The atmospheric boundary layer is a passage between the atmosphere and the Earth, through which atmospheric momentum, energy and various substances are transported upwards and downwards. Turbulent transport in the lower atmospheric planetary boundary layer is an important link for substance and energy exchanges between the atmosphere and the Earth. The Loess Plateau ranges from the Qinling Mountains to the Yin Mountains in a south-north direction, and from the Riyue Mountains to the Taihang Mountains in a west-east direction. The plateau stretches across seven provinces or autonomous regions and has a total area of more than 6.268 × 10⁵ km² and unique land type and ecological environment. It lies on the edge of the southeast monsoon area; i.e., a transition zone from the humid monsoon climate in southeast China to the inland arid climate in northwest China. The average annual precipitation is generally around 400 mm. The ecological environment and agricultural production on the Loess Plateau are sensitive to precipitation change, and therefore, the Loess Plateau is a typical area sensitive to climate change and has vulnerable ecology and agriculture. However, there have been few studies on the surface energy balance for the typical highland terrain of the Loess Plateau, where the underlying surface is distinctively heterogeneous but less affected by human activity.
The diurnal and seasonal variations in energy balance components and energy balance characteristics for semi-arid grassland on the Loess Plateau were studied on the basis of radiation, turbulence flux, soil temperature and soil heat flux data recorded at the Semi-Arid Climate and Environment Observatory of Lanzhou University in 2008. The results show that diurnal variations in energy balance components had the expected single peaks. The yearly net radiation over the grassland was 2.269 × 10³ MJ/m². The yearly sensible heat flux, latent heat flux and soil heat flux were 1.210 × 10³, 1.117 × 10³ and 0.069 × 10³ MJ/m², respectively. The energy balance components had obvious seasonal variations. According to the ratios of energy balance components to net radiation, the sensible heat flux was the main component of net radiation, and the function of surface soil heat flux was the second factor of the energy balance. The energy balance closure averaged 86.8% during the growing season but only 75.5% during the non-growing season. The closure of the surface energy balance for the semi-arid grassland increased 11.3% during the growing season and 12.0% during the non-growing season after considering heat storage from the surface to a soil depth of 5 cm.