Kinematic Structure and Maintenance of the Long-lasting Rainband in the Outer Region of Typhoon Fengshen (2008)

Outer mesoscale convective system (OMCS) is a type of long-lasting rainband occurring in the outer region of tropical cyclones (TCs). Since OMCSs have longer duration than other types of TC rainbands and are critical to TC rainfall forecasting, their kinematic structures and maintaining mechanisms are studied. The OMCS that occurred at about 260 km to the southwest of Typhoon Fengshen (2008) is studied using a set of data simulated by WRF Model. Results show that the inhomogeneous wind field associated with the low-level westerlies and the mid-level northerlies (TC outer circulation) causes the OMCS to organize as a structure of rear-fed inflow and leading stratiform in the cross-line direction (toward the south). A cold pool (Dθ > 3 K) associated with the large stratiform precipitation region is present. As the high-θ_e air associated with the high moisture band located to the west of Fengshen moves southward and enters the OMCS, new cells form continuously on the rear (north) edge of the cold pool. Such process contributes to the long duration of the OMCS. Note that for OMCS, there is no mid-level jet that is present typically in TC primary rainband. However, an along-line surface westerly jet presents in the stratiform precipitation region. Results of momentum budget analyses show that whereas the ageostrophic acceleration is significant in both convective and stratiform regions, the deceleration of horizontal advection effect is large at the middle levels of the convective region but small at the low levels of the stratiform region. Therefore, only the surface jet in the stratiform region becomes organized during developing process of the OMCS.