Analysis of the Formation Mechanism of Overshooting Convection during a Rainstorm Process in the Sichuan Basin

A rainstorm process in the western Sichuan Basin was closely related to the occurrence and development of overshooting convection during 18–22 August 2019. This paper used the ERA5 reanalysis data and high-resolution numerical simulation results of WRF model to reveal the formation mechanism of this overshooting convection. The results indicate that this overshooting convection occurred under a special background, in which a large-scale tropopause folding in the coastal areas of China resulted in the downward stratospheric PV (potential vorticity) over the eastern Sichuan Basin. Meanwhile, a Tibetan Plateau vortex and a Southwest China vortex were observed on the Tibetan Plateau and the Sichuan Basin, respectively. The overshooting convection process can be divided into three stages: (1) Convective initiation. The continental high extended westward, enhancing the pressure gradient near the Sichuan Basin and then causing a LLJ (ow-level jet). The cyclonic shear produced by LLJ strengthened the airflow orthogonal to the southeast of the Tibetan Plateau. In terms of atmospheric instability, the dynamic uplift of terrain and convergence uplift of airflow triggered convection. (2) Development of ascending motion in the upper troposphere. The ascending motion, in this case, was primarily related to the PV air block developing with diabatic heating. The PV air block was generated due to the persistent downward transfer of the stratospheric PV resulting from turbulent activity. The diabatic heating caused by convective development in the western Sichuan Basin enhanced the vertical gradient of temperature net increase in the upper level and then improved the upper-level local positive PV. The right side of the positive PV anomaly under the easterly flow usually exhibits upward movement, which developed and enhanced the ascending motion from 300 hPa to the bottom of the stratosphere. (3) The updraft of the upper–middle–lower level near the basin was coupled and superimposed to form overshooting convection. The stratiform pattern of the mesoscale convective system and evaporative cooling of water vapor in a dry environment caused the downdraft from 300 hPa to 600 hPa. The atmosphere maintained the ascending motion in the upper troposphere while dry intrusion occurred in the middle troposphere, which not only intensified the unstable stratification of “upper-layer dry and lower-layer wet” over the basin but also enhanced the convergence of air flow in the middle and lower troposphere, turning the downdraft near 300–600 hPa in the middle troposphere into an updraft. Because of the slantwise vorticity development in the lower layer, the ascending motion can be maintained. Hence, the ascending motion over the Sichuan Basin appears to be vertical superposition and coupling, which shows the consistent ascending motion from the lower troposphere to the lower stratosphere and the formation of overshooting convection and enhancement of precipitation.