TRANSFORMATION OF A LARGE MONSOON DEPRESSION TO A TROPICAL STORM DURING TCM-93

Data obtained during two aircraft observing periods (AOP) from the TCM -93 mini field experiment are used to describe the transformation betw een 5 degrees and 10 degrees N of a large depression in the western No rth Pacific monsoon trough into a tropical cyclone over a 36-h period. The transformation is defined to occur in three stages. Although a la rge mesoscale convective system (MCS) was present along the eastern pe riphery of the monsoon depression during the preorganization stage cha racterized by observations from the first AOP, the overall convective organization of the broad circulation is weak. The structure of the MC S provided a midlevel subsynoptic contribution to the Vorticity of the monsoon depression and contributed to a shift in the center of the mo nsoon depression circulation between 800 and 600 mb toward the MCS loc ation. However, the presence of unsaturated downdrafts associated with the MCS perturbed the low level thermodynamic conditions and contribu ted to the rapid decay of the MCS. Slow intensification of the monsoon depression circulation during the preorganization stage is primarily due to favorable interactions with large-scale mean and eddy circulati ons at both upper and lower levels. The overall convective signature w as observed in hourly satellite imagery to become more organized durin g a 24-h period between the two AOPs. This organization stage was char acterized by the formation of a new MCS near the midlevel circulation of the decaying MCS from the preorganization stage. Satellite imagery indicates that the broad monsoon depression began to organize around t he new MCS and the outer convection started to be oriented in large pr inciple bands. During the transformation to a tropical storm during th e second AOP, the outer principal bands appear to separate the inner c irculation of the monsoon depression from the large-scale monsoon trou gh environment. Convection rapidly develops along the periphery of the inner circulation that now contains a Vigorous central updraft and hi gh values of equivalent potential temperature that extend to the middl e troposphere. Although several episodes of MCS generation and decay o ccurred throughout the development of the monsoon depression, it is hy pothesized that the subsynoptic processes in the MCS during the first AOP and the MCSs that formed immediately following the second AOP cont ributed to the concentration of the monsoon depression center and tran sformation to a tropical cyclone.