Five Major Environmental Circulation Patterns for Tropical Cloud Cluster Developments in the Western North Pacific

This study analyzes the environmental circulation patterns associated with the development of tropical cloud cluster (TCC) into tropical cyclone (TC) in the western North Pacific (WNP) by using cluster analysis. A TCC, the precursor to a TC, is a well-organized and long-lasting convective system. In the WNP, approximately seventy-three TCCs are identified each year during July–October on average for the period between 1981 and 2009. When the environmental conditions are favorable, TCCs may develop further into TCs. To simplify the different TC developing processes, a non-hierarchical cluster analysis of the synoptic circulation, based on the formation location of the disturbance, is used. Using the sum of squared distance and analysis of variance, forty TC developing processes, composed of eight environmental types of TCC formation and five environmental types of TC formation, are identified. Results show that there are five types of developments that have significant ratios in each type of developing TCCs; three types are in the monsoon-related environments (monsoon trough, monsoon confluence, and north of monsoon trough) and two are in the easterlies-related environments (west of subtropical high and southwest of subtropical high). Based on the analyses of these five developments, the TCCs evolved in the monsoon-related environments require longer developing time to form TCs than in the easterlies-related environments. The averaged genesis potential index around the location of TCC formation shows that the initial environmental conditions are more favorable for TCC development for developing cases than for the non-developing cases in the easterlies-type TCCs. However, no significant difference is observed in the monsoon-type TCCs. On the other hand, the potential vorticity budget analysis shows that the increasing rate of the diabatic heating term (DHT) for each development is similar at the mid- and low-level during the TCC formations. However, during the TC formations, the increasing rate of the DHT at the mid- and low-level in the easterlies-type TCCs is significantly greater than in the monsoon-type TCCs, although the initial DHT is higher in the monsoon-related environments. Furthermore, the angular momentum budget analysis also shows that the low-level inward transport of angular momentum in the easterlies-type TCCs is greater than in the monsoon-type TCCs during the TC developing processes.