In this study, numerical simulations are performed on the formation of Typhoon Robyn (1993) in the western North Pacific (WNP) during the Tropical Cyclone Motion (TCM-93) field experiment. One of hypotheses of the experiment addresses the role of tropical mesoscale convective systems (MCSs) in the development of tropical cyclones (TCs). This TCM-93 case is chosen because additional in-situ and aircraft data are available to define the structure of several MCSs associated with the formation process, and for model initialization. Some conceptual models on cyclogenesis associated with MCSs have been proposed in previous studies. One of the major issues is how a warm-core vortex is generated. Some of the previous studies emphasize the importance of the duration of stratiform rain, some address the importance of successive convection redevelopment, and some focus on generation of low-level potential vorticity anomalies. This study identifies the physical processes responsible for TC formation in the WNP where the background vorticity associated with the monsoon trough and passages of tropical disturbances are both essential factors.

The PSU-NCAR mesoscale model MM5 with nested grids is used for the simulations. The model is initialized with large-scale analyses from the Navy Operational Global Atmospheric Prediction System (NOGAPS). Then the observations from the two intensive observation periods during TCM-93 are nudged into the model fields during the simulation. In the control experiment, the Kain-Fritsch cumulus parameterization is activated on both the coarse grid with 81-km resolution and the fine grid with 27-km resolution. Preliminary results show that the deepening process of the pre-Robyn disturbance in a monsoon depression can be simulated in MM5, but no realistic MCS structures are present. Sensitivity experiments will be performed on the choice of cumulus parameterization, and the addition of a finer grid in which an explicit depiction of the convective clouds can be carried out based on guidance from satellite imagery.