2008年7—9月中国北方不同下垫面晴空陆面过程特征差异

A comparative study of the characteristics of the clear-sky land surface processes over the different underlying surfaces in the northern part of China during July-September 2008

采用2008年7—9月观测的中国干旱/半干旱区实验观测协同与集成研究资料,选取了9个下垫面(包括裸地、草地、森林和农田),分析了中国北方不同下垫面以及不同地区同类下垫面的晴空陆面辐射平衡和热量平衡日变化特征差异。结果表明,不同下垫面以及不同地区同类下垫面的地表辐射和能量过程特征差异明显,而这种差异主要源于下垫面的光学特性、水热特性以及局地陆-气系统中可利用水分的不同。在辐射平衡的比较方面,荒漠沙地发射的长波辐射最大,高寒森林的长波辐射最低,农田下垫面发射的长波辐射总体低于草地;荒漠沙地、草地、农田和高寒森林的反射率依次减弱;荒漠沙地、草地、农田、森林下垫面的净辐射依次增大。在能量平衡方面,荒漠区沙地可利用能量大部分(约80%)用于加热大气,约20%消耗于蒸发和加热土壤;草地下垫面可利用能量中用于加热大气的能量比蒸发水分消耗的能量高,但高寒草甸例外;农田下垫面可利用能量的大部分消耗于蒸发,消耗于加热大气的能量不到20%。水含量越高,潜热通量越大,能量闭合率越低,能量过程也更复杂。

By selecting the 9 land cover types including bare soil,cropland,forest,and grassland,the characteristics of the clear-sky land surface processes over the different underlying surfaces in the northern part of China are investigated compared using the data measured during July to September 2008,provided by the project of the Experimental Co-observation and Integral Research in Semi-arid and Arid Regions over North China.The results show that the pronounced differences in the land surface radiation and energy processes are found within and between the various land cover types.The differences can primarily be attributed to those in the optical,thermal,and hydrologic property of land covers as well as the amount of available water in the land-atmosphere system.For radiation components,upward long-wave radiation increases in the order of forest,cropland,grassland,and bare soil.The reflectivity of the different underlying surfaces reduces in the order of bare soil,grassland,cropland and forest.Land surface net radiation is small in poorly vegetated underlying surfaces such as bare soil and grass land,and is high in cropland and forest.For energy components,80% of the land surface available energy on bare soil is consumed by heating the atmosphere,and the remaining 20% is used for evaporation and heating the soil.For the grassland and forest,higher available energy goes to the atmosphere heating than that lost to evaporation with alpine meadow being an exception.Large part of energy is lost owing to evaporation on cropland,and 20% is used in heating the atmosphere.In addition,for all underlying surfaces,the closure rate decreases with the increasing latent heat flux.