incorrect moist theta?

[gpetty]

I have seen a couple of references in the WRF documentation to "moist theta", which I take to be the virtual potential temperature used in buoyancy or density calculations. In every reference I've seen so far, the expression cited has appeared to me to be incorrect. For example, in the output from ncdump -c, I find the following reference to the perturbation moist potential temperature:
pert moist pot temp=(1+Rv/Rd Qv)*(theta)-T0
The multiplier Rv/Rd = 461.5/287 = 1.61 is incorrect; rather it should be (Rv/Rd-1) = 0.61.

Also, in the README file for namelist variables that accompanies the source code, I find

use_theta_m  = 1        ;   1: use theta_m=theta(1+1.61Qv)
                                        0: use dry theta in dynamics

Once again, the multiplier for Qv is incorrect if the intent is to adjust air density for the presence of water vapor. I don't know whether this expression for theta_m is also used in the actual source code, but if it is, it should be corrected.

[weiwangncar]

Thanks for your comment. The moist theta used in WRF is not virtual potential temperature. It is defined as it is to facilitate solving the equations.

[gpetty]

Thanks for your reply and the explanation. Is there documentation that explains how moist theta is used? This might help me to understand why it has the form that it does. It could also me help me understand the implications of turning 'use_theta-m" on or off.

[weiwangncar]

Here is a presentation that shows what moist theta helped with model solutions:
http://www2.mmm.ucar.edu/wrf/users/workshops/WS2015/ppts/7.1.pdf

[gpetty]

Thank you. It sounds like the option use_theta_m=1 is most important for LES simulations of the boundary layer with sharp vertical moisture gradient. I found that it's turned off in the em_les test case provided with the WRF distribution as well as in the closely related shallow convection simulation, which does have a sharp moisture gradient at the inversion, so it remains a bit unclear when it would be necessary in order to avoid spurious vertical motions and what the drawbacks might be to turning it on when it's not needed.

[weiwangncar]

We probably should have changed use_theta_m option for the LES case prior to V4.0. Since V4.0, this option has become default.

[ewquon]

This is an old thread, but still a source of confusion, I think. For reference, here's a paper that clearly describes the motivation behind the WRF moist theta: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015MS000532