| Tibetan Plateau is the largest and highest plateau in the world and its dynamic effectsexert profound influences on atmospheric circulation and local weather. The mountain dragacting on atmosphere was less studied in Tibetan Plateau. The results of the pressure drag willnot only facilitate to further understand the dynamic effect of Plateau but also provide a goodbenchmark to verify the representation of mountain drag in numerical models.Based on the observational data at six synoptic stations in the southwest of TibetanPlateau, the pressure drag on Gangdise range is firstly estimated. Then the analysis (re-analysis)data at four different resolutions are used to estimate the pressure drag of the main body ofTibetan Plateau in2010. According to the results of the calculation and the topographic scalefeature, the possible drag types in Tibetan Plateau are analyzed. Meanwhile, somecharacteristic analysis of the pressure drag on Plateau is discussed. The main conclusions aresummarized as follows:(1) For the main body of Tibetan Plateau in2010, the zonal pressure torque is positive, whichsignifies a torque upon the earth toward the east. This result implies that the averagedpressure system on Tibetan Plateau will exert an eastward torque that causes the earth toincrease its rate of rotation, imparting angular momentum from the atmosphere to the solidearth. Meanwhile, the atmosphere loses westerly angular momentum, so that the westerlywind is weakened.(2) The pressure drag on the Gandise is mainly contributed between3000-5000meter and itsvalue between5000m and6000m becomes very small. The direction of total drag seemsto be roughly perpendicular to the mountain ridge and it may be connected with themountain range alignment. The seasonal variation of pressure drag is obvious, with amaximum value in February and a minimum value in August. The mountain drag doesn’t necessarily reduce the speed of the airflow, which is the most significantly differencebetween the drag and the friction.(3) The magnitude of the surface pressure drag on the main body of the Plateau increases withthe increase of horizontal resolution of the data. As the horizontal resolution of thetopography increases, more small scale topography and more atmospheric processes areresolved and the magnitude of mountain drag increases correspondingly.(4) Wave drag excited by Rossby wave plays the dominant role in the Tibetan Plateau’s totaldrag; there still exists significant synoptic-scale drag associated with the synopticprocesses of Plateau. As to the meso-scale dynamic influence on the airflow in Gangdiserange, flow spitting is the main type in summer; in winter, surface pressure drag is mainlygenerated by flow splitting below500hPa and by both flow splitting and wave breakingbetween500hPa and200hPa, while the generation of mountain wave and its breaking isthe primary drag type in the stratosphere above200hPa.(5) Through the case studies of the sudden change of the pressure drag, it can be concludedthat the change of pressure drag is closely related with atmospheric activities. Whensynoptic processes pass through plateau, the pressure system on both sides of the mountainwill be changed; meanwhile, the magnitude and direction of the pressure drag will besuddenly changed. Moreover, through the composite analysis of the strong drag eventsunder different resolutions, it can be concluded that the evolution of the strong pressuredrag is a process of the evolution of atmospheric circulation situation and also a process ofthe adjustment of the geopotential height field.(6) Allobaric gradient and topographic gradient are the two influencing factors of the pressuredrag, but the impact of the topographic gradient on pressure drag is finally realized throughthe impact of pressure gradient. |
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