An observational study of the development of a rainband on a Meiyu front causing heavy rainfall in the downstream region of the Yangtze River

Abstract

The mesoscale structure and evolution of a rainband that occurred on 24 June 2001 in the downstream region of the Yangtze River and produced heavy rainfall as much as 110 mm within only two hours in a limited area were studied through detailed analyses of upper-air, surface, and triple-Doppler radar data. The rainband evolved on the north side of a surface Meiyu front. It was oriented in a northeast-southwest direction nearly parallel to the mean storm motion and the low-level vertical shear. The rainband lasted more than 5 hours and propagated slowly southeastward. The environmental conditions ahead of the rainband were characterized by extraordinary stability below and significant conditional instability in a deep layer above the frontal surface that was approximately 0.5 km above the ground. It was found that significant amount of convective available potential energy (1179-370 J kg-1) could be realized by lifting air parcels in a deep layer (0.7-2.5 km) bove the frontal surface. The rainband formed as a core of the storm-relative northeasterly inflow associated with the large-scale environment appeared on the backside around an altitude of 4 km. The rapid development of the rainband was associated with the surge of another storm-relative northeasterly inflow from the rear around an altitude of 2 km. The latter inflow appeared to be related to the development of a mesoscale vortex after the formation of the rainband. The air that fed the updrafts of the rainband came from an elevated layer above the frontal surface, rather than from the boundary layer. On the other hand, downdrafts at low-levels were shallow and weak during the formation and development of the rainband and strong outflows under the rainband were absent. It is likely that the horizontal convergence enhanced by the northeasterly inflow around an altitude of 4 km was important in the formation of the rainband by lifting the elevated air that possessed conditional instability. Meanwhile, the lifting of the most unstable air in the vertical column of the atmosphere by the northeasterly inflow around an altitude of 2 km would have aided the rapid development of the rainband and the heavy rainfall. © 2004, Meteorological Society of Japan.