Synoptic Features of the Second Meiyu Period in 1998 over China

The synoptic circulation features, moisture transport and water vapor budget over the central and eastern China during 21-31 July 1998 are studied by utilizing the daily 1°×1°data of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and observational data of 24-h rainfall recorded by meteorological stations in China. The results clearly indicate that a dew-point front (it was revealed by Gao et al. 2002), marked by a large horizontal moisture gradient in the mid-lower troposphere, located near the periphery of the West Pacific subtropical high (WPSH), can conjugate with the Meiyu front, and form a predominant double front structure (the Meiyu front system),which is related to the intense rainfalls over the Yangtze River Basin in China. The precipitation over China for 21-31 July 1998 was characterized by a narrow intense rainfall zone over the Yangtze River Basin,and it was against the southern side of the Meiyu front and bounded by the dew-point front. The water vapor transport and budget over three domains, corresponding to the location of the dew-point front, the passageway of the Meiyu front and the dew-point front, and the Meiyu front, respectively, bounded by 20°-25°N, 25°-30°N, and 30°-35°N, and 105°-121°E, indicate that the convergence of the moisture flux within the passageway is mainly due to the southwesterly and southerly moisture inflow across the dew-point front.Persistence of the water vapor gradient located at both sides of the passageway is correlated highly with the southwesterly and southerly moisture inflow and its convergence in the passageway. Water vapor budgets over the three domains indicate that the water vapor is mainly converged at lower levels, and is uplifted from low levels to upper levels through convective transport. The passageway has net water vapor budget and is the area corresponding to the most water vapor transport and convergence during the analyzing period. The diagnosis results further manifest that the water vapor mainly comes from the Indian Ocean and the Pacific , and that the water vapor convergence of regional torrential rain is also provided by the semihemisphere scale water vapor transport.