yang_2008()

subroutine module_sf_noahmplsm::yang_2008	(	real (kind=kind_phys), intent(in)	z_0,
		real (kind=kind_phys), intent(out)	zt,
		real (kind=kind_phys), intent(out)	zq,
		real (kind=kind_phys), intent(in)	ustar,
		real (kind=kind_phys), intent(in)	tstar,
		real (kind=kind_phys), intent(in)	qst,
		real (kind=kind_phys), intent(in)	ren,
		real (kind=kind_phys), intent(in)	visc
	)

the original formulation relates the thermal roughness length (zt) to u* and t*:

zt = ht * exp(-beta*(ustar**0.5)*(abs(tstar)**0.25))

where ht = renc*visc/ustar and the critical reynolds number (renc) = 70. beta was originally = 10 (2002 paper) but was revised to 7.2 (in 2008 paper). their form typically varies the ratio z0/zt by a few orders of magnitude (1-1e4).

this modified form uses beta = 1.5 and a variable renc (function of z_0), so zt generally varies similarly to the zilitinkevich form (with czil = 0.1) for very small or negative surface heat fluxes but can become close to the zilitinkevich with czil = 0.2 for very large hfx (large negative t*). also, the exponent (0.25) on tstar was changed to 1.0, since we found zt was reduced too much for low-moderate positive heat fluxes.

this should only be used over land!

Referenced by sfcdif4().

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