| dc.description.abstract | On August 8, 2009, a few strong west-east oriented rain bands associated with Typhoon Morakot formed in southwestern Taiwan. Many deep convection cells were embedded in these rain bands and moved toward the island very quickly. We carried out dual-doppler analysis of Chigu and Magung to retrieve 3-D wind field from 0954 to 1031UTC. The 3-D synthesis wind field revealed that the typhoon northwest wind circulation in Taiwan Strait encountered the southwestern flow to cause a strong convergence zone and form the rain band. The three dimensional reflectivity and flow structures are similar to the conceptual model of hurricane rain band. A low level jet (>30m/s) 10-30km wide, length >100km associated with this rain band.
The hot tower embedded in the rain band is defined by a threshold of reflectivity (dBZ >25) at 10 km height, and compare the features of the hot towers in different stage (1809LST and 1831UTC). The reflectivity pattern was slightly tilting southward and the reflectivity contour of 25dBZ reached 15km. The updraft of the hot tower was near 20m/s. When the cell moved along the rain band, the cell usually accompanied with the meso-γ-scale jet streak (>40m/s, 2-3km width, 5-20km length) and the positive vertical vorticity. Through the vorticity budget, we found the updraft play an important role to enhance the vertical vorticity and the strong jet streak. Along the jet many deep convection cells (hot towers) were moving quickly. Because the jet was blocked by the mountains in southern Taiwan, the sloping updrafts were enhanced to induce heavy rainfall. The prolonged stay of this devastating rain band caused the high accumulation of rain.
We analyzed the distributions of the polarimetric parameters from Magong radar to reveal microphysical structure in the hot tower. The results show that many little supercool drops were carried by the strong updraft into the tower. The low ZDR and KDP indicated the ice particles in the upper level of tower. The lower ρHV proved the mixed phase near 7-9 km height. After compared the hot towers between typhoon Morakot (2009) and typhoon Sinlaku (2008), the former not only had deeper and stronger convection, but also had larger number of raindrops. The huge amount latent heat was released in whole tower, and through cloud dynamic mechanism, it was helpful for maintaining lifetime of hot tower and low level jet.
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