Relation of the second type thermal helicity (H2) to precipitation of landfalling typhoons: A case study of typhoon Talim.

This study utilizes the MM5 mesoscale model to simulate the landfalling process of the Typhoon Talim. The simulated typhoon track, weather patterns, and rainfall process could well represent the observation. Using the numerical simulation outputs, the relation of the second type thermal helicity (H2) to rainfall caused by the landfalling typhoon Talim is analyzed. The results show that H 2 can reflect the heavy inland rainfall well while the relation was not as goo d as that of helicity to rainfall during the beginning landfalling stage. In par ticular, H2 had the highest correlation coefficient with rainfall of Talim 1 h in advance. For 1-5 h prediction times, it had a higher correlation with ra infall than helicity, and thus it showed a better potential in forecasting rainf all intensification. Further analyses have found that when Talim was in the begi nning stage of landfall, the 850-200 hPa vertical wind shear around the Talim ce nter was quite small (about 5 m/s). The highest rainfall was to the right of the Talim track and in the 300 km radius area around the Talim center, and it had no obvious relation with lowlevel temperature advection, low-level air converg ence, or upper-level divergence. But the lowlevel relative vorticity could re flect the rainfall change quite well, which was the main reason why helicity ha d a better performance than （H2） in this period. But after Talim moved inlan d further, it weakened gradually and was affected increasingly by the northern trough, and the vertical wind shear was increasing as well. The left side of the down vertical wind shear direction lies the Lushan and Dabieshan mountain area, which could have contributed to intriguing a secondary vertical circulation, and thus heavy rainfall was produced over there. Hence, H2 showed a better ca pacity to reflect the rainfall change during this stage.