Influence of Leaf Area Index on the Radiometric Resistance to Heat Transfer

Sensible heat flux is an important component of the surface energy balance. Land surface models often use the radiative surface temperature instead of the aerodynamic temperature to predict the surface sensible flux, because the former is much easier to observe by remote sensing or to compute from the surface energy balance equation. Here, measurements from 44 FLUXNET sites are used to examine the stability and leaf area index (LAI) dependence of the radiometric resistance, a resistance that should be included in the bulk transfer method if the radiometric temperature is used for the flux calculation. Results show that the radiometric resistance is much higher under stable conditions than under unstable conditions. In unstable conditions, the radiometric resistance is highly sensitive to LAI, decreasing exponentially as LAI increases. Omission of the radiometric resistance from the bulk transfer method causes a large overestimation in the sensible heat flux, especially for low-LAI surfaces and under unstable conditions.