Abstract

Data obtained during two aircraft observing periods (AOP) from the TCM-93 mini field experiment are used to describe the transformation between 5° and 10°N of a large depression in the western North Pacific monsoon trough into a tropical cyclone over a 36-h period. The transformation is defined to occur in three stages. Although a large mesoscale convective system (MCS) was present along the eastern periphery of the monsoon depression during the preorganization stage characterized by observations from the first AOP, the overall convective organization of the broad circulation is weak. The structure of the MCS provided a midlevel subsynoptic contribution to the vorticity of the monsoon depression and contributed to a shift in the center of the monsoon depression circulation between 800 and 600 mb toward the MCS location. However, the presence of unsaturated downdrafts associated with the MCS perturbed the low-level thermodynamic conditions and contributed 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 circulations at both upper and lower levels. The overall convective signature was observed in hourly satellite imagery to become more organized during a 24-h period between the two AOPs. This organization stage was characterized 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 the new MCS and the outer convection started to be oriented in large principle bands. During the transformation to a tropical storm during the second AOP, the outer principal bands appear to separate the inner circulation of the monsoon depression from the large-scale monsoon trough environment. Convection rapidly develops along the periphery of the inner circulation that now contains a vigorous central updraft and high values of equivalent potential temperature that extend to the middle troposphere. Although several episodes of MCS generation and decay occurred throughout the development of the monsoon depression, it is hypothesized that the subsynoptic processes in the MCS during the first AOP and the MCSs that formed immediately following the second AOP contributed to the concentration of the monsoon depression center and transformation to a tropical cyclone.

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