Cause analysis of a warm-sector excessive heavy rainfall event in Xinxiang on 9 July 2016

Using conventional meteorological observations, observational data from regional automatic weather stations, satellite images, radar data and NCEP reanalysis data, we have conducted a synergic analysis of the cause of a warm-sector excessive heavy rainfall event in Xinxiang on 9 July 2016. The results indicate that this event falls into the category of warm-sector extreme precipitation, and is long in precipitation duration, high in intensity, inhomogeneous in distribution and serious in disaster, whose main weather systems are 500 hPa low vortex and 700 hPa shear line. The southeast warm and wet airflow on the periphery of a typhoon was uplifted at the windward side (western Xinxiang) of the Taihang mountains, which made the deep moist layer over the heavy rain area maintaining for a long time. The superposition of the weak cold advection carried by the low vortex moving southwards at 500 hPa and the warm advection in the low-level over Xinxiang further increased atmospheric instability, which provided water vapor and potential instability conditions for the occurrence of heavy rainfall. Suction effect in the significant diversion area at 200 hPa, topographic uplift effect by Taihang mountains, and low vortex and shear line in the midand low-level caused the vertical upward movement over Xinxiang, which is the dynamic mechanism for heavy rainfall formation. Favorable conditions for high precipitation efficiency in Xinxiang include large-scale high humidity environment in the central and southern part of North China, thick warm cloud and humid layers, abnormally low free convection height and lifting condensation height as well as medium to strong CAPE value. The heavy precipitation center in Xinxiang was caused by the merging of two isolated β-mesoscale convective systems (M_βCS) within which several low-mass center convective cells were generated by a mesoscale convergence system maintained in the east front of Taihang Mountain for a long time. The low-mass center warm cloud precipitation echo and the obvious train effect can be found in the radar basic reflectivity factor chart. It is the long-time retention of mixed convective precipitation echoes dominated by convective echoes that caused the excessive heavy rainfall event in Xinxiang.