The formation and development of dunes depend on wind-blown sand movement which is affected by the characteristics of sand material, topography, wind regimes and other factors. In this paper, we investigated two sand shadow dune groups in Shigatse and Za＇gya Zangbo of Tibet and an individual dune in Da Qaidam of Qinghai, and analyzed their topographies and morphologies, and the physical characteristics of the sand, wind regime and sand transport. Formed under harsh conditions behind hills, these mature sand shadow dunes are hundreds of meters long, have significant ridges and crescent dunes downwind, and have a hill pass on one or both sides. Wind tunnel experiments revealed that the hill gap and wind velocity are important factors in the formation of these dunes Sand shadow dunes formed only when the gap spacing is two-thirds of the hill height. When wind velocities are 20 m/s, the sand body is divided into two parts. The hill pass allows the transport of sand by wind, creating a ＂nar- row-pipe effect＂, which causes the transported material to gradually accumulate in the center of the shadow zone. We observed that the following are needed for sand shadow dunes to form： （1） strong winds, sufficient sand, suitable obstacles and a dry climate; （2） one or both sides of the obstacle forming the shadow zone must have a hill pass; and （3） the windward side of the obstacle must have a wide, flat area, providing adequate spacing for wind flow and transport of material and the leeward side must have a sufficiently broad, flat area to allow the release of the transported material. Research results on these newly discovered dunes on the Qinghai-Tibet Plateau could contribute to the understanding of dune geomorphology.
A V-shaped nylon net fence installed in 1990 on top of the Mogao Grottoes is shown to be effectively resisting aeolian sand damage to the grottoes. The structure guides and causes deposition of sand from westerly wind （the primary hard wind）, but to some extent hinders the inverse function of easterly wind carrying sand away from the grottoes toward Mount Mingsha. The gobi side by the fence experiences higher wind speed, so that on which are easily formed undercutting pits, and the deposited sands on it generally form double-peak structures due to abundant sand sources. If the earth surface characteristics in gobi areas by both sides of the fence are quite varied, the erosion and deposition features of the accumulating sand section are similar in different seasons; however, if the earth surface characteristics are similar, the features become irregular in different seasons. Sand depositions with long slope feet are formed along the south and north sides of V-shaped nylon net fence. Disrupted by strong westerly wind and northwesterly wind, sand accumulations by north of the fence are in form of single peaks. Although the operation duration of the V-shaped nylon net fence has exceeded its design life （10 years）, our observations indicate that it is still effective in reducing wind-driven sand damages to the Mogao Grottoes, so it should not be withdrawn.
The desert and permafrost conditions of the Qinghai-Tibet Plateau are unique.However,the effects of desertification on the ground temperature of permafrost are currently unclear.Recently,understanding this problem has become more urgent because of increasing desertification on the plateau.For this reason,an observational field experiment was undertaken by the authors at Honglianghe on the Qinghai-Tibet Plateau.Thermistor ground temperature probes were used,and synchronized contrasting observations were made in an open area.Observations of the ground temperature of permafrost below sand layers with a range of thicknesses were made from May 2010 to April 2011.The sand layers were found to play a key role in the protection of the underlying permafrost.The ground temperature below a permafrost table overlain by a thick sand layer was lower than that of the average annual temperature for the natural ground surface,and the temperature drop was roughly constant at 0.2°C.During the warmer part of the year (May to September),the maximum temperature drops over the five months were 3.40,3.72,4.85,3.16,and 1.88°C,respectively.The ground temperature near a permafrost table overlain by a thin sand layer was also lower than that of the average annual temperature for the natural ground surface.However,in this case the average of the annual maximum temperature drop was significantly less,0.71°C.The scientific significance of our preliminary conclusions is not only to present an exploration of the interaction between desertification and permafrost,but also to provide new engineering ideas for protecting the permafrost in regions where construction is required on the Qinghai-Tibet Plateau.
Sand/dust storms are some of the main hazards in arid and semi-arid zones. These storms also influence global environmental changes. By field observations, empirical statistics, and numerical simulations, pioneer researchers on these natural events have concluded the existence of a positive relationship between thermodynamic effects and sand/dust storms. Thermodynamic effects induce an unsteady stratified atmosphere to influence the process of these storms. However, studies on the relationship of thermodynamic effects with particles (i.e., sand and dust) are limited. In this article, wind tunnel with heating was used to simulate the quantitative relationship between thermodynamic effects and particle movement on different surfaces. Compared with the cold state, the threshold wind velocity of particles is found to be significantly decrease under the hot state. The largest decrease percentage exceedes 9% on fine and coarse sand surfaces. The wind velocity also has a three-power function in the sand transport rate under the hot state with increased sand transport. Thermodynamic effects are stronger on loose surfaces and fine particles, but weaker on compacted surfaces and coarse particles.
