摘要
从变形场驱动锋生及通过锋生引起倾斜涡度发展的角度对变形场在北京"7.21"暴雨发生、发展过程中的可能作用机制进行了初步探讨。诊断结果发现:北京地区降水产生时,变形向量与等位温线走向一致或有较小夹角,北京地区有较强的变形场局地锋生过程。锋生函数分析发现,变形项对引发暴雨的低层锋生有重要贡献。锋生能够引发大气动力、热力结构的调整,伴随大气锋生过程的高空急流加强和转竖使得北京地区处于高空急流入口区右侧的辐散区中,其带动低层空气辐合,有助于暴雨的加强维持。分析还发现,"7.21"暴雨过程中,垂直涡度存在爆发性发展,尤其是锋面降水阶段,而大气斜压度的增长趋势与垂直涡度增长趋势十分一致。分析全型涡度方程中与变形场有关的斜压度个别变化项发现,与变形场相关的垂直涡度驱动项异常正值区与垂直涡度爆发性增长区相对应,表明变形场在北京"7.21"暴雨过程中对垂直涡度发展有重要贡献。基于变形场沿其压缩轴方向气流汇合的特点,进一步分析了加入水汽作用的水汽通量变形场与暴雨发生、发展的关系。分析结果发现,低层水汽通量变形场的正值区与暴雨具有很强的相关,且水汽通量变形场包含两部分,一部分为比湿平流,其对未来暴雨区位置有很好的指示意义;一部分为变形场项,其对水汽通量变形场分布起主要贡献。
This paper studied the possible mechanism for the deformation field affecting the occurrence and development of torrcntial rainfall,from the perspective of the frontogenesis and slantwise vorticity development.Diagnostic study of the ＂7.21＂ severe rainfall case which occurred in Beijing on 21 July 2012 showed that the deformation tick marks had the same orientations or a small angles with the moist potential temperature contours when the rainfall presented in Beijing, which implied the local frontogenesis.The frontogenesis function calculation results showed that the deformation term of the frontogenesis function had a significant contribution to the low-level frontogenesis which induced torrential precipitation in Beijing.The high-level jet stream then accelerated and veered northernly, which resulted in strong high-level divergence and enhanecm ent of severe precipnation in Beijing.The diagnostic results also showed that the vertical vorticity had an explosive growth accompanying the occurrenec of severe precipitation.Moreover, in the Beijing district, changes in the baroclinicity of the atmosphere had a similar growth trend with the vertical vorticity.According to the slantwise vorticity development theory, the baroclinicity term associated with the deformation in the complete form of vertical vorticity equation was analyzed.The results showed that the deformanon term driving vertical vorticity had positive values in the vertical vorticity growth areas, which implied that the deformation had a significant contribution in the development of vertical vorticity in the ＂7.21” severe precipitation case.In view of the do formation drove air contraction in its confluence axis,the moisture flux deformation was introduced to include moisture into the deformation.It was shown that the low-level positive anomalies of moisture flux deformation corresponded well with the rainfall areas.The moisture flux deformation contained two parts; one was specific humidity advection and the other was deformation.The deformation determined the basic distribution of moisture flux deformation while the specific humidity advection had more rainfall forccasting implication.
出处
《气象学报》
CAS
CSCD
北大核心
2013年第4期593-605,共13页
Acta Meteorologica Sinica
基金
国家重点基础研究发展计划(2013CB430105)
中国科学院重点部署项目(KZZD-EW-05)
灾害天气国家重点实验室开放课题(2011LASW-B15)
公益性行业(气象)科研专项(GYHY200906004)
国家自然科学基金项目(41175060
41075098和41005005)
关键词
变形场
锋生
倾斜涡度发展
水汽通量变形场
Deformation Field, Frontogenesis, Slantwise vorticity development, Moisture flux deformation
