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How should one interpret potential temperature tendencies?

Mingshi Yang

New member
Dear WRF users,

I would like to know how should potential temperature tendencies be interpreted.

I understand that there are several tendency terms attributed to microphysics (h_diabatic) / cumulus (rthcuten) / radiation (rthraten) / PBL (rthblten) etc., plus an advection tendency term (rthften). The sum of these terms are consistent with change of potential temperature ∂θ/∂t.

However, it seems like the change of potential temperature ∂θ/∂t is not straightforwardly partitioned as dθ/dt (sum of first 4 tendencies) - V·∇θ (advection tendency) in the model output, as the advection tendency term seems very different from manually calculated - V·∇θ in three dimensions.

Could anyone explain or clarify what do the terms physically represent? Thank you!
 
Hi,
Apologies for the delay in response. I've been trying to get caught up, and just to let you know, I'll be out of the office from 4/5 - 4/10, but I'll get back to this as soon as I'm able after my return. In the meantime, I have a few questions for you:

Are you calculating your own term? The RTHFTEN would be the flux form ∇. (θ V). Is this being looked at at every time step?
 
Hi,
Apologies for the delay in response. I've been trying to get caught up, and just to let you know, I'll be out of the office from 4/5 - 4/10, but I'll get back to this as soon as I'm able after my return. In the meantime, I have a few questions for you:

Are you calculating your own term? The RTHFTEN would be the flux form ∇. (θ V). Is this being looked at at every time step?
Dear administrator,
Thank you for your reply!

I was trying to compare WRF output heating terms as well as RTHFTEN, with the terms from the equation (dθ/dt = V·∇θ + ∂θ/∂t), which I calculated. I am using 10 minute outputs from a short test simulation to look at the fields.

Given RTHFTEN is in the form ∇·(θ V), does it mean that the heating terms has form dθ/dt + θ(∇·V)? using ∇·(θ V) = V·∇θ + θ(∇·V)
I am still puzzled by the RTHFTEN, because the magnitudes of ∇·(θ V) calculated using θ and V (~10^3 K/day) is much higher than the RTHFTEN in WRF output (~10^1 K/day) at middle troposphere.

In the function "set_tend" I noticed advective tendency is multiplied by a map scale factor when setting tendency with advective tendency. I am not sure if this is relevant to the difference, do I need to do some scaling to compare the result?

Thank you very much for your help.
 
Can you provide a namelist.input file you use to run this test?
Thank you for your help!
The namelist file is attached. It is a test case where I tried to output with high temporal resolution and vertical resolution on pressure level to check the diabatic heating calculation. Please let me know if you need more information.
Regards,
Mingshi
 

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  • namelist.input
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Thanks for providing the namelist file. Part of the problem you're seeing is because the RTHFTEN is only defined when a few cumulus scheme is used. Unfortunately the cu_physics used in the 'conus' suite isn't one of them. So to test your idea, you can change the cu_physics option from current value of -1 to 16 (for new Tiedtke scheme) or 3 or 5 (for Grell type scheme) and see if the values from this array and what you calculated are closer.
 
Thanks for providing the namelist file. Part of the problem you're seeing is because the RTHFTEN is only defined when a few cumulus scheme is used. Unfortunately the cu_physics used in the 'conus' suite isn't one of them. So to test your idea, you can change the cu_physics option from current value of -1 to 16 (for new Tiedtke scheme) or 3 or 5 (for Grell type scheme) and see if the values from this array and what you calculated are closer.
Thank you for your reply!
I forgot to mention that I removed the condition check in the Big Step Utilities code and recompiled the model. I also tried the cumulus schemes you mentioned but the result still does not seem to match. Could you let me know how the heating and advection tendency fields are constructed?
Regards,
Mingshi
 
What model variables do you use to compute advection? Is it U, V and T? Note that T is defined as (theta - 300).
The factor in set_tend is the map scale factor due to map projection from spheric earth to a plane, it should be around 1. You can check it by looking at the output variable MAPFAC_M.
The advective tendency of theta is computed by subroutine call to advect_scalar just a few lines above when set_tend is called. The model's advection is done in flux form and you can find its description in this presentation beginning page 17.
 
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