Contribution of Cumulus Congestus to Tropical Cyclone Formation

The contribution of cumulus congestus to tropical cyclone formation is studied through the analyses of a numerical model simulation and satellite data. The high-resolution numerical model simulation shows that cumulus congestus plays a dominant role in moistening the lower to middle troposphere and spinning up the near-surface circulation before genesis, while deep convection plays a key role in moistening the upper troposphere and intensifying the cyclonic circulation over a deep layer. The transition from the tropical wave stage to the TC stage is marked by a substantial increase in net condensation and potential vorticity generation by deep convection.

The TRMM PR data were further analyzed to confirm some of the findings. The echo top height of the 20-dBZ radar reflectivity is used in combination with the near-surface rain rate to identify the different types of precipitation: i) shallow cumuli; ii) cumulus congestus and iii) deep convection, and iv) stratiform precipitation. The frequency of occurrence of each precipitation type is calculated for 1 day, 2 days and 3 days before genesis, and the relative contributions of different types of precipitation to the total precipitation are examined at various radii with respect to the pouch center. The implication of these findings for a conceptual model for tropical cyclone formation will be discussed.