梅雨锋气旋暴雨的Q矢量分析：个例研究

The Q vector analysis of the heavy rainfall from meiyu front cyclone:a case study.

文中对修改的Q矢量(Q*)进行转化、处理后,所得Q矢量(记为QN矢量)与准地转Q矢量具有类似的计算表达式,但其完全用实际风场资料进行计算。结合1991年7月5日20:00—6日20:00BST的一次典型江淮梅雨锋气旋暴雨过程比较分析表明,QN矢量诊断能力较准地转Q矢量优越,且700hPaQN矢量散度辐合场对同时期地面降水场的水平分布特征具有较好指示作用。将QN矢量沿以等高线为参照线的自然坐标系进行分解(简称为PG分解),所得各项QaNlst矢量(沿流伸展项)、QcNurv矢量(曲率项)、QsNhdv矢量(切变平流项)及QcNrst矢量(穿流伸展项)具有明确物理意义。对1991年7月5日20:00—6日20:00BST此次江淮梅雨锋气旋暴雨过程进行QN矢量PG分解研究表明,QN矢量PG分解可以揭示出天气现象过程中"总"的QN矢量(即QN矢量)难以揭示的潜在物理机制。具体地讲,在梅雨锋气旋不同阶段,QaNlst矢量散度场的水平分布特征都与总QN矢量散度场相似,其散度辐合场在总QN矢量散度辐合场中都占有较大比例,对总QN矢量散度对垂直运动产生的激发与强迫作用贡献大,对梅雨锋气旋引发降水的发生始终都起着主要的促进强迫作用。QcNurv矢量在整个梅雨锋气旋暴雨演变过程中,对降水发生的促进作用逐渐减小,直至起到抑制作用。QsNhdv矢量对降水发生的促进作用则随着梅雨锋气旋发生发展而明显增强,但随着梅雨锋气旋的东移衰亡,其对降水发生的促进作用迅速减弱,直至对降水的发生基本无影响。对于QcNrst矢量来讲,其在梅雨锋气旋的发生发展及强盛阶段对降水的发生基本不起作用,但在梅雨锋气旋衰亡阶段其对降水发生起着主要促进作用。另外,在梅雨锋气旋发生发展及强盛时期,QaNlst矢量与QcNurv矢量、QsNhdv矢量与QcNrst矢量的散度水平分布特征相似,只不过强度上存在差异,但无明显相互抵消现象,而在梅雨锋气旋衰亡阶段就不同了,QaNlst矢量与QcNurv矢量、QsNhdv矢量与QcNrst矢量的散度水平分布特征基本相反,且存在明显的相互抵消现象。可见,通过QN矢量PG分解可以揭示出梅雨锋气旋不同阶段降水的强迫因子是不同的。

After the modified Q vector （ Q^ ＊ ） is transferred and handled, a kind of Q vector （Q^N） is derived, whose calculating formula is similar to counterpart of quasi-geostrophic Q vector （Q^G）, however, Q^N vector is calculated by real wind. The diagnostic abilities of Q^N vector and Q^G vector are compared on the basis of a typical Changjiang-Huaihe Meiyu front cyclone （MYFC） rain storm occurred during 5 - 6th July 1991, the results show that the diagnostic ability of Q^N vector is better in comparison to the counterpart of Q^G vector, in the meanwhile, the convergence filed of Q^N vector divergence at 700 hPa can serve as a good indicator to the horizontal distribution feather of the simultaneous real precipitation. Then Q ^N vector is partitioned into Q ^Nalst （the ＂alongstream stretching＂ component that represents the intensification/weakening of the alongflow thermal gradient by contraction/stretchiong of isotherm spacing）, Q^Ncurv（the ＂curvature＂ component that represents the curvature effect according to which a downstream increase （decrease） in the cyclonic curvature of the isohypses induces subsidence （ascent））, Q ^Nshdv（ the ＂shear advection＂ component that represents the thermal advection by horizontal wind shear）, and Q ^Ncrst （the ＂crosstream stretching＂ that represents the effect of confluence and diffluence of the wind, i. e. , the intensification/weakening of the cross-flow thermal gradient forced by confluence and diffluence of the wind） four components in a natural coordinate system that follows the isohypses （hereafter referred to as PG partitioning）, and the four terms have specific physical meanings respectively. The application of Q^N PG partitioning to analyze the MYFC torrential rain indicates that Q^N PG partitioning can disclose the potential physical mechanism of synoptic process, which is difficult for ＂total＂ Q^N （which is equal to Q^N） to display. Specifically, the horizontal distribution characteristic of 2 △↓· Q^ Nalst is always similar to counterpart of 2 △↓· Q^N and accounts for high proportion on the differing stages of MYFC, which means that the former has large contribution to the latter in the context of exciting and forcing vertical motion generation and plays a primary forcing role in occurrence of MYFC precipitation. During the whole evolution of the heavy rain of MYFC, the effect of Q^Ncurv on the genesis of rain gets little by little until plays a restrained role. While the counterpart of Q^Nshdv gets strong with developing of MYFC and gets weak rapidly until almost disappears as MYFC moves eastward to sea and decays. To Q^Ncrst, it has no effect on the generation of the precipitation during the developing and strong stages of MYFC in such a manner that it plays a leading role on the MYF decaying stage. In addition, Q^Nalst and Q^Ncurv, Q^Nshdv and Q^Ncrst are analogous respectively on the basis of the divergence horizontal distribution feathers with only difference in intensity and no obvious mutual cancellation during the MYFC developing and strong stages, however, they are basically on the contrary and have marked mutual cancellation on the decaying stage of the MYFC. Obviously, the Q^N PG partitioning can disclose clearly the fact that the forcing factors of precipitation generation are different on differing stages of MYFC.