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-b-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-b-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.''

Oceanic influence on the Baiu frontal zone in the East China Sea

“…During the cruise, we observed two distinct frontal structures in the BFZ similar to the frontal structures reported by Moteki et al [2004aMoteki et al [ , 2004bMoteki et al [ , 2006. However, their analyses were limited to atmospheric data and they did not investigate oceanic influences on frontal structures of the BFZ.…”

“…The BFZ is accompanied by characteristic frontal structures in the lower troposphere, which together are called the Baiu front, and can be identified by sharp gradients of specific humidity and equivalent potential temperature [Ninomiya, 1984] and a horizontal wind shear line [Akiyama, 1973]. The front shows multiscale structures from meso to synoptic, and interactions among various timescales are believed to be essential to develop and maintain the BFZ [Ninomiya and Akiyama, 1992;Moteki et al, 2004aMoteki et al, , 2004b. Mesoscale disturbances frequently develop within the BFZ, accompanied by intense convection and heavy rain, occasionally causing flooding and mudslides [e.g., Ogura et al, 1985;Kato, 1998;Davidson et al, 1998;Ninomiya and Shibagaki, 2007;Maeda et al, 2008;Manda et al, 2014].…”

Seasonal Migration of the Baiu Frontal Zone over the East China Sea: Sea Surface Temperature Effect

“…During several of these heavy rainfall events, a localized cold high over the Yellow Sea has been confirmed on the surface weather map (Moteki et al 2004a(Moteki et al , 2004b. These authors note that a cold dome originating from the Yellow Sea contributes greatly to the behavior of the Baiu front, which induces rainfall events over Kyushu.…”

Environment and morphology of mesoscale convective systems associated with the Changma front during 9–10 July 2007

“…This study utilised Doppler radar, which provides datasets with high spatial and temporal resolutions of the internal structure of precipitation. In the past, dual-Doppler radar observations have provided in-depth information on structures of the meso-α and meso-β scales, including their development in monsoon frontal systems (Yamada et al, 2003;Moteki et al, 2004;Geng et al, 2004). You et al (2010) proved that deep warm-air advection (WAA) supports the maintenance of a convective system for a longer time and results in greater rain intensity, producing drops of larger sizes.…”