碩士 / 國防大學中正理工學院 / 應用物理研究所 / 94 / A mesoscale convective system induces a heavy rainfall event while propagating to southern Taiwan area on 7 June 2003. The precipitation at Ma-chia station accumulates 379.5 mm rainfall per day, which reaches extremely torrential rainfall level defined by Central Weather Bureau. A line echo organized by several convective cells was observed by Ken-ting radar since 0000 UTC. The line system bends and evolves into a line-echo wave pattern (LEWP) gradually. In LEWP, there existed three wave-like echoes during the evolution of the system. The longest lifetime of these wave-like echoes is over 4 hours while whole system sustains over 6 hours.
Some characteristics of WRF model simulation are listed as below:
(1) Three wave-like echoes are located further south comparing to that in the observation.
(2) A cyclonic vortex appears at the northern end of those wave-like echoes from system-relative speed at 2 km level, but it can’t find the anti-cyclonic vortex at the southern.
(3) A rear-inflow in the vertical structure of the wave-like echo, which is colder and dryer, propagates from 3 or 5 km level behind the system, and descends to ground inside the convective zone. However, the front to rear flow is absent.
(4) The vorticity of those cyclonic vortex are over 10-3 s-1 which are weaker than that of midlatitude counterpart.
(5) The average vertical shear between 0 and 4 km in altitude is 2.75×10-3 s-1, which is also weaker than that in midlatitude environment. Consequently, the characteristics of the simulated case study show that the present wave-like echo is different from the bow echo and a squall line existed in midlatitude.
(6) The results analyzed from a simulation of 9 km horizontal grid show that the cyclonic vortex existed at the northern end of wave-like echo is collocated with the mesovortex associated with Mei-Yu front. A dry cold flow exists at the northwest of the vortex, and a moist warm southerly wind exists at the southeast of the vortex. Therefore, the dynamic mechanism of horizontal shear associated with mesovortex triggers the wave-like echo.
A sensitivity test of microphysics illustrates that:
(1) The Kessler scheme can not provide a generation of wave-like echo. The Lin et al. scheme can trigger only one wave-like echo in the southwest Taiwan area.
(2) The WSM 3-Class scheme parameterization is valid for the precipitation distribution, however the total rainfall rate is less than that of observation. And only one wave-like echo shows up propagating toward south with longer life cycle.
(3) The WSM 5-class scheme parameterization is valid for three wave-like echoes which are comparable with real observations. And the precipitation distribution is also better than other schemes.
A sensitivity test for precipitation evaporation shows that:
(1) Below 50% evaporation rate, the wave-like echo can not be triggered.
(2) With the WSM 5-Class scheme, the changes of evaporation rate only affect the temperature field in the lower atmosphere.
(3) With the 80% evaporation rate, those wave-like echoes have the same lifetime and accumulating rainfall rate as the real case.
(4) The averaging moving speed of the wave-like echo B and cold pool have never been affected due to different evaporation rate.
(5) In principle, the decreasing evaporation rate by applying WSM 5-class scheme will not influent moving speed of line echo and formation of wave-like echoes.
| Identifer | oai:union.ndltd.org:TW/094CCIT0504006 |
| Date | January 2006 |
| Creators | Lu Hua-pai, 陸華白 |
| Contributors | 朱昌敏，何台華 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 78 |
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