Structure and Development of Two Merged Rainbands Observed over the East China Sea during X-BAIU-99 Part I: Meso-.BETA.-Scale Structure and Development Processes

Abstract

This paper is the first in a two-part series in which the multiscale structures and development of oceanic precipitation systems observed in the Baiu frontal region during a field experiment of X-BAIU-99 are investigated. The meso-β-scale structure, and development of two merged rainbands on 27 June 1999, were observed by dual Doppler radars over the eastern part of the East China Sea. A southern rainband called LINE1 formed in a southwesterly wind field associated with a weak convergence line in a layer of 1-2 km. A northern rainband called LINE2 formed along the Baiu front ahead of cold northwesterly winds. LINE2 moved southeastward and merged into the quasi-stationary LINE1. As the merging occurred, LINE1 developed rapidly resulting from an intensification of low-level convergence associated with the cold northwesterly winds in LINE2. The two merged rainbands were reproduced well by a 5 km-resolution non-hydrostatic model, and the thermodynamic and moisture structures of the circumstances of the rainbands before the merging were analyzed in detail. The weak convergence line of LINE1 in the southwesterly wind field had a large water vapor gradient in a layer of 0.5-1.5 km. This line with the large water vapor gradient distinct from the Baiu front is named a “water vapor front” in this study. The present meso-β-scale analyses showed that two distinct fronts without any land effects existed in the Baiu frontal region: the “water vapor front” (LINE1) and the Baiu front (LINE2). The rapid intensification of the rainfall in LINE1 was induced by a combination of a strong convergence in the Baiu front, and a large water vapor supply from the south in the “water vapor front.”