I am relatively new to the WRF model but am currently doing some WRF-LES runs. My question is, when I go back to compute my SGS fluxes, I noticed something odd in the module_diffuse_em.F file. It computes the temperature gradient (dthrdn) at the lowest model point (temp gradient between the surface and second model level) using two rdzw distances to compute this gradient. If I understand this correctly, I think this is not the correct distance, as this would be greater than the distance between these points. Is there a reason the gradient is computed with this distance rather than the actual distance?

What I would expect it to be is something like: (0.5*(theta(k=1)+theta(k=2))-thetasfc)/(1/rdzw)

The above would give you the gradient at that lowest center point. However, this suggests I could use linear interpolation at the surface where the temperature gradient is typically non-linear. Perhaps this is why dthrdn is calculated as it appears in the module_diffuse_em.F file? If someone could explain this decision or point me in the direction of somewhere where it is presented, it would be greatly appreciated.

Thanks,

Jason