Large-Scale Patterns Associated with Tropical Cyclogenesis in the Western Pacific

Abstract

Five characteristic, low-level, large-scale dynamical patterns associated with tropical cyclogenesis in the western North Pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis. An 8-yr analysis set for the region is used to identify, and create composites for, the five characteristic patterns of monsoon shear line, monsoon confluence region, monsoon gyre, easterly waves, and Rossby energy dispersion. This brings out the common processes that contribute to tropical cyclogenesis within that pattern, which are described in detail. A 3-yr set of satellite data is then used to analyze the mesoscale convective system activity for all cases of genesis in that period and to stratify based on the above large-scale patterns. It is found that mesoscale convective systems develop in all cases of genesis except one. Seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44% of cases developed multiple mesoscale convective systems at a single time. Stratification by pattern type indicates some differentiation in mesoscale convective activity and it is inferred that this is due to the large-scale processes. Two of the five patterns, the monsoon shear line and the monsoon confluence region, had more than the average amount of mesoscale convective activity during the genesis period. These patterns also account for 70% of the total genesis events in the 8-yr period. The analysis for the other three patterns exhibit less mesoscale convective system activity during genesis. This may indicate either that genesis processes for these patterns are not as dominated by mesoscale convective system activity, or that genesis occurs more rapidly in these cases.