| Meiyu is the basic characteristic of East Asian subtropical summer monsoon.Climatologically,accompanied by the seasonal northward shift of the East Asian summer monsoon,the meiyu rainfall usually starts in early-June and ends in mid-July,so the Meiyu season rainfall accounts for more than half of the total annual rainfall over Yangtze-Huaihe Basin.As the primary weather system generating heavy rainfall,the Meiyu Front triggers extreme rainfall events such as persistent and short-term heavy rainstorms almost every year during the Meiyu season,leading to heavy flooding and other disasters.Under the background of global warming,the occurrence frequency of persistent rainstorms and extreme rainfall events tend to increase over the Yangtze-Huaihe Meiyu domain,leading to more severe losses to the local economy.Potential vorticity（abbreviated as PV）is an important physical quantity in atmospheric dynamics,comprehensively representing the dynamic and thermal characteristics of atmosphere and has significant advantages in diagnosing weather systems and their evolutions.Therefore,studying the characteristics of the variations of heavy rainfall and the physical mechanisms of extreme rainstorms over the Yangtze-Huaihe Meiyu domain based on the theory of PV is helpful in improving the understanding of the causes of extreme rainfall events and the ability of weather prediction.Based on the latest development of PV theory,the multiscale variation characteristics of heavy rainfall in the Yangtze-Huaihe Meiyu domain and the physical mechanisms of some typical extreme rainstorm events are studied from the perspective of the relationship between PV and vertical motion changes.The data used in this paper include rainfall data from Chinese stations,CMORPH and station-merged high-resolution rainfall data,reanalysis data from ERA5 and CMIP6 numerical simulation products.In this paper,the long-term variation trend of rainfall intensity during the Meiyu season is analyzed.On the basis of the increasing trend of the Meiyu rainfall,the phase dependence relationship between heavy rainstorm events over the Yangtze-Huaihe Meiyu domain and the Asian Summer monsoon intraseasonal oscillation is explored through synthetic analysis.The differences in circulation structures between persistent rainstorms and extreme rainfall events are also compared,and the role of PV anomalies in vertical velocity changes during extreme events is revealed.Considering the importance of the meridional water vapor gradient in the formation of the Meiyu front during Meiyu season,based on the constraint relationship between vertical motion and PV related to potential temperature,the constraint equation for moisture potential vorticity（MPV）and vertical motion based on the equivalent potential temperature is derived by using the thermodynamic equation containing water vapor.Using the newly derived equation,the physical mechanisms of PV and vertical motion evolution during several typical rainstorms in 2015 and 2020 are revealed.By combining qualitative analysis with quantitative diagnosis of vertical velocity,the differences in the physical mechanisms of vertical velocity variations during the evolutions of three extreme rainfall events during the longest Meiyu season in 2020 are also revealed.The main conclusions are as follows:（1）Attribution analysis of long-term trends in rainfall in the eastern and western Yangtze-Huaihe Meiyu region.During the Meiyu season from 1979 to 2020,there was a significant increasing trend in total rainfall over the Yangtze-Huaihe Meiyu domain（28°–34°N,110°–122.5°E）.The trend of increasing rainfall in the eastern Meiyu region（28°–34°N,116°–122.5°E）is slighter larger than western Meiyu region（28°–34°N,110°–116°E）,while the causes of the increasing trends over the eastern and western Meiyu region are not consistent.The increasing trend in rainfall over the western Meiyu region is mainly related to the long-term increasing trend of local evaporation,while the increasing trend in rainfall over the eastern Meiyu region is mainly affected by the long-term increasing trend in the vertically integrated anomalous moisture advection by the climatological vertical velocity.The anomalous anticyclonic circulation trend on the northwest side of the climatological western North Pacific subtropical high enhanced the gradient of vertically water vapor advection in the eastern Meiyu region,leading to the increasing trend in the eastern Meiyu region.The increased evaporation over the western Meiyu region is related to the increased saturation water vapor pressure resulted from global warming,and the decreased actual water vapor pressure is caused by the movement of the western Pacific subtropical high.Accompanied by the linear increasing trend in total rainfall during the Meiyu season from 1979 to 2020,the occurrence frequency of rainstorms and extreme rainfall also show significant increasing trends over the Yangtze-Huaihe Meiyu domain.（2）Comparative study of typical characteristics and physical mechanisms of persistent rainstorms and extreme rainfall events over the Yangtze-Huaihe Mieyu domain.Statistical analysis shows that persistent rainstorms(the daily precipitation amount exceeds 50mm day-1 for at least 9 neighbouring stations and 3 consecutive days)and extreme rainfall events（the daily precipitation amount exceeds 95%threshold value for at least 9 neighbouring stations）during the Meiyu season occur more frequently in the 4th phase of the 30–60-day period of the Asian summer monsoon intraseasonal oscillation（BSISO1）,and almost never occur in the 7th phase.In phase 4,tropical convection is mainly located in the western equatorial Pacific,while the western North Pacific subtropical high extends into the South China Sea,forming a meridional vertical circulation of downwarding over the South China Sea and ascending over the Meiyu region,providing a favorable circulation background for the development of upward motion,thus contributing to the occurrence of heavy rainfall.In addition,the phase 4 of BSISO1 shows significant increasing trend from 1979 to 2020,indicating that the increasing trend of persistent rainstorms and extreme rainfall events mainly result from the increasing trend in phase 4.During persistent rainstorms,the high-value PV from the north of north of the Meiyu region invading downward into the Meiyu region from the upper atmosphere are favorable for the concave bending of isentropic surfaces and the intrusion of dry and cold air from the upper troposphere into the Meiyu region,promoting the dynamic lifting of warm and humid air in the middle and lower troposphere.In contrast to persistent rainstorms,the north of the Meiyu region is significantly influenced by negative PV anomalies in the upper troposphere during extreme rainfall events.The strengthened northwesterlies at the edge of the anomalous anticyclonic enhance the strong divergence in the upper troposphere,contributing to the development of intense upward motion during extreme rainfall events.Diagnostic analysis shows that the key factor of the development of vertical velocity during extreme rainfall events is the increase of PV advection with increasing height resulted from the negative horizontal PV advection in the lower troposphere and the positive PV advection in the upper troposphere.The positive PV advection in the upper troposphere is related to the transport of high-value PV by the northerlies forced by the negative PV anomaly during extreme rainfall events.（3）Development of the vertical velocity decomposition equation based on the equivalent potential temperature and its application.Accompanied by heavy water vapor transport and strong meridional gradient of water vapor,Meiyu rainfall tends to be associated with the Meiyu front.Considering the contribution of water vapor variation to the vertical motion,and in order to separate the contribution of the rainfall-released latent heating and other diabatic heating to the vertical motion during extreme rainfall events,the PV and vertical motion constraint equation proposed by Wu et al.（2020）is further developed in this study from the equivalent potential temperature perspective using the thermodynamic equation.Then,the newly derived MPV and vertical motion constraint in this study extends the theory of PV.In the vertical motion decomposition equation based on the equivalent potential temperature,the impact of the variations of the isentropic surfaces on the vertical velocity components in the vertical velocity decomposition equation proposed by Wu et al.（2020）is replaced by the impact of the variations of the moist isentropic surfaces.The vertical velocity component related to diabatic heating only includes the contributions of sensible and radiation heating,and the contribution of rainfall-released latent heating is no longer included,contributing to exploring the dynamic mechanisms of vertical velocity evolution in near-saturated extreme rainfall events.The equation based on equivalent potential temperature comprehensively reflects the contributions of moist isentropic-gliding,moist isentropic-displacement,sensible and radiation heating,and the convective stability of the atmosphere to the evolution of the total vertical velocity.Therefore,the developed vertical motion decomposition equation is particularly useful in analyzing extreme rainfall events which occur in the moist atmosphere.It can be found that the extreme rainfall event associated with the Meiyu front which occurred in late June 2015 in the Yangtze-Huaihe Meiyu domain was accompanied by a strong meridional equivalent potential temperature gradient and a tilted equivalent potential temperature belt in the lower and middle troposphere.The vertical velocity evolution mechanism in this event is diagnosed using the vertical velocity decomposition equation based on equivalent potential temperature,suggesting that the upward motion along the tilted moist isentropic surfaces is found to occur before the actual ascending motion and the occurence of rainfall,triggering this extreme rainfall event.（4）Comparative study on the influence of different circulation characteristics on the evolution of vertical velocity during extreme rainfall events.During the longest Meiyu season in 2020,the Meiyu region experienced three extreme rainfall events.Due to the gradually eastward-moving Southwest Vortex,ascending moist isentropic displacement vertical velocity in the convectively unstable atmosphere in the lower troposphere is induced in the first extreme rainfall event,triggering the first event.The ascending moist isentropic displacement vertical velocity in the lower troposphere,along with the ascending vertical velocity along the moist isentropic surfaces in the convectively stable atmosphere in the middle troposphere resulted from the eastward-moving Tibetan Plateau Vortex,jointly contributed to the development of actual ascending motions and rainfall.The development of ascending motions during the second extreme rainfall event was mainly related to the downward intrusion of the large-value PV on the north of the rainfall key region.The downward intrusion of the large-value PV contributed to the downward intrusion of dry and cold air from the middle and upper troposphere along the sloping moist isentropic surfaces to the rainfall key region,strengthening the convergence lifting and increasing the meridional water vapor gradient in the middle and lower troposphere.Therefore,the developed ascending vertical velocity along the moist isentropic surfaces in the convectively stable atmosphere in the middle troposphere led to the development of total ascending motion and this extreme rainfall event.The key region of the third extreme rainfall event was located in front of the upper trough,mainly controlled by the southwesterlies in the lower troposphere.The upper trough deepened and moved eastward gradually,enhancing the cyclonic shear in the lower troposphere over the eastern upper trough,and the lower trough behind the key region gradually strengthened and moved eastward.The rainfall key region was gradually controlled by positive horizontal PV advection in the upper troposphere and negative horizontal PV advection in the lower troposphere,leading to increase PV advection with increasing height,thus contributing to the development of c and this extreme rainfall event.Although the physical mechanisms of vertical velocity evolution in the three extreme rainfall events were not consistent,the vertical structure of increase PV advection with increasing height was present in all three events,leading to the development of increase PV advection with increasing height. |
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