The organization and intensity of convections in a synoptic-scale tropical disturbance are critical in tropical cyclone formation. Aircraft radar observations during special field experiments (TCS-08 or PREDICT) enabled more detailed kinematic and dynamic structures of the mesoscale convective systems (MCSs). However, the observations of the convections just prior to tropical cyclone formation (e.g., within 48 h prior to a tropical depression formation) are absolutely rare.
Satellite passive microwave 85-GHz polarized corrected temperatures (PCT) images have been used to investigate the evolution of convective features related to TC formation. In particular, the PCTs on the NRL TC website have been analyzed qualitatively but a quantitative guide for analyzing the PCT images is also necessary. Therefore, this study calculates number of MCSs evolving within tropical disturbances, size of each MCS, minimum and average PCT values for each MCS, and etc. to characterize the MCSs related to tropical cyclone formation.
Multi-years analyses of the quantitative feature of the PCT images indicate that the sizes of the largest MCS within a tropical disturbance are significantly increased after about 48 h prior to the tropical depression declaration time for TY-developing cases. The result implies that the dominant scale of the MCS has changed from meso-≤ to meso- ± scale. The occurrence of the most intense convection (that is, the deepest hot tower or vertical hot tower) is not necessarily at the nearest TC formation time. That is, the deepest hot towers can exist earlier than the time the transformation to larger scale happened. In addition, our results show that the size and minimum PCT of the largest MCS before tropical depression declaration time are different in TY developing and TY non-developing cases. Larger MCS for TY developing cases were detected by TMI observation than in TY-non-developing cases.