| With the use of conventional and automatic weather stations observations, FY-2E infrared satellite imagery, Doppler radar and NCEP reanalysis data, synoptic analysis is conducted of the Chongqing severe convection event that occurred on5May2010, focusing on the evolutions of different weather systems as well as their configurations and interactions. A conceptual model of synoptic background for this case is proposed. With the knowledge of large scale environment, mesoscale processes and mechanisms responsible for the formation of conditional unstable lapse rate and low level lifting are further investigated with the results from a high resolution numerical simulation employing the Weather Research and Forecasting (WRF) model. Sensitivity experiments concerning the role of solar radiation and local topography of western Chongqing in convection initiation (CI) process are also performed in this study.This event occurred under the background that midlatitude cyclone, anticyclone and upper trough were growing. Between the surface cyclone and anticyclone, a cold front emerged and intensified during southward propagation. Meanwhile, in Southwest China located another cyclone. At the northeast of it, cyclonically sheared flow comprised of southerly and northeasterly winds dominated the low levels of Chongqing. Moreover, this particular cyclone with the extratropical surface pressure systems and the subtropical high made up a stretching deformation field, whose axis of dilation was in close proximity to the cold front in the western part of it. In the midtroposphere, the trough in the north and the ridge in the south tended to be in phase gradually during their eastward propagations, forming a midlevel jet streak north of Yangtze River. Chongqing was just in the right entrance region of this jet and upstream of the lower latitude broad ridge. The developments of the lower-to-mid level weather systems were accompanied by the eastward propagation of an upper level jet.During this process, the deformation field at low levels caused frontogenesis along the axis of dilation and the cold front, strengthening associated frontal circulation. Interestingly, this secondary circulation happened to coincide with that in the entrance region of the midlevel jet, resulting in positive feedback interactions between the low level frontogenesis and the midlevel jet via this ageostrophic vertical circulation, which enhanced lower-to-mid level large scale lifting over Chongqing remarkably. The ascending motion is found to be crucial in lapse rate increase by stretching effects, which in combination with diabatic heating due to solar radiation caused continuous decrease of static stability in low levels and increase of convective available potential energy (CAPE). In the meantime, the cyclone in Southwest China transported abundant moisture northward by the southerly wind in the eastern part of it, which enabled accumulation and deepening of low level moisture in Chongqing. Besides, an east-west-oriented mesoscale convergence line formed in the horizontally sheared flow few hours before the occurrence of convection in the southwest of Chongqing, and then propagated northward steadily. Since the air at both sides of the line possessed high water vapor mixing ratios, significant boundary-layer moisture convergence occurred, providing favorable environment for the initial development of convection. Furthermore, the mesoscale convergence line and the cold front approached to each other during their propagations in opposite directions. As both lines had similar vertical circulation patterns, that is, boundary-layer air converged and ascended along the lines, diverging at the height of about2km, two cells collided in the process. The divergent flows at2km from both cells caused convergence and associated lifting, providing the triggering mechanism for the convection in this case. The results from the sensitivity experiment show that the local topography of western Chongqing played little role in CI process. |
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