Study On The Dynamic Mechanism Of Formation Of Mesoscale Weahther Systems Triggered By Wind Perturbations

The formation and development mechanism of heavy rainfalls and other severe convective weather have been an emphasis and difficulty problem in the mesometeorological field for their sudden and disastrous features. There are many important mesoscale weather systems resulting in heavy rainfalls and other severe weather such as mesoscale vortex (MV) and squall lines. These weather systems could be mainly classified as two types:point-like system and line-like system. MVs which have been observed with mesoscale data or numerical simulated in mesoscale model usually form in the col fields and within shear lines. However, the formation reasons of the MV and the surface pressure systems of the squall line are not clear. In this paper, the dynamic mechanism of formation of mesoscale weather systems is studied by theoretical analyses and numerical simulations.The observational facts of meso-βscale vortex (MβV) in two typical col fields causing the famous "98.7" torrential rainfall over eatern Hubei province and the extremely heavy rainfall on 6 July 2001 over Shanhai are analysed, respectively, and the dynamic mechanism of the formation of the MV is disccused by use of the theory that a vortex coplet can be triggered by a point vorticity. The winds triggered by the vorticity source are superimposed on the different wind backgrounds. It is shown that there are only some curves without any vortex in the field when the westerly background is strong with wind velocity of 10 m s-1, while in the weak wind background with wind velocity of 2 m s-1, there is a very clear MβV with the size of cyclonic circulation less than 50 km. When the background is dilatation axis, the size of MβV is about 80 km, and the MβV is rather strong as the background filed is replaced by a col field in which a cyclonic and an anticyclonic circulations have sizes of greater than 100 km. It is suggested that the weak wind circumstance is favorable for the formation and development of the MβV.A two-dimensional shallow model is employed to simulate the formation of the MβVs triggered by different mesoscale wind perturbations in a col field. It is shown that, all types of wind perturbations could trigger meso-βscale vortex couplet embryo (MβVCE). The MβVCEs (or one of the vortices) could develop as they move toward the col point or the dilatation axis except for those trigged by the easterly and westerly perturbations. The northerly and the southerly perturbations could trigger stonger vortex couplets. Moreover, with the concurrence of northerly and southerly perturbations, a very strong cyclonic circulation with the size of about 200 km forms as the the triggering vorticices merge near the col point. The effecs of the col point on the formation of the MβV are discussed. It is suggested that the existence of strong wind (or LLJ) which is located far from the col point could produce wind perturbations and trigger the MβVCE. The MβVCE is forced to move toward col point or dilatation axis in the special flow of col field, and the weak wind background near the col point and dilatation axis is favorable for the development of MβVCE, and the MβVCE may be forced to stagnate at col point or to move along the dilatation axis away from the col point.The transient eddy (TE) of meridional component of wind during the "010805" heavy rainfall event caused by a tropical low (TD) over Shanhai is analyzed. The results show that the TD weakens as it makes landfall on Taiwan. When the retainment of the TD with the strong southeasterly TEs moves into the col field near Tawan Strait, the TD develops again, and the redeveloped TD is moving slowly northward along the dilatation axis and develops continuously. This may indicate that the col field and the dilataion axis are mainly responsible for the redevelopment and intensification of the TD.An advanced mesoscaleη-coordinate REM model is used to simulate the "98.7" torrential rainfall over eastern Hubei provinve. Without considering the latent heating feedback, a pure col field background is obtained. Then the formation of the MβV triggered by a mesoscale LLJ (mLLJ) is numerical simulated by adding an ideal mesoscale perturbation to the south of col point. The results show that there is a close relationship between the large- and meso-scale LLJ and the latent heating feedback. Without the latent heating feedback, the LLJ would have a significant weakening, and the MPV would not form, as well as the torrential rainfall. In the experiment of no latent heating feedback, though the wind perturbation could trigger a MβV near the 700-hPa col point, the weak could not be long lived. While in the experiment of considering the latent heating feedback, the intensification of the MβV and the rainfall are stronger, and could maintain for a long time.It is shown by analyzing the simulated temperature and geopotential height that the mLLJ triggering the MβV forms under the circumstance that the latent heat release in the intense precipitation region leads to strong ageostrophic winds duing to a locally strong pressure drop. The analysis of generalized scalar frontogenesis function shows that the effects of deformation flow leads to the convergence of water vapor. The perturbation triggers the onset of rainfall in stratification unstable warm ant wet region. The released latent heat warms the air and causes a low-level wind convergence, and the increase of total deformation and positive vorticity, and then leads to the formation of mesoscale cyclone. The triggered mesoscale cyclone intensifies the frontogenesis function, and further intensifies the rainfall. A conceptual model of the formation of the "98.7" torrential rainfall is summed up:under the favorable thermodynamic conditions in a stable col field, the low level mesoscale wind perturbation triggers strong rainfall; The latent heating release from low-level to upper-level strengthens the mLLJ, and then triggers the formation of the MβV. There is a positive feedback between the heavy rainfall, mLLJ and the MβV.Based on the theory of stream field triggered by singularities in a two-dimensional potential flow, the wind field caused by vorticity lines in an incompressible flow is deduced. By use of a two-dimensional shallow-water model, the flow and pressure fields are numerical simulated in a weak wind background under the influence of mesoscale line-like perturbation similar to outflows of thunderstorm high. The results show an elliptic cyclonic and anticyclonic circulation in association with a positive and a negative vorticity line, respectively. In the case of two vorticity lines, one positive and the other negative, a mesoscale long and narrow 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 presquall mesolow in a mature squall line system are produced. The theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall line may be partly attributed to the dynamical effects of the ageostrophic outflows. By use of the scale separation method, this dynamical mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.Using the mesoscale modle REM, the squall line passed through Jiangxi province on 12 April 2003, resulting in serious disasters with gale, thunder, hail and heavy rainfall is simulated. The formation and development of the mesoscale cyclone group (MCG) in squall system and the relationship between the MCG and LLJ are analyzed. The impact of mesoscale topography, surface character, and solar radiation on precipitation and squall line is studied by sensitivity experiments using factor-separation technique. It is shown that the formation of MCG has a close relationship with the southwesterly and northeasterly LLJ to the south and north of the shealine. Except the thermal effect, the coupled dynamic effect of the LLJs is also very important. The strong horizontal wind velocity shear to the left of the LLJ core is favor of the formation of mesoscale cyclone, and the coupled positive vorticity of the LLJs within the shear line intensifies. Once a perturbation triggers the severe convection, the feedback of the latent heat release to the dynamic field would lead to the maintanence of the LLJs.The sensitive experiments results show that the surface mesocyclones in the experiment without mesoscale topography and solar radiation will weaken obviously in association with a significant reduction of precipitation. These three factors have important impacts on the location and intensity of rainfall and surface mesocyclones. Their mutual interactions have the synergistic contributions. The rainfall in Yiyang of Jiangxi province is distinctly affected by the interaction between surface character and solar radiation. The factor-separation technique allows for a quantitative isolation of the effects due to several factors, so it can be used to determine the contributions of various physical processes, as well as their mutual interactions.