Key synoptic-mesoscale coupling factors in extreme rainfall from the merger of two Meso-β convective systems in Henan, China

On 21 July 2021, extreme rainfall hit central and northern Henan, China, with Xinxiang recording an hourly rainfall of 149.9 mm and 10-h accumulations exceeding 400 mm (the 7.21 Xinxiang rainstorm). This rainstorm, a critical episode in the “21·7” Henan rainstorm event occurring one day after the high-impact 7.20 Zhengzhou rainstorm, was triggered by the development and merger of two meso-β-scale convective systems (NMCS and SMCS). Using convection-permitting ensemble simulations and targeted data assimilation, this study first examines the roles of key mid-to-lower tropospheric synoptic systems (which influenced the 7.20 Zhengzhou rainstorm) in the subsequent 7.21 Xinxiang rainstorm, then identifies and verifies critical mesoscale factors governing the two MCSs' development and merger. Results indicate that a mid-level low-pressure system (corresponding to a lower-level trough) and a high-pressure ridge extending from the southwestern periphery of the western Pacific subtropical high were critically important synoptic-scale systems influencing both consecutive rainstorm events. For the 7.21 Xinxiang rainstorm, the low primarily governed the total precipitation amount, while the ridge mainly influenced the rainfall spatial pattern in central Henan by modulating the SMCS organization. Furthermore, two mesoscale shear lines are found to drive convection evolution. One, formed by the near-surface southeasterly flow ahead of the trough turning into a northeasterly barrier flow parallel to the terrain, influenced the southern tip of the NMCS. The other, from converging ridge-edge southwesterlies and southeasterlies, drove SMCS development. Radar radial velocity assimilation confirms that refining these pivotal shear lines can enhance convective and quantitative precipitation forecasts.