| The atmospheric boundary layer is the main channel of material and energy exchanged between the earth and atmosphere. It is also the major place for human life and production, being closely related to the development of human beings and national economy. As a result of particular climate environment and atmospheric circulation background in arid region of Northwest China, the atmospheric boundary layer there is obviously deeper than general areas. In this article, based on the field intensive observation data of atmospheric boundary layer in Duhuang during the period of June to July 2006 and Jan. 2007, the structural characteristics and variation regularities of main physical factors over extreme arid region of Northwest China in winter and summer clear sky days are analyzed. Meanwhile using the routine sounding data from 5 stations: Dunhuang(extreme arid region),Minqin and Jiuquan(arid region),Yuzhong and Pingliang(semi-arid region) in January,April,July and October, which present winter,spring,summer and autumn respectively, the structure of atmospheric boundary layer over the five different drought degree regions are studied. Moreover, the temporal and spatial evolution characteristics of atmospheric boundary layer there are compared. What's more, utilizing the flux data and automatic weather station data during Dunhuang intensity observation period, the correlations between thermal factors,wind shear and the height of atmospheric boundary layer are discussed. At last the formation mechanism of atmospheric boundary layer is expounded preliminarily.The main conclusions are as follows:(1) Dunhuang locates in extreme arid region of Northwest China. In this area, atmospheric boundary layer in summer is generally thicker than that in winter. In daytime of summer clear days, the mean value of maximum height of convective boundary layer is about 3000m and in nighttime the max depth of stable boundary layer can reach 900m generally. Whereas in winter, the mean value of max height of convective boundary layer and the stable boundary layer are 2350m and 500m lower than that in summer, respectively. In the two seasons, structure of atmospheric boundary layer is similar, that is, the characteristics of convective boundary layer (stable boundary layer) in daytime (nighttime) are both obvious. According to the specific humidity profiles characteristics, whether in summer or winter the specific humidity has a significant diurnal variation and in summer the specific humidity is much bigger than that in winter.As to the vertical wind profiles characteristics in arid region of Northwest China, both in summer and winter, the wind speed increases with height rising from surface during the whole day, showing low-level jet feature. Besides, both in summer and in winter the wind velocity always shows Ekman spiral characteristics from daytime to nighttime.(2) Minqin,Jiuquan and Yuzhong,Pingliang locate in arid and semi-arid regions of Northwest China, respectively, where both the characteristics of mixed layer in daytime and the residual layer in nighttime are remarkable. However, the depth of atmospheric boundary layer in Minqin and Jiuquan is higher than that in Yuzhong and Pingliang.(3) In regard to the seasonal variation of atmospheric boundary layer height over Northwest China, the maximum height of atmospheric boundary layer occurrs in summer, the next is in spring and the minimum appears in winter in the five regions. As to the spatial variation, the depth of atmospheric boundary layer is associated with different drought degrees and geographical positions. Among the five regions, the height of atmospheric boundary layer in Dunhuang is the highest and in Pingliang is lowest.(4) It is certain that characteristics of atmospheric boundary layer have a close relationship with land surface processes. Compared with other areas, particular thermal factors in Dunhung desert region provide favorable energy conditions for the formation and development of much deeper atmospheric boundary layer, at the same time, large convection and turbulent motion also create a positive dynamic environment for the uplifting of atmospheric boundary layer. The development of deep convective boundary layer is mainly affected by thermal factors, whereas dynamic elements play dominant roles on the development of stable boundary layer.(5) The formation mechanism of convective boundary layer and stable boundary layer is different. It is found that the deep atmosphere boundary layer phenomena in arid region of Northwest China is correlated to many complex elements, sucn as strong solar radiation flux, favorable dynamic environment, special surface characteristics and so on. In a word, under the particular climate environment, thermal and dynamic factors play key roles together, leading to the supper depth of atmospheric boundary layer occurring in arid region of Northwest China.
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