Observation analysis of land‐atmosphere interactions over the Loess Plateau of northwest China

Abstract

Arid and semi‐arid areas of the Loess Plateau over northwestern China are one of the dust aerosol source regions featured by its unique underlying surface. These areas, suffering the severe aridity trend in past decades, are also known as the transitional zone of climate and ecosystem change. To better understand the basic characteristics of the land surface energy budget, seasonal and diurnal variations of moisture and heat flux over this region, field observations collected at the Semi‐Arid Climate and Environment Observatory of Lanzhou University (SACOL, 35°57′N, 104°08′E, Elev. 1965.8 m) from January 2007 to December 2008 were analyzed systematically, especially focusing on land surface energy partitioning and energy balance. The results indicate that all four radiative components had distinct seasonal and diurnal cycles, except for the diurnal variation of downward longwave radiation. They maintained high values during the growing season and low values during the non‐growing season. The highest daily mean value of DSR (downward shortwave radiation, 369.2 Wm−2), DLR (downward longwave radiation, 386.8 Wm−2) and ULR (upward longwave radiation, 484.2 Wm−2) measured in summer while the highest daily mean value of USR (upward shortwave radiation, 150.1 Wm−2) occurred in winter as the snow cover. The highest surface albedo was also found in winter as a result of the snow cover. Surface albedo was lower in the growing season (wet season) due to the larger vegetation fraction and wetter soil. The components of the land surface energy budget varied seasonally except for the surface soil heat flux, and all showed strong diurnal cycles. Net radiation increased from winter to summer and decreased from summer to winter associated with the variation of DSR. Sensible (latent) heat flux was the main consumer of the available energy in winter and spring (summer and autumn). The energy imbalance problem was also identified. When the soil heat storage in the surface soil and vegetation canopy was neglected, the energy imbalance ratio was about 22%. While given the surface heat storage calculated by the thermal diffusion equation and correction method (TDEC), the imbalance ratio was only 14%. Furthermore, taking the soil heat storage into account, this ratio was only 8% in spring, and 15% in summer and autumn. Compared with the bare surface layer in spring, it is likely that a part of energy was stored in the vegetation canopy in summer and autumn. In addition, the sensible and latent heat fluxes over different land surface types of the Loess Plateau are analyzed. Sensible and latent heat fluxes are utterly different substantially over those different underlying surfaces due to the factors such as vegetation, precipitation, and soil moisture.