Why CLM updates surface fluxes by multiplying surface temperature changes with the derivative of fluxes wrt surface temperature?

[QINKONG]

Hi! This is Qin. It seems that CLM update surface sensible or latent heat fluxes by multiplying the surface temperature change since previous time step with the derivative of surface fluxes wrt surface temperature. This derivative is calculated by solving surface resistances from the Monin-Obukhov similarity equations at the previous time step.

Just out of curiosity, why take this approach? Since we can directly get surface fluxes by solving the Monin-Obukhov similarity equations, why not just take this surface flux values (rather than defining resistances and updating fluxes using the approach above)?

Thank you!

Best,
-Qin

 

[QINKONG]

After some thinking, I guess it's related to the coupling between land and atmosphere component. Before land updates surface fluxes and pass it to the atmosphere, all it has is still atmosphere temperature/humidity from the previous time step. So, CLM cannot solve the Monin-Obukhov equations with both land and atmosphere T/Q at the current time step.