Abstract
Torrential rainfall associated with the 23rd Typhoon Fitow in 2013 hit the Yangtze River Delta (YRD) region of China, resulting from mutual effects of residual low pressure cyclonic circulation of Fitow and the 24th Typhoon Danas, which imposed great challenge to forecasters. In this study, the Weather Research and Forecasting (WRF) model was used to simulate the rainstorm under the background of binary typhoons of Fitow and Danas. Three sensitivity experiments of typhoon intensity changes of binary typhoons were carried out. It was found that the Typhoon Danas was the main factor in this torrential rain event, in which its accompanied strong eastward low-level jet was the major moisture conveyor belt through which the warm and moist air was brought into the heavy rainfall zone and the static instability was maintained and enhanced over the YRD. The convergence line formed by periphery easterly flow of Typhoon Danas and southward cold air, together with the local frontogenesis mainly due to convergence, was an important trigger factor of this rainstorm. The large scale forcing was the major uplift mechanism, and the underlying frontal uplift played a secondary role for rainstorm in the north YRD, while uplift mechanism for rainstorm in the southern YRD is mainly local underlying frontal uplift induced large CAPE release resulting in local strong buoyancy uplift that led to strong upward motion. Not only did the convergence of twin typhoons directly provide dynamic conditions for the rainstorm, but also the dynamic lifting was enhanced by binary typhoons through strengthening the coupling of upper-level and low-level jet. The sensitivity tests revealed that the rainstorm in YRD was sensitive to both typhoons' intensity, and the rainfall in the south YRD was more sensitive than that in the north. A conceptual model of YRD rainstorm under binary typhoon situation was proposed based on the above-mentioned factors.
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