| dc.description.abstract | Abstract
According to the statistics of the accumulated rainfall at northern station in Taiwan during the Mei-Yu season 2007, there were obvious heavy rainfall events from June 5th to 10th . After thoughly investigating the data of CWB, it exhibited that a squall line moved slowly from southeast coast of China to Taiwan. We have used NCU C-band dual-polarization (C-Pol) radar to confer the structure of squall line in this thesis. In addition to the study of the inner flow structure characteristics, we also added the discussion of the cloud physical phenomena revealed by the dual-polarization variables. It is the first time in Taiwan to analyze cloud microphysics of the Mei-yu front precipitation systems using dual-linear polarization Doppler radar .
From weather analysis, the surface weather map show there was a well defined frontal system influencing Taiwan during the whole period. We detected a LLJ in 850mb weather map, a short wave trough in 500mb and divergence in 200mb. The system on top of the Taiwan island enhanced with the above-mentioned advantage conditions for meso-scaled development.
The vertical wind structure showed that there was warm and wet south-west flow moving into the southwest leading edge convective cells on the squall line. This flow provided the main inflow for the developed system, and the wind on north side of the main convective cell was colder and dryer northeast wind. When this northeast flow encounter with the southwest flow, the flow will move upward along the northeast in southwest flow and across it. It enhanced the new convective cells by the release of convective instability which was provided by the unsteady flow in this system.
From the vertical cross-section of the retrieved DSD parameters (NW and D0), the precipitation systems were dominated by relatively less drops number and smaller drops size at the initial stage of a convective system.
Nevertheless, the DSD parameters: Nw and D0 increase notably at the mature stage of convective system. In this stage, the precipitation system was composed by two different characteristic of the DSD. The leading edge (southern part) of the convective cells accompanied with less drops number and larger drops size. However, the relatively higher concentration of raindrops and relatively smaller drop size at rear part (northern part) of convective cell was observed from NCU CPOL.
As a result we concluded that the stronger upper motion associated with the front to rear air flow will cooperate with the downward motion. Consequently, the rain droplets will be carried toward the rear part of the flow, and only relatively big drop can reach the surface. Since the rear part of the system has less upper movement, the droplets will have the tendency to descent. Therefore, the results agree with the wind field and the polarimetric measurements. It also reflects that the dynamical and microphysical DSD consistent which were retrieved from the original polarization parameter. | en_US |