Climatic characteristics and environmental conditions of the tornado occurrences within tropical cyclones over Guangdong province，China

To investigate the general characteristics of tornado occurrences within tropical cyclones (TCs) over Guangdong province, this study primarily analyzes the differences in some main thermodynamic and kinematic environmental indices between tornadic and non-tornadic TCs, and also the strongly and weakly tornadic TCs that have similar tracks during the period from 1961 to 2022. The analysis is conducted using the Full Collection of Meteorological Disasters in China (1959—2000), the Reports of Meteorological Disasters in Guangdong (2002—2022), and the ERA5 atmospheric reanalysis data. Results show that TC tornadoes in Guangdong are spatially concentrated over the Pearl River Delta, Leizhou peninsula and Chaoshan plain. TC tornado occurrences account for nearly 42% of all the tornado reports in Guangdong with a peak frequency in August, which is positively correlated with the number of TCs affecting Guangdong. On diurnal cycle, they are characterized by two diurnal peaks at 14:00—18:00 BT and 06:00—10:00 BT. Tornadoes mainly occur in the northeast quadrant of the TC center, the rear right sector relative to the TC's movement, and the upper shear region relative to the deep-layer shear vector. The time window of 2—18 h after TC’s landfall is the tornado active period, during which the parent TCs are characterized by intensities of tropical storm and tropical depression. The dominant synoptic patterns can be classified into two types: The southerly jet and northerly jet on upper levels. The tornado locations associated with the two weather types agree well with the large-value areas of supercell composite parameters (SCP) and significant tornado parameters (STP). When comparing the environmental conditions between tornadic and non-tornadic TCs and between the TCs with strong and weak tornadoes, it is found that the tornadic environments are characterized by significantly larger values of entrainment convective available potential energy (E-CAPE) and 0—1 km storm relative helicity (SRH1) in the northeast quadrant of the TC center than those of non-tornadic TCs (at the 95% significance level). The combination of E-CAPE and SRH1 can better indicate the potential of tornadoes caused by TCs. The mixed layer CAPE (MLCAPE) and STP of Guangdong strong tornadoes in TCs are comparable to those of the strong tornadoes in the United States while SRH1 is significantly smaller. The E-CAPE, 0—6 km vertical wind shear (SHR6), 0—3 km storm relative helicity (SRH3), and STP can better distinguish strong and weak tornadic TC environments in Guangdong. Comparing the TC tornadoes between Guangdong and Jiangsu, as well as the Liaoning tornadoes under the cold vortex background, the average CAPE of Guangdong TC tornadoes is approximately half of that in Jiangsu and Liaoning. However, tornadoes in Guangdong have significantly stronger dynamic environmental conditions, with SRH1 more than twice that of the Liaoning tornadoes under the cold vortex background. These conclusions are conducive to better understanding of the environmental conditions of tornadoes in Guangdong under the TC background, and provide clues to potential forecasting on TC tornadoes.