ABSTRACT The third precipitation episode of China＇s great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an...ABSTRACT The third precipitation episode of China＇s great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an important role in the rainfall over south- ern China. With the eastward movement of the trough, the jet shape changed from two straight jets to a tilting jet over China and then it moved southward. With these variations, the south-north movement of ascending flow and precipitation area over southern China occurred.展开更多
Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow,the wind field caused by vorticity lines in an incompressible flow is deduced in this paper.The result...Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow,the wind field caused by vorticity lines in an incompressible flow is deduced in this paper.The result shows an elliptic cyclonic（anticyclonic） circulation in association with a positive（negative） vorticity line.By use of the shallow-water model,the flow fields are simulated in a weak wind background under the influence of mesoscale vorticity lines.In the case of two vorticity line,one positive and the other negative,a mesoscale vortex couplet forms in the flow.When three vorticity lines are considered,three mesoscale circulations develop,and a mesohigh and two mesolows similar to the thunderstorm high,wake low and pre-squall mesolow of a mature squall line are produced.Theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall lines may be partly attributed to the dynamical effects of the ageostrophic outflows.The strong downdrafts under the thundercloud base of the squall line lead to surface ageostrophic outflows,and produce positive-negative-positive arranged vertical vorticity bands（VBs） along the direction normal to the squall line,then the mesoscale circulations develop and mesoscale pressure systems form or strengthen during the geostrophic adjustment.By use of the scale separation method,this dynamic mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.展开更多
基金supported by the National Basic Research Project of China (Grant Nos.2013CB430105 and 2012CB417201)the National Natural Science Foundation of China (Grant No.40930950)+1 种基金Chinese Academy of Meteorological Sciences State Key Laboratory of Severe Weather (LaSW) (Grant No.2011LASW-A01)the Key Research Program of the Sciences (Grant No.KZZD-EW-05-01)
文摘ABSTRACT The third precipitation episode of China＇s great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an important role in the rainfall over south- ern China. With the eastward movement of the trough, the jet shape changed from two straight jets to a tilting jet over China and then it moved southward. With these variations, the south-north movement of ascending flow and precipitation area over southern China occurred.
基金Supported jointly by the National Fundamental Research "973" Program of China under Grant No. 2004CB418301the National Natural Science Foundation of China under Grant Nos. 40775034 and 40830958
文摘Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow,the wind field caused by vorticity lines in an incompressible flow is deduced in this paper.The result shows an elliptic cyclonic（anticyclonic） circulation in association with a positive（negative） vorticity line.By use of the shallow-water model,the flow fields are simulated in a weak wind background under the influence of mesoscale vorticity lines.In the case of two vorticity line,one positive and the other negative,a mesoscale vortex couplet forms in the flow.When three vorticity lines are considered,three mesoscale circulations develop,and a mesohigh and two mesolows similar to the thunderstorm high,wake low and pre-squall mesolow of a mature squall line are produced.Theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall lines may be partly attributed to the dynamical effects of the ageostrophic outflows.The strong downdrafts under the thundercloud base of the squall line lead to surface ageostrophic outflows,and produce positive-negative-positive arranged vertical vorticity bands（VBs） along the direction normal to the squall line,then the mesoscale circulations develop and mesoscale pressure systems form or strengthen during the geostrophic adjustment.By use of the scale separation method,this dynamic mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.
