The southward movement preference of large-scale persistent extreme precipitation events over the Yangtze River Valley during Mei-yu period

Abstract

Among all rainfall events, heavy rainfall that affects large areas and persists for days can cause serious flooding, severe casualties, and substantial economic losses. In this study, we identify the extreme precipitation events that affect a large area and have persistence of more than three days (LPEPEs) in China during 1961–2013. The LPEPEs incline to occur in summer and over southern China, coinciding well with the rainy season in China. The movement of LPEPEs is dominated by the tendency to extend southward, especially for the LPEPEs during the Mei-yu period (LPEPEs-M). The dynamical composite analyses on the southward-moving LPEPEs-M show that the southward extension is generated by the combined effect of the large-scale circulation configuration and the diabatic heating caused by large-scale condensation. The latent heating results in a cyclonic anomaly from the middle to the lower levels. Therefore, westerly and easterly anomalies are produced to the south and north of the heating center, respectively. The two anomalies manipulate the vertical distribution of zonal winds which is dominated by the large-scale circulation to both sides of the rainband. This kind of anomaly configuration, together with the quasi east–west zonal distribution of diabatic heating, creates a pattern with positive and negative PV tendency to the south and north of the heating center, leading to LPEPE system development to the south of the precipitation center. Therefore, the LPEPE rainband has a preference to move southward over the Yangtze River Valley during the Mei-yu period. We also test the dynamical mechanism with a numerical sensitivity experiment using the WRF model. In the experiment without latent heating feedback, the Mei-yu rainband moves northward and extends to central and North China. While in the experiment with the latent heating feedback, the rainband intensifies and extends southward. The southward extension preferences of the LPEPE can provide an internal dynamical of the stagnation of East Asian summer evolution.