Pattern Classification of Heavy Rainfall in Jianghuai Region and Associated Circulations

Newly reconstructed dataset of regional historical heavy rain events, daily rainfall data of 2474 observational stations in China and NCEP/NCAR reanalysis data during 1981-2016, 1-10 d rainfall products of 2016 from ECMWF and T639 models are used to study heavy rainfall events in Jianghuai Region.Firstly, typical rain patterns are refined by empirical orthogonal function (EOF) based on 72 heavy rainfall cases. And then the classification of the regional heavy rainfall patterns for all heavy rainfall cases is done objectively through taking corresponding circulation patterns of basic rain patterns as targets with an analogue method.Furthermore, circulation characteristics of heavy rainfall patterns in Jianghuai Region are investigated. From the prophase of heavy rainfall at 500 hPa height fields, for Type Q_Ⅰ there is an enhancing large low vortex and it splits into two centers forming a circulation type of two troughs and one ridge, with marked anomalies but small amplitudes at low latitudes near Jianghuai Region. Both Type Q_Ⅱ and Type Q_Ⅲ is dominated by meridional circulation, but Type Q_Ⅱ is dominated by negative anomalies over mid-high latitudes of Asian, and then gradually one trough and one ridge develop, with the western Pacific subtropical high(WPSH) extending further westward at low latitudes. Type Q_Ⅲ is dominated by two ridges and one trough forms at mid-latitudes, with the WPSH weaker and unremarkable. However, synoptic systems of all three patterns move slowly, and Jianghuai Region is under the control of a relatively low system by the northwest side of WPSH, which is prevail to the heavy rainfall over this region. The jet stream locates to the north of Jianghuai Region with an anomalous divergence over the region at 200 hPa, but the strength of the jet stream is different for different patterns.Finally, the relationship between circulation patterns and related heavy rain are investigated. The circulation fields are estimated using height field analogue method with different leading time from 10 d to 0, and correlation coefficients between these parameters and the observation are considerable high, such as the height of 500 hPa, meridional wind component at 850 hPa, relative humidity at 700 hPa, and zonal wind at 200 hPa. Threat scores (TS) of the rainfall for days with daily rainfall more than 25 mm and 50 mm are also checked. TS in independent experiments of different lead time from 1 d to 10 d are higher than those from ECMWF and T639 models for events with daily rainfall more than 25 mm.