Mesoscale and convective-scale characteristics of an incipient tropical depression observed over the western North Pacific

The role of small-scale processes, such as the evolution of mesoscale convective systems and mesoscale vortices, in a tropical cyclone formation is recently focused in numerical studies using high resolution models. The present study aims at understanding the role based on observations, carried out using Doppler radars over the western North Pacific. The case studied is an incipient tropical depression, which was characterized by a peak of convective activity 1.5 days before the lowering of sea-level pressure. This formation occurred when a tropical disturbance in an easterly trade flow approached a monsoon trough ahead of a low-level monsoonal southwesterly flow. During the genesis stage, one of mesoscale vortices in stratiform echo regions was replaced by a group of deep convective cells possessing mesocyclones (see an attached figure). The organization of this convection led a low-level cyclonic circulation on a system scale (~100 km). In terms of morphology and kinematics, this evolution of cyclonic circulation is quite similar to that induced by vortical hot towers, simulated in Montgomery et al. (2006). The environment of the convective system was characterized by the rapid destabilization, which resulted from the confluence of undisturbed air in the lower troposphere. These results suggest that only a mesoscale vortex in the special environment, characterized by low-level convergence and strong latent instability, in addition to synoptic-scale cyclonic vorticity and weak vertical shear, has a potential to grow into a circulation center of a tropical cyclone.