Analysis on the formation mechanism of an elevated thunderstorm over the middle and lower Yangtze Basin in February 2013

An elevated thunderstorm process occurring in the middle and lower reaches of the Yangtze River region in February 2013 was investigated using conventional observation data, reanalysis data and Doppler radar data. The results show that this elevated thunderstorm formed and remained in the upper air behind the surface cold front. Its evolution process is described as follows. The air parcel in the vicinity of the cold front was lifted by the topographic and frontal forcing first. However, the air parcel could only reach the lower height of neutral buoyancy due to the convective inhibition in a stable atmospheric environment. As the parcel ascended, its potential temperature and geostrophic absolute momentum were conserved, thereby leading to an absolute momentum anomaly (ΔM) between the parcel and the atmosphere at the level of neutral buoyancy in the baroclinic atmosphere. The parcel then experienced an inertial acceleration and the inertial adjustment occurred along the neutral buoyancy surface, which forced the parcel to move along a slantwise path and the ΔM-adjustment circulations formed. Once the warm moist air reached 700 hPa level, the slantwise motion further strengthened by the shear line and the coupling of the two jets at the upper and lower levels. Accompanied with the ΔM-adjustment, the conditional symmetric instability (CSI) generated. The ΔM-adjustment and CSI circulations both promoted the development of slantwise convection above the cold front. In a word, the interwoven effects of the topographic and frontal forcing, the atmospheric stability, ΔM-adjustment, the shear line and the coupling of the upper and lower level jets, and CSI greatly promoted the development of slantwise convection, which released the slantwise convective available potential energy and caused this elevated thunderstorm process.