Observational analysis of the topographic effect of Mount Tai on an extreme rainfall event occurring at the edge of the subtropical high

Intense observations of precipitation around the Mount Tai during an extreme heavy rain event in autumn 2022 in Shandong province by regional automatic weather stations, radars, wind profilers and satellites are analyzed and possible reasons for the precipitation distribution are explored. The results are as follows: (1) The heavy rain event in Shandong occurred under the background of strong southerly flow in the middle and lower troposphere, and the period of heavy rainfall was concentrated from 23:00 BT 1 October to 02:00 BT the next day. The 100 mm rainfall contour showed a "reverse bow" shape, stretching across the north and west sides of the Mount Tai, with over 170 mm of precipitation at each center. In contrast, rainfall on the south side of the Mount Tai was significantly weaker. (2) Heavy rain belts corresponded to the convergence line-mesoscale vortex system on the ground. The mesoscale vortex on the west side of the Mount Tai formed due to the encounter of the cold flow around the north side of the mountain and the warm flow around the south side. It resulted in a strong precipitation center with single-peak precipitation on the west side of the Mount Tai. The convergence line on the north side of the mountain was sustained and rebuilt, resulting in longer precipitation time and greater accumulated precipitation on the north side of Mount Tai. Hourly precipitation on the north side exhibited a double peak pattern. (3) The two precipitation peaks observed on the north side of the Mount Tai corresponded to the two parallel echo bands of radar reflectivity. The first echo band was located on the north slope of the Mount Tai and remained quasi-stationary for a long time, which corresponded to the ascending branch of the horizontal vorticity circulation on the north side of the Mount Tai. Its formation mechanism is the strong development and maintenance of horizontal vorticity due to the southwesterly low-level jet with strong vertical shear and the northeasterly airflow obstructed by the mountain at low levels in nighttime. The second precipitation echo band corresponded to a cold front cloud system. When it approached the north side of the Mount Tai, it was influenced by the leeward upslope southwesterly low-level airflow, resulting in an increase in the radar reflectivity factor. The corresponding ground wind field was featured by a reconstruction process of the convergence line. (4) On the west side of the Mount Tai, the ground convergence line moved southeastward under the drive of low-level cold air, causing the echo band to gradually evolve into a "reverse bow" shape and the heavy rain band also exhibited a "reverse bow" distribution. The south side of the Mount Tai is located under the subsidence branch of the horizontal vorticity formed by strong vertical shear low-level jet, where precipitation was significantly less compared to that in the north and west sides.