Abstract:
The multi-scale characteristics of an extreme rainstorm in North China on July 2023 were studied using ERA5 reanalysis data with the spatial resolution of 0.25° × 0.25° and JMA GSMaP precipitation data with the spatial resolution of 0.1° × 0.1°. Results showed that the extreme rainfall episode in North China was formed under the combined effect of monsoon vortex in the Bay of Bengal, remnant of Typhoon Doksuri, Typhoon Khanun, upper- and low-level jet, high blocking and terrain elevations. The configuration of multi-scale weather systems was unlike any previous extreme rainstorm process. The Bay of Bengal monsoon and Typhoon Khanun provided favorable conditions for transportation of water vapor during the continuation of precipitation. A high blocking to the north of North China caused the accumulation of water vapor from low latitudes to gather at east of the Taihang Mountains. The remnant vortex of Doksuri obtained water vapor and was situated at the southern foothills of the Taihang Mountains. The horizontal potential gradient formed by this remnant vortex, along with the blocking high pressure to its north and the western North Pacific subtropical high to its east, promoted the development and intensification of a low-level jet. The coupling of 850 hPa low-level jet and 200 hPa high-level jet continuously generated mesoscale convective clouds on the windward slopes of the Taihang Mountains. When ground winds in North China were nearly perpendicular to topography, pronounced forced lifting motion due to topography further organized and developed mesoscale convective clouds, resulting in short-term heavy precipitation. The first stage of intense precipitation occurred when atmospheric stratification was vertically convectively stable. However, intensified boundary layer low-level jet promoted inclined upward flow over windward slopes, triggering release of conditionally symmetric instability energy and leading to heavy precipitation. The second stage mainly occurred in warm sectors with atmospheric stratification showing vertical convective instability triggered by terrain-induced dynamical forcing.