On the Genesis of Tropical Storm Eugene (2005) associated with the ITCZ Breakdowns

In this study, tropical cyclogenesis from merging mesoscale convectively generated vortices (MCVs) associated with breakdowns of the Intertropical Convergence Zone (ITCZ) is investigated using the NCEP reanalysis, satellite data, and 4-day multi-nested cloud-resolving simulations of Tropical Storm Eugene (2005) with the Weather Research and Forecast (WRF) model.

Both observational analyses and GOES-10/12 satellite images reveal that two MCVs under study were initiated on the eastern ends of the ITCZ breakdowns that occurred more than 5 days and 1000 km apart before the genesis took place. The WRF model reproduces their different movements, intensity and size changes, and merging processes at nearly the right timing and location at 39 h into the integration as well as the subsequent track and intensity of the merger in association with the polarward rollup of the ITCZ. Model results show that the two MCVs are merged in a coalescence and capture mode due to their different larger-scale steering flows and different sizes.

Examination of the genesis of Eugene shows that axisymmetrization, a process by which any vorticity anomaly is quickly advected around and enhances the mean vortex via wave dynamics, is barely observed. Although necessary conditions for axisymmetrization are met frequently (except during the merging period), Eugene shows little intensification before two MCVs merge together. This appears to differ from the axisymmetrization-induced mechanism for cyclogenesis from bottom up. For Eugene, the main mechanism responsible the genesis of Eugene is due to the merger of two MCVs, a result that is consistent with recent statistical study of cyclogenesis in the Eastern Pacific during 1999-2003 active seasons. As the midlevel potential vorticity of the MCVs is consolidated with a shrinking area, the merged vortex is stretched downward to the surface, leading to a subsequent rapid intensification. Here, merging mechanism is strong enough to trigger the air-sea interaction before the rapid growth of the storm can take place.

Based on the results, we conclude that the ITCZ provides a favourable environment with dynamical instability, high humidity and background vorticity, but the merger of the two MCVs is critical for the genesis of Eugene. The storm decays as it moves northwestward into an environment with increasing vertical shear, dry intrusion, and colder sea surface temperatures. The results appear to have important implications to the high frequency development of tropical cyclones in East Pacific.