The Mesoscale Processes Leading to the Formation of Super Typhoon Megi (2010)

Through the cloud-resolving simulation with the Weather Research and Forecast (WRF-ARW) model, this study examines the mesoscale processes leading to the formation of Supper Typhoon Megi (2010), which includes the evolution of midtropospheric cyclonic vortex (MCV), occurrences of shallow and congenstus convection, and burst of deep convection. As deep convection bursts in the environment preconditioned by the tropical waves, substantial downdrafts initially fueled by evaporative cooling and then forced by strong near-surface divergence induces a MCV in Megi’s precursor. When the MCV intensifies, a low-level warm and dry core as well as near-surface anticyclonic circulation accompanying with low sea-level pressure develop below the MCV. The unbalanced wind and pressure fields and the wind surge from the environment cause updrafts below the MCV, which increases the relative humidity in the lower troposphere, promoting shallow, moist convection. The shallow convection enhances near-surface vorticity, and meanwhile, assurances the persistent increase of atmospheric unstable energy. Subsequently, congestus convection becomes dominant in Megi’s precursor, which together with the mid-to-upper-level dry air intrusion, enhances the potential instability in the mid-to-upper levels. Furthermore, the congestus convection strengthens low-level vorticity and leads low-level jets to penetrate into the inner core of Megi’s precursor. Finally, deep convection reinvigorates in Megi’s precursor to spin up the storm. The deep convection bursts intermittently with the period of ~24 hours. During the interlude of deep convection, the processes similar to that in shallow and congestus convection episodes also occurs more or less. Besides the diurnal cycle, the adjustment induced by deep convection may be important to the periodical burst of deep convection in Megi’s precursor. The role of the MCV in the formation of Megi is also discussed. It is suggested that the MCV may only be the derivative of deep moist convection and not be critical to the formation of Megi.