Roles of Synoptic to Quasi-Biweekly Disturbances in Generating the Summer 2003 Heavy Rainfall in East China

During the Meiyu season of summer 2003, the Yangtze and Huai River Basin (YHRB) encountered anomalously heavy rainfall, and the northern YHRB (nYHRB) suffered a severe flood because of five continuous extreme rainfall events. A spectral analysis of daily rainfall data over YHRB reveals two dominant frequency modes: one peak on day 14 and the other on day 4, i.e., the quasi-biweekly and synoptic-scale mode, respectively. Results indicate that the two scales of disturbances contributed southwesterly and northeasterly anomalies, respectively, to the Meiyu frontal convergence over southern YHRB (sYHRB) at the peak wet phase. An analysis of band-pass filtered circulations shows that the lower and upper troposphere was fully coupled at the quasi-biweekly scale, and a lower-level cyclonic anomaly over sYHRB was phase-locked with an anticyclonic anomaly over the Philippines. At the synoptic scale, the strong northeasterly components of an anticyclonic anomaly with a deep cold and dry layer helped generate the heavy rainfall over sYHRB. Results also indicate the passages of five synoptic-scale disturbances during the nYHRB rainfall. Like the sYHRB rainfall, these disturbances were originated from the periodical generations of cyclonic and anticyclonic anomalies at the downstream of Tibetan Plateau. The nYHRB rainfalls were generated as these disturbances moved northeastward under the influence of monsoonal flows and higher latitude eastward-propagating Rossby-wave trains. It is concluded that the sYHRB heavy rainfall resulted from superposition of quasi-biweekly and synoptic-scale disturbances, whereas the intermittent passages of five synoptic-scale disturbances led to the flooding rainfall over nYHRB.