wind profile in this context?
Best,
Weiyi
I'm certainly not an expert on this so maybe someone can chime in who has more expertise. But my very vague understanding the layer between the lowest model level and the surface is assumed to be a layer of constant flux (of heat, momentum or whatever) following a flux-gradient relationship and the profile of how quantities vary within this surface layer is part of that iterative process to get the surface fluxes and then the fluxes following this flux-gradient relationship within that layer to match up. The Monin-Obhukov theory describes the formulation of these profiles as a function of stability. So exactly how something like wind speed is going to vary within the surface layer is going to be a function of the stability at any given time.
As mentioned, U10 is a function of stability and thus depends on the lowest level temperature and moisture as well. Over oceans, the computation is based on Large and Pond 1981 (Open Ocean Momentum Flux Measurements in Moderate to Strong Winds) and 1982 (Sensible and Latent Heat Flux Measurements over the Ocean), which might be helpful resources here.
It's a bit different over land, though still following the assumptions of MO Theory. U10 is still dependent on atmospheric stability but has a few more classes of stability for which the calculation varies slightly (in other words, it's not just stable vs. unstable but different levels of instability that matter). You can find some more details in the CLM technical note (2.5. Momentum, Sensible Heat, and Latent Heat Fluxes — ctsm release-clm5.0 documentation), which should have other sources with a bit more detail as well.
Hi Meg,
Thanks for your detailed introduction. I have refered to the note you mentioned in your reply and found the equation to calculate the 10-m wind is based on (2.5.61) attached below. I am curious why CESM, also with other climate models, especially global climate models, they mostly have a bad ability to capture the observed long-term trend (at monthly/annual scale), do you have any idea on how to improve the performance of the simulations in CESM?
