A study of the parameterization of land surface processes over the natural vegetation surface of Loess Plateau

Based on the data observed at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL), the physical parameters related to land-surface processes over the natural vegetation surface of Loess Plateau are calculated. The paper studied averaged diurnal variations and the frequency distributions of the bulk transfer coefficients in the different seasons, and analyzed the trend of the surface roughness as well as effects of precipitation on the roughness. Monthly averages of the roughness in a normal year of precipitation have more changes compared with those in a dryer year since precipitation increasing tends to increase the roughness. The roughness in a normal year is 9×10-3m, while the roughness in a dryer year decreases to 6×10-3m. The rainfall raises the roughness by increasing the vegetation cover and height. Relationships among the bulk transfer coefficients and the two factors including surface roughness and the Richardson number are discussed. The role of dynamical transfer in land-atmosphere energy exchange over the Loess Plateau is dominant. Additionally, the neutral bulk transfer coefficient for momentum is close to that over the movable dune at Naiman in Inner Mongolia and the neutral bulk transfer coefficient for sensible heat is close to that over Gobi. We also analyzed effects of solar elevation angle and soil moisture on surface albedo and preliminarily a multiple factorial parameterization formula of surface albedo is suggested. Generally, the albedo over the Loess Plateau is smaller than that over the desert of Dunhuang and larger than that over pine forests in Changbai Mountain. The various vegetation covers and soil types in the three regions lead to differences of albedo. By testing the simulation results of the albedo formula, it is found that the albedo with low solar elevation angles is sensitive to the other factors except soil moisture and solar elevation angle, while soil moisture and solar elevation angle affect the albedo with high solar elevation angles significantly.What’s more, the soil thermal parameters including soil thermal conductivity and thermal diffusion are calculated as well. In the same soil moisture, the soil thermal conductivity is larger than that in desert of Dunhuang. In the end, a parameterization formula of soil thermal conductivity is gotten by fitting the thermal conductivity and the soil moisture.