Characteristics of Momentum and Heat Transfer over Semiarid Grasslands with Different Grazing Intensities in Inner Mongolia, China

The drag coefficient (Cd) and heat transfer coefficient (Ch) with the bulk transfer scheme are usually used to calculate the momentum and heat fluxes in meteorological models. The aerodynamic roughness length (z0m) and thermal roughness length (z0h) are two crucial parameters for bulk transfer equations. To improve the meteorological models, the seasonal and interannual variations of z0m, z0h, coefficient kB-1, Cd, and Ch were investigated based on eddy covariance data over different grazed semiarid grasslands of Inner Mongolia during the growing seasons (May to September) from 2005 to 2008. For an ungrazed Leymus chinensis grassland (ungrazed since 1979), z0m and z0h had significant seasonal and interannual variations. z0m was affected by the amount and distribution of rainfall. kB-1 exhibited a relatively negative variation compared with z0h, which indicates that the seasonal variation of z0h cannot be described by kB-1. To parameterize z0m and z0h, the linear regressions between ln(z0m), ln(z0h), and the leaf area index (LAI) were performed with R2=0.71 and 0.83. The monthly average kB-1 was found to decrease linearly with LAI. The four-year averaged values of Cd and Ch were 4.5×10-3 and 3.9×10-3, respectively. The monthly average Cd only varied by 8% while the variation of Ch was 18%, which reflects the different impacts of dead vegetation on momentum and heat transfer at this natural grassland. Moreover, with the removal of vegetation cover, grazing intensities reduced z0m, z0h, Cd, and Ch.