A Multi-scale Dynamic Study Of Two Extreme Warm Region Torrential Rain Cases

As a key problem in meteorology,the dynamical study of warm-sector rainstorms has been of continuing interest.Using the new functional analysis apparatus,namely,multiscale window transform(MWT),and the MWT-based localized energetics analysis and theory of canonical transfer,the two extreme warm-sector rainstorm events on May 7,2018,in South China and July 21,2012,in Beijing are diagnosed for an understanding of its underlying dynamics.The main conclusions are as follows:(1)First,the atmospheric fields from the ERA5 data are reconstructed onto three scale windows,namely,the background window,quasi-biweekly oscillation(QBWO)window and rainstorm window.From the reconstructed fields the upper-layer and lower-layer jets are well represented on the background window,and the vertical motion is clearly seen on the rainstorm window.(2)Previously it has been believed that the warm-sector rainstorms are characterized by weak baroclinicity;but here baroclinic instability matters as well as barotropic instability.(3)It is found that the dynamical processes differ with height.In the lower layer,barotropic instability dominates,and the barotropic canonical transfer directing to the rainstorm window from the QBWO window is more important than that from the background window.In the middle layer,the instability is mixed;apart from barotropic instability,the baroclinic canonical transfer also brings available potential energy from the background window to the rainstorm window,which is then converted into kinetic energy,maintaining the rainstorm-scale motion in the middle layer.As the lower layer,the upper layer sees only the canonical energy transfer from the background window to the rainstorm window.(4)By the comparison of the two rainstorm events in China,it is found that the dynamical processes at different heights are similar and the difference lies in the dominated time scales affecting the middle and lower troposphere.In the lower troposphere,the QBWO window is more important than the background window in the case of Xiamen,while in the case of Beijing,it is completely dominated by the QBWO window.In the middle troposphere,the Xiamen case is influenced by the background scale and the QBWO scale,while the Beijing rainstorm is still affected by the QBWO scale,just like the lower layer.(5)By the comparison with the second stage of the North China case,the frontal rainstorm is mainly influenced by the background window via baroclinic instability,while the warmsector rainfall involves much more complex interactions among different scale windows,making its forecasting a difficult task.