An Intensification of the Warm and Moist Conveyor Belt of the Asian Summer Monsoon in the “21.7” Henan Rainstorm and Its Key Circulation from the Quasi-geostrophic Potential Vorticity Perspective

This study uses rain-gauge observation data, the fifth reanalysis dataset of the European Center for Medium-Range Weather Forecasts, and the piecewise quasi-geostrophic potential vorticity (QGPV) inversion to mainly investigate the intensification of the warm and moist conveyor belt of the Asian summer monsoon of the Henan extreme rainstorm and its key circulation during July 18–21, 2021. The result shows that the continual westward extension of the subtropical high covered eastern China, on whose southwestern flank, broad southwesterlies transported not only a warm and moist air mass but also a high QGPV over Henan to northwestern China. Meanwhile, in northwestern China, the sensible heating of the Alxa Plateau maintained and deepened a local thermal low, which generated high QGPV anomalies in the near-surface layer of the low-pressure center and the middle and lower tropospheres (750–650 hPa) over the Hetao region. Thus, an extensively high QGPV was formed with low-pressure circulation over Henan, which yielded an extensive confrontation with the subtropical high. The QGPV inversion results show that this meteorological circulation pattern in the middle and lower tropospheres intensified the southerly over Henan. The amplified southerly effectively transported a hot and humid air mass of the warm and moist conveyor belt to the Henan region, which was a key factor for the extreme downpour on July 20. The southerly over Henan on July 20 was primarily contributed by the subtropical high, with secondary contributions from the positive QGPV anomaly at the middle and lower tropospheres over Hetao, while the contribution of the local low-pressure circulation over Henan was slightly smaller.