A Study On Parametric Feature Of The Surface Roughness And Albedo In North China

Land surface process is the bond that unites the climate and people’s living environment. The land-atmosphere interaction is one of the two important aspects to improve numerical simulation capability. Its spatial difference can drive mesoscale circulations and sometimes can affect climate and change the ground-surface conditions. The land-atmosphere interaction influence atmospheric-circulation through changing a series of feedback processes of the surface fluxes, radiation and soil heat exchange, etc, and the surface roughness and albedo are two significant parameters involved in these processes. By using the data provided by "Experimental Co-observation and Integral Research in the Semi-arid and Arid Regions over North northern China" from July to September,2008and the data collected at the Semi-arid Climate and Environment Observatory of Lanzhou University(SACOL) from June2006to December2012, the characteristics of land surface process parameters(the surface roughness and albedo) over different underlying surfaces in the arid and semi-arid regions of north China were analyzed in the paper. The main conclusions are as follows:(1)Roughness length changes a little along with wind direction in forest underlying surface; the wind direction is well-distributed and the roughness length varies somewhat along with wind direction in farmland underlying surface; the roughness length is discrete and changes greater along with wind direction and in grassland underlying surface, and the values of roughness length in the two main wind direction intervals are different. This is related to the concrete condition of the underlying surfaces. Even in the same wind direction, the roughness length varies dynamically along with wind speed, et al. In the whole, the mean surface roughness increases with the vegetation height raising and for vegetated underlying surface, vegetation height basically determines the magnitude of roughness length. The roughness decreases with the wind speed increasing. At constant wind velocity, the roughness length increases with the friction velocity increasing while the it decreases with the wind speed increasing at constant friction velocity, and the higher the friction velocity is, the better the correlation is. The roughness length will vary little along with wind speed and friction velocity when the wind speed increases to some extent. The roughness increases with the vegetation height increasing over different vegetated underlying surface in the condition of the calm wind, and as well as the extent of the vegetation effected by the wind.(2) It has previously been shown that aerodynamic roughness length changes significantly along with near-surface atmospheric thermodynamic state; however, at present this phenomenon remains poorly understood, and very little research concerning this topic has been conducted. In this paper, by using the data of different underlying surfaces provided by the Experimental Co-observation and Integral Research in Semi-arid and Arid Regions over North China, aerodynamic roughness length values in stable, neutral and unstable atmospheric stratifications are phylogenetically compared with one another, and the relationship between aerodynamic roughness length and atmospheric thermodynamic stability ζ0is analyzed. It is found that aerodynamic roughness length shows great differences among the stable, neutral and unstable atmospheric thermodynamic states, with the maximum relative deviation between aerodynamic roughness length under full thermodynamic stability, and it under the neutral condition it may reach60%of mean aerodynamic roughness. Furthermore, in the wind speed interval, in which the data is less sensitive to aerodynamic roughness length, the surface aerodynamic roughness length changes more significantly along with the atmospheric thermodynamic stability, and is highly correlated with both the Monin-Obukhov stability ζ0and overall Richardson number Rib, with correlation coefficients greater than0.71and0.47in the stable and unstable atmospheric stratifications, respectively. The empirical relation fitted with the experimental observations is quite consistent with the Zilitinkevich theoretical relation in the stable atmospheric state, but the two are quite distinct in the unstable atmospheric state, and even show opposite variation tendency. In addition, according to the verification of the empirical fitted relations, performed by using the experimental data, it is shown that the application effects of the fitted relation are slightly stronger than those of the Zilitinkevich theoretical relation in stable atmospheric stratification, but the former are much more suitable in unstable atmospheric stratification.(3) Taking into account the impact of terrain, vegetation, and precipitation and thermal conditions, annual change characteristics of roughness in the southeast and northwest and its influence mechanism are analyzed, and fitting relationships between normalized roughness and time in the two directions are given. Studies have shown that for heterogeneous underlying surface, the difference of roughness length in different wind directions caused by undulating terrain and vegetation difference is very significant. Selecting the southeast and northwest directions, both magnitude and change trades of the two directions are remarkable different. And due to differences in terrain and vegetation, the annual variation trend of roughness length is consistent with the annual variation of atmospheric stability in southeast and precipitation in northwest, and the roughness length has a certain relationship with atmospheric stability in southeast and has a good relationship with precipitation in northwest.(4) The characteristic of land-surface albedo and the response to precipitation are analyzed, and the relationships between land-surface albedo and solar altitude angle and soil humidity are studied. According to the average albedo, the sequence of albedo over all the underlying surfaces are arid meadow in the northeast of China>alpine meadow in the northwest of China>oasis farmland in the northwest>natural vegetation on the Loess Plateau>semi-arid meadow in the northeast of China>farmland in the northeast of China> fruit bearing forest in the northeast of China. And the albedo is smaller when the precipitation is much while it’s bigger when the precipitation is less on thw whole. The diurnal variation curves of albedo have three types: symmetry type, bigger in the morning and smaller in the noon and afternoon, biggest in the afternoon and smallest in the morning. In the selected6stations, the valley of albedo basically occurs in rainy days and the peak occurs in days which are not rainy and solar radiation is strong. The land-surface albedo decreases along with the solar altitude angle increasing and tends to remain when the solar altitude angle is bigger than40°. The land-surface albedo decreases obviously with the soil moisture increasing in Yuzhong and farmland of Tongyu, then the grassland of Tongyu. And the land-surface albedo of Maqu, Miyun, Jinzhou has no significant change along with the soil moisture.