Tropical cyclones (TCs) in a monsoonal environment may have heavy rain events associated with long-lasting mesoscale convective systems separate from the eyewall rainfall. Two types of long-lasting rainbands in western North Pacific TCs interacting with the East Asia summer monsoon during 1999-2009 are studied and the effects of these rainbands on TC size and intensity changes are examined.
Outer mesoscale convective systems (OMCSs) are long-lasting, heavy rainfall events separate from the eyewall rainfall that have previously been shown to occur in 22% of western North Pacific tropical cyclones (TCs). Environmental conditions accompanying the development of 62 OMCSs are contrasted with the conditions in TCs that do not include an OMCS. The development, kinematic structure, and maintenance mechanisms of an OMCS that occurred to the southwest of Typhoon Fengshen (2008) are studied with Weather Research and Forecasting model simulations. Quikscat observations and the simulations indicate the low-level TC circulation was deflected around the Luzon terrain and caused an elongated, north-south moisture band to be displaced to the west such that the OMCS develops in the outer region of Fengshen rather than spiraling into the center. Strong northeasterly vertical wind shear contributed to frictional convergence in the boundary layer and then the large moisture flux convergence in this moisture band led to the downstream development of the OMCS when the band interacted with the monsoon flow. As the OMCS developed in the region of low-level monsoon westerlies and mid-level northerlies associated with the outer circulation of Fengshen, the characteristic structure of a rear-fed inflow with a leading stratiform rain area in the cross-line direction (toward the south) was established. A cold pool (î∏ > - 3K) associated with the large stratiform precipitation region led to continuous formation of new cells at the leading edge of the cold pool, which contributed to the long duration of the OMCS.
From 1999 to 2009, 80 south-type ERBs and 85 south-type OMCSs that were associated with TCs influenced by southwesterly monsoon flows have been analyzed to understand their correlations on TC intensity and size. Whereas OMCSs usually developed from distant rainbands between 200 km and 700 km from the TC center, ERBs usually developed from the primary rainband between 100 km and 300 km radius. The VWS orientation may be one of the important factors determining the azimuthal variations of these long-lasting rainbands that OMCS (ERB) was favored in the downshear-right (downshear-left) quadrant. Furthermore, the OMCSs typically propagated more outward with respect to the TC center, and the ERBs usually moved more cyclonically. The primary conclusion is that the occurrence of a long-lasting ERB coincides with an increase in TC size and maintenance in TC intensification (rather than previous suggestions for an intensity decrease). These ERBs are more likely to occur in larger tropical storms (TSs) and tend to lead to larger typhoon (TY) sizes than in a sample of typhoons that did not have an ERB during the intensification from TS to TY. By contrast, the long-lasting OMCSs did not lead to significantly larger TC sizes, and tended to be associated with a reduced TC intensification rate.