In monsoon season, flash floods in metropolitan cities caused by organized thunderstorm complex are common.Due to short period of life cycle and high variability of rainfall intensity, the prediction skills of which are very low.In this paper, observations and polarimetric signatures of urban flash flood thunderstorm complex systems are investigated using dense meteorological surface stations and newly upgraded WSR-88D dual-polarized radar.In this paper, a flash flood case caused by the afternoon thunderstorm complex (14 June 2015) over Taipei basin is examined.Observations from dense network of automatic rain gauges and meteorological surface stations over Taipei basin are analyzed.It is found more than 190 mm rainfall has been recorded by Kong-Quan (C1A730, near NTU campus) station between 2-4pm.There are several stations on the SE district recorded 3 hourly rainfall greater than 100mm.Surface analysis indicates there are sea breezes from both river mouths, the Tanshui river and Keelong river.Pronounced convergence zone can be identified.The environment was moist and unstable and suitable for thunderstorm to develop.Initially, the storms were generated in the mountain region south of the basin and propagated northward.It is found storm cell merging process is important in producing concentrated enhanced rainfall.Cell after merge in average has a larger horizontal extend, stronger vertical velocity, and enhanced rainfall intensity.Convergence related to sea breeze and cold outflows from existing storms is the major cause for the cell merge.It is also found that the cell movement was from SW to NE parallel to the averaged mean wind and also parallel to the mountain range before merging.However, the tracks have been changed toward NW after merging.The change of cell track after merging seems to indicate the importance of interaction of outflows from existing cells and moisture-abundant sea breeze.Structures of thunderstorm complex before and after merge are analyzed using data observed by the WSR-88D(NEXRAD) polarimetric radar in northern Taiwan.The signatures of flash-flood producing storms revealed from the polarimetric observables are examined and will be discussed in the presentation.