Long-lasting convective systems in the outer region of tropical cyclones in the western North Pacific



[1] Long-lasting mesoscale convective systems (MCSs) may occur in the outer region of tropical cyclones in the western North Pacific, especially in conjunction with the southwest monsoon as in the case of Typhoon Morakot that caused great flooding and landslides in Taiwan. These “outer-MCSs” are linear convective systems that develop from distant rainbands, have a large cold cloud shield, and last more than six hours. These outer-MCSs are important for typhoon rainfall forecasting because of the torrential rainfall when they interact with land and terrain to produce serious flooding that is separate from the rainfall near the center. A total of 109 outer-MCSs that occurred during 1999–2009 are identified using infrared and passive microwave images. About 22% of all typhoons in the western North Pacific have at least one outer-MCS during their life cycle. In 85% of the QuikSCAT oceanic 10-m wind observations of outer-MCSs, positive shear vorticity on the left side of mesoscale surface jets below the stratiform precipitation regions may be contributing to the continuous formation of new convective cells.