## 4 The Dynamic Effects
### Frontogenesis Dynamic Effect  
#### 4.2.1 Impacts on Frontogenesis  
Significant effects of aerosols on cloud microphysics and dynamics of MCSs have been found [25; 30; 77]. For the Mei-Yu front, a synoptic-scale system, whether or not, and how, did aerosols associated with thermodynamic and dynamic effects influence the frontal structure and intensity, have not been studied yet. A significant character of Mei-Yu front is strong humidity gradient across the front. Therefore, it is more suitable to describe the intensity of Mei-Yu fronts by using virtual potential temperature gradient variations [60]. The vertical cross sections of zonal mean virtual potential temperature perturbation (\(\theta_{v}=\theta(1+0.61q_{v})\), \(\theta^{\prime}{}_{v}=\theta_{v}-\overline{\theta}_{v}(z)\)), the tendency of the virtual potential temperature gradient (\(\frac{\theta}{\partial t}(\
abla\theta_{v})\)), and cross-front wind perturbation vectors (\(v^{\prime},w\)) (here, \(v^{\prime}=v-V_{S}\)) are shown in Figure 13. During the initial stage of precipitation (t = 15 h), the differences of the frontal boundaries and the cross-front circulation between the two cases are negligible. However, there is a slight difference in the intensity of the front (Figure 13a,b). Comparing the tendency of the virtual potential temperature gradients in the two cases, the difference is obvious at high levels around 6 km. While there is less of difference at low levels below 2 km. This is in agreement with the latent heat distribution. Figure 11: Vertical cross sections of zonal-mean Latent Heating (The warm color shadings show heating greater than 1 K h\({}^{-1}\); the cold color shadings show cooling less than \(-0.5\) K h\({}^{-1}\)) and cross-front wind perturbation vectors (\(v^{\prime},w\)) at t = 21 h. The blue thick line indicates the front. Only a 500 km \(\times\) 15 km of the domain is presented. The left panel shows C-case, the right panel shows P-case. Figure 12: Vertical cross sections of zonal-mean Divergence at t = 21 h (The contours show divergence greater than \(10^{-5}\) s\({}^{-1}\), the color shadings show convergence less than \(10^{-5}\) s\({}^{-1}\)). Only a 500 km \(\times\) 15 km of the domain is presented. The left panel shows C-case, the right panel shows P-case. Figure 13: Vertical cross sections of zonal mean virtual potential temperature perturbation (\(\theta^{\prime}v\), 1.0 K contour interval), the tendency of the virtual potential temperature gradient (shading shows greater than 3 k/100 km \(\cdot\) h) and cross-front wind perturbation vectors (\(v^{\prime},w\)) at (**a**,**b**) t = 15 h, (**c**,**d**) 18 h and (**e**,**f**) 21 h. Negative contours are solid. The blue thick solid line highlights -0.5 K isopleth of virtual potential temperature perturbation in order to approximately delineate the leading edge of the front in the lower troposphere. Only a 900 km \(\times\) 10 km of the domain is presented. The left panels show C-case, the right panels show P-case. During the developing stage of precipitation (t = 18 h), the frontal boundaries of both cases move around 20 km to the south. Low-level convergence, high-level divergence, and cross-front ageostrophic circulation all intensify (Figure 13c,d). At this time, the greatest virtual potential temperature perturbations are above 2 K, which is at a height of 6-8 km in both cases. With development of the precipitation system, both high-level heating and low-level cooling increase further, causing the tendency of the virtual potential temperature gradient to increase at both high and low levels. In comparison, the tendencies of virtual potential temperature gradient at a height of 6 km and below 2 km enhanced significantly in P-case. It is noted that the maximum tendencies of virtual potential temperature gradient also exceeds 10 k/100 km \(\cdot\) h at low levels. Thus, Frontogenesis associated with Mei-Yu front strengthens significantly at low levels. At t = 21 h (Figure 13e,f), the frontal boundary in both cases moves slightly to the south.