Characteristics Of Land Surface And Dew Over Loess Plateau, China

Land surface processes over Loess Plateau of China play an important role in climate system as the particular underlying surface. The dew which is a part of land water, is very important not only to energy exchange between land and atmosphere, but also about water resources utility in Loess Plateau region. So it should be paid more attention to land surface processes and dew.Based on the observation data at Dingxi station, Pingliang station and Qingyang station,the characteristics of land surface in winter are analyzed.It is show that Atmosphere is heated by surface at most time of a day(from 17:00 to next 9:00).Each radiation value on clear day in winter is almost an half of that in Summer. The average radiation is close to that on clear day. Whether on clear day or average condition, the sensible heat flux in winter, which is an half of net radiation, is heavy proportion in surface energy. The value of the latent heat flux is only 25% of the sensible heat flux and the soil heat flux is middle between the sensible heat flux and the latent flux. The turbulent exchange in nighttime is very weak. Average value of albedo is 0.22 during the no snow period time and that is 0.28 caused by sonw, which is almost increasing 30%. It needs about 7d when Albedo restore to new quasi-equilibrium state. So the snow process effect on albedo is very complicated.it is one of reason that is difficult to parimaterization albedo.It is an interesting thing that the CO2 in Dingxi is and source in daylight, but meanwhile, that at Pingliang and Qingyang station show as the sink.Maybe it suggest that the soil respiration over semi-humidity region of Loess Plateau is stronger than that in semi-arid region in winter.The results about dew show that in the semi-arid regions in Loess Plateau, the daily maximum amount of condensation can reach 0.33mm; the daily minimum is 0.09mm; and the average daily amount is 0.23mm. No obvious correlations are found between the amount of condensation and wind velocity, relative humidity, temperature, or land-air temperature difference. During the process of dew formation, the relative humidity which supplies the water vapor for condensing is the internal factor. The temperature which is power for condensing process is a key factor. The wind speed has an important affect and is uncertainty factor, which both promotes the dew formation and counteracts that. It is also found that the following favor conditions for the condensation process is at wind speed of 0.5-2m/s, relative humidity>80% and land-air temperature difference<±2℃.The value at different height is different. Maybe the layer close to the surface should be more researched because the meteorological factors such as wind speed, air temperature and water vapor flux etc are uncertain. The value of evaporation or evapotranspiration by eddy covariance method is higher than that.on surface It is show that the difference can reach the 20% ratio between at surface layer and on the surface. The error is caused by evaporation value at surface layer stead of that on surfaceLast, the imbalance problem about in surface layer is discussed.it is show that the vertical speed plays an important role. The hide reason should be found at continuous work.