Long-lasting rainbands associated with the interactions between tropical cyclones and their environment

• Main Authors: Buo-Fu Chen, 陳柏孚
• Other Authors: 李清勝
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/89769281591975999401

博士 === 國立臺灣大學 === 大氣科學研究所 === 103 === This study analyzes two types of long-lasting rainbands associated with tropical cyclones (TCs) in the western North Pacific. These rainbands are separated from the eyewall convection and form due to the interactions of TCs and the monsoonal environments. The first type is the outer mesoscale convective system (OMCS), which is a linear convective system with a large cold cloud shield that develops from the TC distant rainband (e.g. OMCS embedded in Typhoon Morakot 2009). The second type is the enhanced rainband (ERB) that develops from TC principal rainband and is accompanied with active and long-lasting convections. A total of 109 OMCSs and 90 ERBs that occurred in the western North Pacific during 1999-2009 are identified using infrared and passive microwave images. About 22% (21%) of all TCs have at least one OMCS (ERB) during their life cycle. Eighty five OMCSs and 80 ERBs that developed in the southern part of TC are further analyzed. Results show that the south-type OMCSs developed at 200-700 km radii from the TC center and moved predominantly outward. The south-type ERBs developed at 100-300 km radii from the TC center and moved predominantly cyclonically. The TC intensification rate decreased but the rate of TC size change did not change when after an OMCS was present. However, the TC size increased significantly after an ERB was present. Seventy percent of very large typhoons (radius of 15 ms-1 wind > 3.6°) had an ERB during the period when they intensified from tropical storms to typhoons. The Weather Research and Forecasting (WRF) model was used to simulate the development and maintenance of an OMCS that occurred to the southwest of Typhoon Fengshen (2008). Results show that the WRF does a reasonably-well simulation except that the OMCS is shifted southward by 100 km because the simulated TC outer circulation is larger. The low-level TC circulation of Frngshen is deflected by the Luzon terrain causing a westward shift of an elongated north-south moisture band which then converges with the low level monsoon southwesterly flows. As a result, the OMCS develops at the outer region of Fengshen instead of being spiraled into the inner-core region. The formation of this moisture band is attributed to i) the moisture from the southwest monsoon that is transported cyclonically around the TC center, and ii) the tilting of the TC due to the strong northeasterly vertical wind shear (VWS). The TC-VWS interactions result in down-shear frictional convergence associated with asymmetric vorticity in the boundary layer and the formation of the moisture band. The OMCS develops when this moisture band interacts with the low-level southwesterly flow. Results also show that a characteristic structure of rear-fed inflow with leading stratiform in the cross-line direction (toward the south) is established when the OMCS becomes mature and moves southward. Such a structure contributes to the long duration of the OMCS because the high-CAPE air in the moisture band keeps feeding into the system and new cells form continuously at the trailing (north) edge of a cold pool (Δθ < - 3K) associated with the large stratiform precipitation. The synoptic conditions of all 85 south-type OMCS formations are examined. Results show that 51% of these OMCSs that formed at the intersection of an elongated moisture band in the TC northerly circulation and the southwest monsoon flow have similar feature with that of the OMCS embedded in Fengshen.