Multi-Scale Features of the Meiyu-Baiu Front and Associated Precipitation Systems

Abstract

The multi-scale features of the Meiyu-Baiu front (MBF) and associated precipitation systems are described based on the recent observational studies for the intense rainfalls period in July of 1991.The MBF extends from the Yangtze River basin to the Japan Islands along the northwestern rim of the westward protruding North Pacific subtropical anticyclone. A blocking ridge develops over Primorskiy Kray of Russia, while a cold upper low develops over Mongolia during this period. The low-leveljet stream, nearly moist neutral stratification, and a strong gradient of the specific humidity characterize the MBF. The thermal gradient in the Meiyu front is weak, whereas the relatively large thermal gradient in the Baiu front is sustained between the tropical maritime airmass and the polar maritimeairmass.The strong meridional convergence of the moisture flux sustains the large precipitation in the MBF. The differential advection of the equivalent potential temperature generates convective instability against the stabilizing effect of the cumulus convection, and sustains moist neutral stratification and intense precipitation. The northward ageostrophic winds from the northwestern rim of the westward protruding subtropical anticyclone causes strong low-level convergence in the MBF.The short-wave trough that propagates along the southern rim of the upper cold low couples with the short-wave trough in the MBF. The coupling of the troughs causes the development of the subsynoptic-scale depression (SD) and the associated subsynoptic-scale cloud system. Successively with the development of the SD, a series of the meso-α-scale cloud systems are formed along the long trailing portions of the SD. Thus the ”cloud system family“ with a length of ∼2000 km, which consists of a subsynoptic-scale cloud system and a few meso-α-scale cloud systems aligned along the trailing portion of the SD, is formed along the MBF. Features and the development process of the subsynoptic- and meso-α-scale cloud systems are studied in detail.