Origin and Maintenance of the Long-lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008)

An outer mesoscale convective system (OMCS) is a type of long-lasting rainband with heavy precipitation that occurs in the outer region of western North Pacific tropical cyclones (TCs). Lee et al. (2012, Geophysical Research Letters) defined OMCSs as linear convective systems that develop in the distant rainband region of a TC, have a large cold cloud shield (area of the 208K cold cloud shield must exceed 72000 km2), and persist for more than six hours. Lee et al. documented 109 OMCSs that occurred from 1999 to 2009 in the western North Pacific, and were embedded in 22% of all TCs. Since OMCSs have longer durations than other types of TC rainbands and are critical to TC rainfall forecasting, the initiation processes and maintenance mechanisms of an OMCS that occurred within the outer circulation of Typhoon Fengshen (2008) are studied with Weather and Forecasting Model simulations. The OMCS occurred at about 260 km to the southwest of the TC center when Fengshen interacted with the monsoonal circulation. The monsoonal environment contributed to the northeasterly deep-layer (850 mb to 200 mb) vertical wind shear (VWS) and via large moisture fluxes from the ocean in the southern through eastern quadrants of Fengshen. A moisture budget indicates moisture associated with the TC/monsoon interaction is transported cyclonically around the TC and becomes concentrated in an elongated band with large moisture flux convergence located well to the west of the TC center. This moisture band has very large convective available potential energy (3600~4000 J/kg) and a low level of free convection (< 400 m) in the down-shear right to down-shear quadrant. The OMCS then occurs in the downstream side of the moisture band (in the down-shear quadrant of the TC) as the thermal conditions become good for the development. It is proposed that the vorticity asymmetry of the TC induced by the VWS contributes to the formation of the elongated moisture band that is enhanced by frictional convergence in the boundary layer. The results of a model sensitivity test with 30% and 150% topography height indicate that the deflection of the TC low-level circulation by the terrain caused the elongated moisture band to be displayed to the west such that the OMCS develops in the outer region of the TC. Lee-side subsidence may have also contributed additional moisture convergence to the elongated moisture band. Therefore, the Luzon terrain had a key role in creating the OMCS. Note that since 0~6 km local VWS vectors within this elongated band has a direction shift of about 90 degrees to the low-level winds, thus is favorable for large stratiform precipitation region to develop relative to the east-west oriented convective line. As the OMCS developed in the region of low-level westerlies and mid-level northerlies associated with TC outer circulation, the characteristic structure of a rear-fed inflow with a leading stratiform occurred in the cross-line direction (toward the south) was established.