Evaluating parameterizations of aerodynamic resistance to heat transfer using field measurements

Parameterizations of aerodynamic resistance to heat and water transfer have a significant impact on the accuracy of models of land – atmosphere interactions and of estimated surface fluxes using spectro-radiometric data collected from aircrafts and satellites. We have used measurements from an eddy correlation system to derive the aerodynamic resistance to heat transfer over a bare soil surface as well as over a maize canopy. Diurnal variations of aerodynamic resistance have been analyzed. The results showed that the diurnal variation of aerodynamic resistance during daytime (07:00 h–18:00 h) was significant for both the bare soil surface and the maize canopy although the range of variation was limited. Based on the measurements made by the eddy correlation system, a comprehensive evaluation of eight popularly used parameterization schemes of aerodynamic resistance was carried out. The roughness length for heat transfer is a crucial parameter in the estimation of aerodynamic resistance to heat transfer and can neither be taken as a constant nor be neglected. Comparing with the measurements, the parameterizations by Choudhury et al. (1986), Viney (1991), Yang et al. (2001) and the modified forms of Verma et al. (1976) and Mahrt and Ek (1984) by inclusion of roughness length for heat transfer gave good agreements with the measurements, while the parameterizations by Hatfield et al. (1983) and Xie (1988) showed larger errors even though the roughness length for heat transfer has been taken into account.

The Roughness Length for Heat of Sparse Vegetation

A dual-source model that solves the energy balance over vegetation and soil separately can be inverted to obtain the roughness length for heat z0h of a single-source model. Model parameters for the dual-source model were taken from previous analysis of data from a sparse canopy in semiarid terrain. In these circumstances, the value of z0h, is shown to be dependent on the humidity deficit, the available energy, the vegetation fraction, and the surface resistance of the soil and the vegetation.