A case study of the July 2021 Henan extreme rainfall event: From weather forecast to climate risks

Abstract

During 17–22 July 2021 a rainfall event in Henan Province of China has delivered over 820 mm precipitation around Zhengzhou with hourly intensity exceeded 201 mm. This paper presents a systematic case study of its driving processes, predictability, and future climate risks. The key aspects of this event are 1) geographically stationary and 2) temporally persistent. A sustained low-level moist easterly jetstream resulting from between typhoon In-Fa and a northward displaced subtropical high and orography in the region appear to have played a major role in the event. The event was, overall, well predicted in global operational 5-day weather forecast, though the details may not be very accurate. From a climate perspective, the large-scale low-level circulation pattern was like the August 1975 catastrophic floods in the same region, but opposite to the 2020 summer anomalous extreme Meiyu situation. Both weather patterns were rare during the past 47 years, together accounting for less than 10% of low-level daily weather patterns. The orographic features around Henan make it vulnerable for floods. A risk assessment based two sets of ensemble climate model simulations suggest the probability of such events occurring in the future will increase under the IPCC RCP8.5 scenario.