Composite Analysis of Long-Lived Mesoscale Vortices over the Middle Reaches of the Yangtze River Valley: Octant Features and Evolution Mechanisms

Abstract

ABSTRACT A 14-yr climatology is presented of the mesoscale vortices generated in the vicinity of the Dabie Mountains [Dabie vortices (DBVs)] in the Yangtze River valley. Analyzing these vortices using the Climate Forecast System Reanalysis (CFSR), DBVs were found to be a frequent type of summer mesoscale weather system, with a mean monthly frequency of 12.2. DBVs were mainly located in the middle and lower troposphere, and ~92% of them triggered precipitation. Most DBVs were short lived, and only 19.5% persisted for more than 12 h. Latent heat release associated with precipitation is a dominant factor for the DBV’s three-dimensional geometry features, life span, and intensity. The long-lived DBVs, all of which triggered torrential rainfall, were analyzed using a composite analysis under the normalized polar coordinates. Results indicate that these vortices generally moved eastward and northeastward, which corresponded to the vortices’ orientation, divergence, vorticity budget, and kinetic energy budget. The evolution of long-lived DBVs featured significant unevenness: those octants located at the front and on the right side of the vortices’ moving tracks were more favorable for their development and maintenance, while those octants located at the back and on the left side acted conversely. Convergence-related shrinking was the most favorable factor for the vortices’ development and persistence, while the tilting effect was a dominant factor accounting for their attenuation. Long-lived DBVs featured strong baroclinity, and the baroclinic energy conversion acted as the main energy source for the vortices’ evolution. In contrast, the barotropic energy conversion favored the vortices’ development and maintenance at first, and later triggered their dissipation.