A Numerical Study of Mesoscale Vortex Formation in the Midlatitudes:The Role of Moist Processes

In this study, a three-dimensional mesoscale model was used to numerically simulate the well-known "98.7" heavy rainfall event that affected the Yangtze Valley in July 1998. Two experiments were conducted to analyze the impact of moist processes on the development of meso-β scale vortices(MβV) and their triggering by mesoscale wind perturbation(MWP). In the experiment in which the latent heat feedback(LHF) scheme was switched off, a stable low-level col field(i.e., saddle field—a region between two lows and two highs in the isobaric surface) formed, and the MWP triggered a weak MβV. However, when the LHF scheme was switched on as the MWP was introduced into the model, the MβV developed quickly and intense rainfall and a mesoscale low-level jet(mLLJ) were generated. The thickness of the air column and average temperature between 400 and 700 hPa decreased without the feedback of latent heat, whereas they increased quickly when the LHF scheme was switched on, with the air pressure falling at low levels but rising at upper levels. A schematic representation of the positive feedbacks among the mesoscale vortex, rainfall, and mLLJ shows that in the initial stage of the MβV, the MWP triggers light rainfall and the latent heat occurs at low levels, which leads to weak convergence and ageostrophic winds. In the mature stage of the MβV, convection extends to the middle-to-upper levels, resulting in an increase in the average temperature and a stretching of the air column. A low-level cyclonic circulation forms under the effect of Coriolis torque, and the m LLJ forms to the southeast of the MβV.

A Study on the Dynamic Mechanism of the Formation of Mesoscale Vortex in Col Field

The mesoscale vortex associated with a mesoscale low-level jet （mLLJ） usually causes heavy rainfall in the col field. The col field is defined as a region between two highs and two lows, with the isobaric surface similar to a col. Using a two-dimensional shallow water model, the meso-β scale vortex couplets （MβVCs） induced by eight types of mesoscale wind perturbations in an ideal col field were numerically simulated. With the sizes of -100 km, the MβVCs induced by northerly perturbation （NP） and southerly perturbation （SP） moved toward the col point. The sizes of MβVCs induced by southwesterly perturbation （SWP）, southeasterly perturbation （SEP）, northwesterly perturbation （NWP）, and northeasterly perturbation （NEP） were relatively small for the perturbations moving toward dilatation axis. The MβVC induced by easterly perturbation （EP） and westerly perturbation （WP） could not develop because they quickly moved away from the col point, before the circulation could form. The size of the circulation was determined by the distance between the vortex and the col point. The closer to the col point the vortex was, the larger the size of vortex. The comparisons of maximum vorticity and vorticity root mean square error （RMSE） of the NP, the SWP, and the WP show that the maximum vorticity and the vorticity RMSE of the NP decreased slower than other perturbations. Therefore, the weak environment of the col field favors the maintenance of vorticity and the formation of vortex. When a mesoscale vortex forms near the col point or moves toward the col point, it may maintain a quasitationary state in the stable col field.

Numerical Simulation on the Formation of Mesoscale Vortex in Col Field

There exist typically two kinds of low-level col fields over the middle and lower reaches of the Yangtze River of China during summer. ＇One is associated with the mesoscale vortex embedded in the Meiyu front; the other is related to tropical cyclones making landfall over eastern or southern China. The first one is the focus of this study. The meso scale vortex （MβV） causing heavy rainfall usually forms in a col field or within a shear line associated with the mesoscale low-level jet （mLLJ）. The MβV, triggered by mesoscale wind perturbation in a col field, is simulated by using a three-dimensional q-coordinate mesoscale model. This col field represents the circumstance of the 98.7 heavy rainfall event over eastern Hubei Province. The results show that the MβV triggered by wind perturbation was weak and maintained only several hours if the latent heat feedback was switched off. The wind perturbation also weakened rapidly. However, when the latent heat feedback was included, precipitation became more intense and the mLLJ and MβV quickly developed. The MβV maintained quasi-stationary during its life cycle under the stable col field. The MβV triggered by the southwesterly perturbation was located closely to that by the northeasterly perturbation. They were both located in the weak wind region near the col point. The stronger the perturbation was, the more intense and longer the dynamic MβV lived. The 24-h accumulated precipitation in different experiments showed a similar pattern, which indicates that the relatively stable intensity and range of precipitation were the intrinsic characters of the stable col field. Furthermore, it is found that mesoscale perturbations had some impacts on the location and intensity of the rainfall. The fluctuation of large-scale LLJ to the south of the col field might produce a perturbation, causing instable stratification and rainfall within the low-level col field or shear line. The mLLJ near the rainfall was enhanced due to the latent heat feedback and then caused formation of the MβV. The stable col field provided favorable thermodynamic conditions for the formation and development of the MβV. Therefore, it is an ＂incubator＂ for MβVs and the associated consecutive heavy rainfall.

The Effects of Strong Ageostrophic Outflows on the Formation of Surface Mesoscale Pressure Systems in Squall Lines

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.