A study of the formation and development of a mesoscale convergence line within Typhoon Rananim (0414)

Rananim （0414） has not only caused huge catastrophes for the coastal provinces in China, but also produced excessive rainfall in the inland areas, resulting in flash floods and geological disasters. Based on the satellite, surface and sounding observation data, 1°×1° NCEP ananlysis data and 20 km×20 km regional spectral model data published by the Japan Meteorological Agency, the formation and development of a mesoscale convergence line （MCL） within the circulation area of Typhoon Rananim (0414), which eventually leads to torrential rainfall over inland China, was investigated. The results show that (1) the midlatitudinal cold air intrudes on the typhoon circulation area and meets the easterly warm wet airflow in lower levels, which leads to the formation of MCL on its northwestern quadrant; (2) the MCL occurs in the lower troposphere below 700 hPa, with an ascending airflow inclined to the cold air, and forms a secondary vertical circulation across the MCL near which meso- scale convective cloud clusters generate and develop before merging into typhoon remnant clouds; (3) convective instability and conditionally symmetric instability (CSI) appear simultaneously near the MCL, which is favorable for the development of convection and, the increase of atmospheric moist baroclinicity is a key factor for the occurrence of CSI within the slantwise updraft of MCL; (4) the diagnosis analysis of the interactions between the MCL and the typhoon remnant circulation indicates that the MCL obtains kinetic energy and positive vorticity for its further development from the remnant at lower levels, and, the development of the MCL provides, in turn, the remnant with kinetic energy and with positive vorticity at upper levels, which may slow down the decay of typhoon. The MCL not only produces heavy rainfall in a direct manner, but also creates a condition that is favorable for the maintenance of typhoon, and hence sustained precipitation.