Precipitation processes of a thunderstorm occurred on 19 august 2014 in northern taiwan documented by using a high resolution 4dvar data assimilation system

The evolution of a heavy rainfall event that occurred on 19 August 2014 in northern Taiwan is investigated using observed data and analyzed using a newly developed system, namely, IBM_VDRAS. This system is based on a four-dimensional (4D) Variational Doppler Radar Assimilation System (VDRAS) that is capable of assimilating radar observations and surface station data over a complex terrain by adopting the Immersed Boundary Method (IBM). This event has precipitating processes and track different from those frequently observed in northern Taiwan. From the surface observations and the high spatiotemporal resolution analysis fields generated by IBM_VDRAS, it has been found that the rainfall process started with two single convective cells triggered by the interaction between land–sea breeze and terrain in two different cities (Taoyuan and Taipei). The outflow of one of the convective cells developed in Taoyuan City at an earlier time along with the outflow of another convective system that developed in the Taipei Basin; the former provided favorable conditions to intensify the latter. The enhanced major convective cell moved to the Taipei City metropolitan area and produced 80-mm precipitation within approximately 2.5 h. The kinematic, thermodynamic, and microphysical fields of the convective cells were analyzed in detail to explain the mechanisms that helped maintain the structure of the rainfall system. Sensitivity experiments of quantitative precipitation forecast revealed that the terrains prevent the location of major rainfall from shifting outside of the Taipei Basin. By assimilating the surface data, the model can better predict the rainfall position.