Contrastive Analysis of Two Intense Typhoon-tornado Cases with Synoptic and Doppler Weather Radar Data in Guangdong

Conventional observations, Doppler weather radar and NCEP/NCAR reanalysis data are used to analyze two strong tornado events originated in the outside-region of typhoons on 4 October 2015(EF3) and 4 August 2006(EF2) contrastively. Results show that two strong typhoon-tornado events both occur in the northeast quadrant of landfalling typhoons, with some similar environmental conditions including low-level convergence, upper-level divergence and superimposition of strong southeast jet at mid and low level over the Pearl River Delta. The difference of circumstances is that two typhoons are in different weakening phase. The former typhoon has just landed and is more organized and stronger than the latter. Environmental parameters are shown to be relatively moderate convective available potential energy, low convection inhibition, low condensation uplift height, strong deep (0-6 km) and low level (0-1 km) vertical wind shear and large storm relative helicity (SRH). Storm relative helicity is a good indicator for the occurrence of supercell or mesocyclone. The larger the SRH is, the more likely a supercell or mesocyclone may form. Combining the northeastern quadrant of typhoon with the high SRH area, the area where typhoon tornadoes may occur could be determined to a certain extent. The two tornado storms are mini-supercells, and the radar base reflectivity factors of two tornadoes are similar to features of classical supercell such as the low-level warm-humid inflow gaps and hook echoes. The former (tornado parent storm on 4 October 2015) has stronger echo and more apparent hook echo features. Strong mesocyclones and tornado vortex signature (TVS) can be observed on radar speed chart in both tornado events, and mesocyclones form at mid-low level firstly, then develop to the lower level, resulting in tornadoes finally. TVS is observed either synchronously with the tornado touchdown or a volume scanning ahead. Vertical vorticity of TVS in the center of low-level mesocyclones is strong, and the bottom and top heights are very low in the mesocyclone and TVS. The difference of bottom/top height of the mesocyclone and TVS between two cases is that, the former presents an abruptly-drop phenomenon whereas the latter (tornado parent storm on 4 August 2006) maintains at low level before and after the tornado touchdown. Before and after tornadoes touchdown, the strongest wind shear of storms both increase sharply, but the wind shear in TVS is larger, which is about twice of the latter.