The rapid and slow intensification of western north Pacific tropical storms is studied from an observational and modeling perspective, with the objective of decribing how common large scale flow patterns affect core convection and intensity change.

The large scale flow patterns occurring around future rapidly intensifying and slowly intensifying tropical storms were defined using scalar EOF analysis using NCEP/NCAR reanalysis data (1975-2001). All analyses are performed on storm centered grids. 88 cases intensified by greater than 30kts in 24hrs from an initial intensity of 35kts, according to JTWC best track data. This represents the top 12% of 24hr intensification rates from weak tropical storm stage and is the criteria for the definition of rapid intensification. As a comparison 58 cases of slow intensification (10kts/24hrs) were included in the study.

Flow patterns elucidated by this method include variations of the monsoon shear line and confluence patterns. It is found that 13/18 cases of extreme rapid intensification (40kts/24hrs +) are associated with a form of the monsoon confluence region

The character of trough interactions preceeding and during rapid intensification are examined in an isentropic potential vorticity (PV) framework. The approach (before intensification),partial superposition and weakening of a small scale PV anomaly (during intensification) is observed for 15 cases, very similar to the picture that emerges from work done in the Atlantic giving more confidence in the universal inporatance of this interaction.

Observational results are used to design numerical experiments (TCM3 -Yuqing Wang) to examine the sensitivity of tropical cyclone intensification rates to variations in realistic western north Pacific flow patterns. Variations of the monsoon shear line and confluence patterns are studied. The work's emphasis is on how core convective asymmetries are induced by these differing flow patterns and how this feeds back on intensity. Significant asymmetries are found for the monsoon shear line cases in the mature stage and work is underway to find the influence on intensity change. Initial results show the importance of low level convergence of the environmental flow near the tropical cyclone in producing the most rapid intensification rates.