Numerical Simulation And Diagnostic Analysis For The Large-scale Dust-storm Weather In Northern China During 9～(th)-11～(th) April 2006

In this paper, firstly we take an analysis for the duststorm route and the duststorm types of this large-scale duststorm process based on the surface observation data, and then we give duststorm weather model through the upper-level influential systems by using the upper air observation data. Using the GRAPES-MESO model ,we do numerical simulation on this dust storm process.According to model output results, we study the whole duststorm process in both dynamic and thermal aspect.especially, we explores the evolutional characteristics of the initial stage of the two route duststorm. Base on these studies, we obtain some important results, which provide a certain scientific basis for further studying and forecasting duststorm weather. These results are as follows:(1) This duststorm process occurs under very advantageous climatic background and weather background. Before this process, the higher temperature, the less little precipitation and the frequent cold air activity are beneficial climatic background. The middle-high latitude shows inverse "Ω" stream pattern before this process, in which the transversal trough and the frontal zone are major upper influential systems. The surface influential systems are mainly cold front and Mongolia cyclone.(2)Through analysis for the surface synoptic systems influencing this duststorm process, this process is divided into west route and northwest route. The cold front I around Mountain Altai and Mongolia cyclone before cold front give rise to the west route duststorm, which mainly influences Mongolia ,Inner-mongolia, North china, Northeast china; the cold front II around Mountain Tianshan lead to northwest route duststorm, which influences the East Xinjiang, Qinghai province, Gansu province, Ningxia autonomous region, Shaanxi province.(3) There are two strong duststorm stages, the first stage appears during 11:00 to 20:00 9^th, the second stage appears during 08:00 to 20:00 10^th, the period of 20:00 9^th to 08:00 10^th is in decreasing stage.During the first stage of the west route, the strong upper divergence, potential vorticity pouring-downward and warm advection exert important dynamic and thermal roles on continuous development of Mongolia cyclone. From upper systems, the duststorm occurs in upper trough before frontal zone and jet stream area; From surface wind field, duststorm mainly occurs in west wind of south side of the cyclonic circulation. The duststorm during this period belongs to Mongolia cyclone category.During the second stage of the west route, it is the mainly surface influential systems of the second duststorm eruption of the west route that the cold air invasion after cold front and Mongolia cyclonic maintenance before cold front. The strong northwest wind before cold front and the strong southwest wind of the south side of the Mongolia cyclonic circulation provide horizontal dynamic condition for the second duststorm eruption. The duststorm during this period belongs to mixed category of the cold front and Mongolia cyclone.(4) The whole duststorm of northwest route belongs to pure dry-cold front. The strong cold advection in frontal zone after the transversal trough turning-vertical gives rise to the northwest route duststorm, the duststorm mainly takes place in strong northwest wind before cold front. The coupling effect of upward branches of secondary circulation formed by upper jet stream and the secondary circulation before cold front provides major vertical dynamic condition. There is the most important contribution of the cold advection on 700hPa on the northwest route duststorm, the rapid increasing of cold advection in Hexi corridor offers a good indication for eruption time of duststorm. The appearance and motion of PV2 on 700hPa can take trace as low jet stream in Hexi corridor, meantime, which also can be as the important forecasting index for occurrence of the northwest route duststorm.(5) This whole duststorm process occurs in stable atmospheric stratification state. There are close relation between the divergence, vorticity field distribution and occurrence and development of duststorm. The whole duststorm takes place in the broad ladder trough locating head of the inverse "Ω" stream pattern on height field, there exists strong positive vorticity below 500hPa before trough and exists negative vorticity above 500hPa.It makes the duststorm to maintain with a long time that convergence alternating with divergence and the more convergence and the less divergence in middle-low level. The initial stage of the two route duststorm eruption both show evident increasing divergence. The upper jet stream plays a major role on occurrence and development of the duststorm, the Ferrel circulation below upper jet stream can play a role which carries the momentum to low level and strengthens the frontal zone; the Ferrel circulation is the important dynamic and thermal mechanism to bring about duststorm.