ABSTRACT
The largest uncertainty in estimating LE comes from computing H. A simple form to express and examine the relationship between H and the surfaceair temperature difference is via a resistance relationship (e.g., Ref.[5]),
H ¼ rCP TRðyÞ TA RA þ REX ð2Þ
In this equation, TA is the near-surface air temperature, r, the air density, CP, the specific heat of air, RA, the aerodynamic resistance and REX, the so-called ‘‘excess resistance,’’ which addresses the fact that momentum and heat transport from the roughness elements differ.[6] The method offers the possibility of mapping surface heat fluxes on a regional scale by using radiometric temperature observations, TR(y) (converted from satellite brightness temperatures) if RA and REX can be estimated appropriately. REX has been related to the ratio of roughness lengths for momentum, zOM, and heat, zOH, and the friction velocity u
having the form[5,6]
REX ¼ k1 ln zOM zOH
u1 ð3Þ
where k ¼ 0.4 is von Karman’s constant. While addressing the well-known differences in efficiency between momentum and heat transport from natural surfaces, this model is just one of several that have been developed (e.g., Refs.[5,7]). There have been numerous efforts in recent years to apply Eq. (2) and hence determine the behavior of REX or zOH for different surfaces, but no universal relation exists for land surfaces with large spatial and temporal variations in the magnitude of zOH having been documented.
