Volume 82 (2004) Issue 1 Pages 45-65
In this paper, the synoptic- and meso-α scales structure of two oceanic rainbands, and build-up processes of the frontal convergence, are investigated with the use of a regional spectral model (RSM) and a nonhydrostatic model (NHM). Over the eastern part of the East China Sea on 27 June 1999 during the X-BAIU-99, we observed that a northern rainband (LINE2) merged into a southern rainband (LINE1), and LINE1 developed rapidly. Before their merging, LINE1 and LINE2 developed along the “water vapor front” and the Baiu front, respectively. The “water vapor front,” found by the meso-β-scale analyses of Part I of this series, is a weak convergence line with a large moisture gradient in a southerly wind field. In the RSM-simulated field, the “water vapor front” extended from the eastern coast of China with a length of about 1000 km. The NHM simulation revealed that the “water vapor front” is a convergence line with oceanic southwesterly and continental westerly flows. The convergence line formed a meso-α-scale rainband (LINE1) in the southerly wind field over the ocean without any land effects. Because an abundant moisture supply with the oceanic airflow was blocked at the “water vapor front” by the continental airflow, the rainfall intensity was quite weak along the western part of the Baiu front. The lowlevel convergence of the eastern part of the “water vapor front” was quite weak, the oceanic air had partially advected into the eastern part of the Baiu front, and LINE2 developed strongly. In a synoptic-scale cold air mass to the north of the Baiu front, a meso-α-scale cold pool formed in a stratiform precipitation region. The cold pool expanded to the south, and pushed rapidly southward the Baiu front, about 50 km in 2 hours. It was confirmed that the cold pool was created by an evaporation cooling of raindrops from a sensitivity experiment. The experiment revealed that the cold pool continuously accelerated northerly winds, and intensified the low-level convergence along the Baiu front. In the areas of the southern side of the Baiu front, and the “water vapor front,” southerly winds were greatly accelerated over 5 m s-1 in 4 hours by the pressure gradient force enhanced near the convective rainfalls of LINE1 and LINE2. The frontal convergence along both fronts were greatly enhanced by the acceleration of the southerly winds.