Study on Formation and Development of a Mesoscale Convergence Line in Typhoon Rananim

This study investigated the formation and development of a mesoscale convergence line (MCL) within the circulation of Typhoon Rananim (0414), which eventually led to torrential rainfall over inland China.The study is based on satellite, surface and sounding data, and 20 km￡20 km regional spectral model data released by the Japan Meteorological Agency. It is found that midlatitude cold air intruded into the typhoon circulation, which resulted in the formation of the MCL in the northwestern quadrant of the typhoon.The MCL occurred in the lower troposphere below 700 hPa, with an ascending airflow inclined to cold air, and a secondary vertical circulation across the MCL. Meso-βscale convective cloud clusters emerged and developed near the MCL before their merging into the typhoon remnant clouds.Convective instability and conditional symmetric instability appeared simultaneously near the MCL, favorable for the development of convection.Diagnosis of the interaction between the MCL and the typhoon remnant implies that the MCL obtained kinetic energy and positive vorticity for its further development from the typhoon remnant in the lower troposphere. In turn, the development of the MCL provided kinetic energy and positive vorticity at upper levels for the typhoon remnant, which may have slowed down the decaying of the typhoon.