Convective Characteristics and Formation Conditions in an Extreme Rainstorm on the Eastern Edge of the Tibetan Plateau

From 7 July to 11 July 2013, an extreme rainstorm occurred in the Sichuan Basin (SCB) of China, which is located at the eastern edge of the Tibetan Plateau, causing severe floods and huge economic losses. The rainstorm event was associated with mesoscale convection systems (MCSs). In this paper, we analyze the evolution characteristics and formation conditions of the MCSs, and the results show that: (1) the continuous activity of MCSs was a direct cause of the formation of extreme rainstorms. Under an “east high and west low” circulation mode, the MCSs formed a “cloud cluster wave train” phenomenon from the plateau to the basin; that is, the MCSs over the basin developed strongly in the process of the MCSs over the plateau area weakening. (2) The activities of MCSs over the rainstorm area was related to ascending branches of the two vertical circulations and topographic gravity wave. Under the influence of meridional vertical circulation, MCSs could move in the south–north direction in the western SCB, while under the influence of zonal circulation, it was difficult for MCSs to develop in the descending airflow east of 106°E. (3) In the mountainous area of the western part of the SCB, the gravity wave stress was obvious and its direction was opposite to the direction of the lower southeast warm–moist airflow. This configuration was able to form a drag effect in the low-level airflow, which was conducive to the convergence of the wind field and strengthening of the vertical ascending movement. These findings help in further understanding the effects of vertical circulation and terrain on MCSs and extreme rainstorms.

Soil erosion and sediment interception by check dam in watershed under extreme rainstorm on the Loess Plateau, China

<p>The magnitude of soil erosion and sediment reduction efficiency of check dams under extreme rainstorms are long-standing concerns. This paper aims to use check dams to deduce the amount of soil erosion under extreme rainstorms in watersheds and to identify the difference of sediment intercepting efficiency of different types of check dams. Based on the sediment deposition of 12 check dams with 100% sediment intercepting efficiency and sub-catchment clustering by taking 12 check dams-controlled catchments as standard separately, the amount of soil erosion caused by an extreme rainstorm event on July 26<sup>th</sup>, 2017 (named “7·26” extreme rainstorm) was deduced in the Chabagou watershed in the hill and gully region of the Loess Plateau. The differences of sediment intercepting efficiency among check dams in the watershed were analysed according to the field observation 17 check dams. The results showed that the average erosion intensity under the ‘7·26’ extreme rainstorm was approximately 2.03×10<sup>4 </sup>t·km<sup>-2</sup>, which was 5 times that in the second erosive rainfall in 2017 (4.15×10<sup>3 </sup>t·km<sup>-2</sup>) and 11-384 times that in 2018 (0.53×10<sup>2 </sup>t·km<sup>-2</sup> - 1.81×10<sup>3 </sup>t·km<sup>-2</sup>). Under the ‘7·26’ extreme rainstorm, the amount of soil erosion in the Chabagou watershed above Caoping hydrological station was 4.20×10<sup>6</sup> tons. The sediment intercepting efficiencies check dams with drainage canals (including the destroyed check dams) and with drainage culverts was 6.48% and 39.49%, respectively. The total actual sediment amount trapped by the check dam was 1.11×10<sup>6</sup> tons, accounting for 26.36% of the total soil erosion amount. In contrast, 3.09×10<sup>6</sup> tons of sediment was inputted to the downstream channel, and the sediment deposition in the channel was 2.23×10<sup>6</sup> tons, accounting for 53.15% of the total amount of soil erosion. The amount of sediment transport at the hydrological station was 8.60×10<sup>5</sup> tons. The sediment delivery ratio (SDR) under the “7·26” extreme rainstorm was 0.21. The results indicated that the amount of soil erosion was huge, and the sediment intercepting efficiency of check dams was greatly reduced under extreme rainstorms. It is necessary to strengthen the management and construction technology standards of check dams to improve the sediment intercepting efficiency and flood safety in the watershed.</p>