| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 99 === One of the important processes of concentric eyewall formation is the axisymmetrization of outer region asymmetric convection zone by center vortex of typhoon. Before the formation of concentric eyewall, we find a large outer region asymmetric convection zone in every concentric case from radar echoes or microwave images. For instance, typhoon-Lekima(2001) ,Dujuan(2003), and Imbudo(2003). There are 69 concentric cases at North-west Pacific Ocean in the period of 1997 to 2009. We use the cases’ microwave data to discuss about the relationship between outer region convection area and temporal and spatial distribution. Moreover, the study discusses about the relationship between concentric eyewall formation and monsoon index.
At first, we calculate the area of outer convection zone that is smaller than 230K in the 9°x9° region of microwave image, and then we subtract the center vortex which is defined as the sum of inner eyewall and moat width at the time of concentric eyewall formation. We also classify the sizes of area into three scale including small(2 to 5 million kilometers), medium(6 to 10 million kilometers), and large(11 to 17 million kilometers) by average and one standard deviation. The result shows that the large convection area cases are mostly happened in June to August and forming in the region between 123°E~135°E , and 13°N~27°N in North-west Pacific Ocean; small convection area cases are mostly happened in August to September and forming in the region between 126°E~154°E , and 13°N~27°N in North-west Pacific Ocean. Moreover, we classify the position of the outer region convection zone into two kinds- south dominate and north dominate. From all cases we choose, the number of south dominate cases is two times greater than the north dominate cases. South dominate cases were mostly formed on the west side of 135°E in North-west Pacific Ocean, and most of them have larger convection area at the outer region than north dominate cases.
We use information of vertical wind shear between 200-850hPa from SHIPS (Statistical Hurricane Intensity Prediction Scheme) to analyze the formation mechanism of outer region convections by counting the area of convection at down shear side. We find that the outer region convection happen mostly at down shear left sides. Further, we use water vapor parameters to find the relationship with convection area. For south dominate cases, we find that the area of outer region convection zone has great thing to do with the water vapor flux in the south-west of the typhoon. On the other hand, for the north dominate cases, we find that the area of outer region convections zone is better related to the convergence of water vapor flux in the north part of the typhoon.
North-west Pacific monsoon index is defined as the zonal wind speed difference at 850hPa in the 110-140°E, 5-30°N. East Asia region has cyclonic circulation when the index is positive. Among 47 cases of concentric typhoons which happened in the same domain as monsoon index during 13 years, 70.5% of them happened with monsoon index up to 4m/s. Especially during June to September, the percentage of certain cases is up to 93.5%. The fact indicates that during June to September, the environment with monsoon index higher than 4ms-1 is a benefit to concentric eyewall typhoons.
Last but not least, the analysis showed that the intensity of concentric eyewall typhoon formation is correlated to the upper ocean heat content (UOHC), with R-square value of 0.15, and it has nothing to do with SST and vertical wind shear.
|
|---|
