| Precipitation is one of the most important meteorological elements,and is also an important basic link in the process of water cycle.At present,there has been a lot of researches to understand precipitation from the synoptic and climatological perspective,but the research on precipitation under complex terrain is still a research difficulty.Qilian Mountain is series of mountain systems on the northeast edge of Qinghai-Tibet Plateau,which is an important water supply place in Hexi Corridor and even northwest China.In recent ten years,there have been many uncertain changes in precipitation in this area under the background of global climate change.Therefore,it is urgent to study the climate characteristics and formation mechanisms of precipitation in this area.Firstly,based on high-precision observation data and NCEP/NCAR reanalysis data during 2009-2019,this paper statistically analyzes the temporal and spatial distribution characteristics of precipitation in Qilian Mountain in recent ten years and the periodic characteristics of rainy season,which accounts for more than 90% of the annual precipitation.In view of the density of observation stations,the influence of terrain on precipitation is preliminarily discussed in sections,and representative stations are selected to analyze the relationship between altitude and precipitation.Then,the regional precipitation events in July and August with the most precipitation are selected to synthesize and analyze the atmospheric annular flow field and water vapor field of precipitation.Based on those foundations,a case of precipitation with obvious synoptic systems on July 28,2019 is selected for WRF numerical simulation.The model simulation results are compared with the observation data,and then the three-dimensional variational data assimilation system(3DVAR)is used to assimilate the wind data,and its impact on the simulation results and possible causes are evaluated.Finally,the dynamic diagnosis methods including quasi geostrophic vorticity equation,creepage and bypass equation and water vapor budget equation are selected to analyze the mechanism of this case.The following conclusions are drawn:(1)In recent ten years,the annual cumulative precipitation of Qilian Mountain and its surrounding areas generally presents the spatial distribution characteristics of more in the east and less in the west,more in the south and less in the north.Compared with the precipitation climate in recent ten years,there is an obvious phenomenon of excessive precipitation in the study area.It is the most obvious in2015,and the precipitation in the south of the mountain is generally more than that in the north.The annual precipitation in recent ten years shows an oscillatory increasing trend as a whole,with the most precipitation in summer and the least in winter.The total precipitation in wet season(from May to September)accounts for more than 90%of the annual precipitation,and July and August are the periods with the most precipitation.The periodic signal of "quasi one week oscillation" is the most significant in the whole wet season,and the signal of "quasi two week oscillation" is the most important in July and August.The atmospheric circulation characteristics of excessive precipitation in Qilian mountain area are mainly as follows: "one trough and one ridge" in middle and high latitudes,Balkash Lake trough in middle latitudes,bay of Bengal cyclone and stable subtropical high in low latitudes.On the basis of the above conditions,favorable terrain,better water vapor conditions and the existence of Northwest vortex are the reasons for the precipitation in the south of Qilian Mountain is generally more than that in the north in July and August.(2)The simulation results of WRF model on this precipitation case show that the simulated high empty slot system at 500 h Pa has a good corresponding relationship with those from the reanalysis data,and the simulated local wind field strongly affected by the terrain on 700 h Pa also shows a good simulation ability.In the precipitation field,although the simulated precipitation in the precipitation center is too large,the simulated rain belt position and distribution can be consistent with the actual situation in both the initial stage of precipitation and the stage of heavy precipitation.The results of data assimilation show that the data assimilation system with three-dimensional variation in WRF model can effectively weaken the overestimation error of precipitation before assimilation.650 h Pa is the key layer of creeping flow.The upper flow field shows that the precipitation center simulated is located in front of the windward slope of tall mountains regardless of whether there is data assimilation or not.After assimilation,the wind field has been significantly improved.The creeping flow near the ground in the precipitation center is significantly weakened,which weakens the upward movement,and the precipitation triggered by convection is greatly reduced.Because the topographic forced creeping flow is only a part of the upward movement,the direct vertical velocity can more reflect the development of the upward movement in the process of precipitation.After data assimilation,the vertical velocity over the precipitation center is indeed significantly weakened,and the corresponding precipitation is also reduced.(3)Based on the reliable simulation results of WRF model,the formation mechanism of this precipitation is analyzed.Using the quasi-geostrophic vorticity equation to diagnose the large-scale trough ridge system in the middle and high layers,it is found that the large value area of positive absolute vorticity is located in the front of the trough in the whole precipitation process,and the position of the trough indicates the position of the rain belt.Using the flow around and the flow over equation to diagnose the small and medium-sized local dynamic conditions in the lower atmosphere,it is shown that near the precipitation center in the heavy precipitation stage,when the creeping flow is large,the creeping flow is relatively small,and vice versa.The local convergence of the full wind speed wind field mainly comes from the contribution of the flow around,but the flow over with less contribution plays an important role in the vertical transport of water vapor.The analysis of water vapor transport characteristics in horizontal and vertical directions shows that the water vapor source of this precipitation process is mainly the Sichuan Basin,and some water vapor is transported from the bay of Bengal and the Western Pacific to Qilian mountain area,which are significantly strengthened in the Sichuan Basin.The circumfluence will transport abundant water vapor for the precipitation area,and the creeping flow will promote the water vapor to transport along the topographic gradient and converge and rise in the precipitation center.The water vapor budget near the precipitation center in the heavy precipitation stage shows that the vertical transportation of water vapor plays a leading role in this precipitation stage,the convergence of water vapor is slightly lower than the former,and the local variation of water vapor can be ignored.The analysis of mesoscale stratification instability shows that in this precipitation process,when both water vapor and upward motion conditions are conducive to precipitation,the instability of middle and low stratification in the troposphere is more likely to trigger convection and enhance precipitation. |
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