Limitations of MOST and z0T

Two of the “understandings” common in today's models are examined, in the light of recent data. First, the limits of applicability of the Monin-Obukhov similarity theory (MOST) are quantified using a statistical examination of data from a 2014 field study in Texas. The data indicate that MOST is relevant primarily within the stability range -0.1 < z/L < 0.1. Outside this range, free convection reigns in daytime and intermittency at night, if conditions are right. Independent datasets demonstrate that nighttime intermittency is most common on cloudless nights, but do not permit detailed examination of possible causes. Second, new data are used to examine expressions relating the roughness length associated with heat transfer (z0T) to the familiar roughness length (z0) through a dependence on the surface roughness Reynolds number (Re*). Randomizing data leads to fictitious results that mirror the relationships often assumed, of the general form ln(z0/z0T) = a.Re*b. The practical applicability of these relationships is therefore in doubt. New data support the proposal that the ratio z0/z0T can be approximated as about 7 for vegetated surfaces. Expectations that this must depend on the nature of the surface have yet to be substantiated with confidence, but it appears that Re* is not a good defining property on which to rely.