Diagnostic analysis of a record-breaking rainstorm process caused by a weak tropical depression in Guangxi coastal

Based on atmospheric thermal and dynamic diagnostic equations, the cause of the record-breaking tropical depression rainstorm process in the coastal area of Guangxi and the possible mechanism of the maintenance of the depression during 7—9 June 2023 are analyzed using multi-source observations and ERA5 reanalysis data. The results are as follows: (1) Under the background of the interaction between the mid- and high-latitude and low-latitude weather systems, the tropical depression moved slowly and even remained stationary over Guangxi, which contributed to the occurrence of the heavy precipitation process. The atmospheric thermal elements and water vapor content showed significant anomalies during the process. Normalized anomalies of the divergence of vertically integrated vapor flux and precipitable water and K index reached −5.5, 3.2 and 1.3, respectively. (2) Precipitation is strongest from night to early morning. The quasi-stationary mesoscale cloud clusters and the "echo training" in the convective system caused continuous local accumulation of precipitation and resulted in record-breaking rainfall. (3) From the night of 7 (8) June to the early morning of 8 (9) June, the warm shear line (boundary layer jet) to the southeast (east) of the tropical depression and land surface friction provided the dynamic condition for the occurrence and development of heavy precipitation. The convergence of the boundary layer jet on 9 June was stronger than that of the shear line on 8 June, and the frontogenetical forcing was stronger. The increase in pressure gradient force caused by the strengthening of tropical depression system and the pressure gradient force impact caused by meridional advection of potential height promoted the growth of local kinetic energy, which is the reason for the development of boundary layer jet. (4) The tropical depression circulation continuously sucked water vapor from the South China Sea, and the convergence and vertical transport of water vapor increased the wet layer depth, which is conducive to the increase of precipitation efficiency. Continuous warm and moist airmass transport was conducive to the maintenance of unstable stratification, and the convective unstable structure in the lower atmosphere maintained throughout the whole process. The configuration of vertical wind shear and atmospheric baroclinicity made the stratification more unstable on 9 June than that on 8 June, which, combined with the dynamic conditions, led to more intense precipitation on 9 June. (5) The maintenance of the tropical depression system was mainly supported by the convergence of horizontal winds. The contribution of the geostrophic wind component is relatively weak in the whole process, while the horizontal divergence term of the ageostrophic wind component led to changes in local vorticity. This is related to the convergence in the tropical depression center after the inertia rotation of the ageostrophic wind.